History and Basic Design of .223 and 5.56 Ammunition.
 

Q. What is the history behind the development of the .223/5.56mm round?

Studies of the fighting in WWII determined that most of the infantry fighting took place at distances under 200 yards, and those figures have not changed much in modern conflicts.⁽¹⁾  This was a revelation at the time and a controversial one, as ever since the development of smokeless powder, the long distance capabilities of military rifles had been stressed.  It was common for rifles designed in the 1890s through the 1940s to have sights adjustable out to 1,000 or even 2,000 yards, and often not having an adjustment below 200 or 300 yards.  Obviously, there was a discrepancy between the design of these rifles and how they were most often used.

Following WWII, the US military decided it needed a select-fire, detachable-magazine rifle.  (The WWII-era M1 Garand had originally been designed with a detachable magazine, but at the time, the military decided they were a liability for a standard, front-line infantry rifle and had the M1 redesigned.)  During this period during the late 40s and early 50s, many nations were experimenting with smaller-caliber rifles that were controllable in full-auto and allowed more rounds to be carried.  The US military insisted on a 30 caliber rifle, though, and merely shortened the existing .30-06 Springfield (7.62x63mm) round to create the 7.62×51mm round, which Winchester released commercially as the .308 Winchester.  The US also forced this round onto the newly-formed NATO, over protests that it was too much cartridge, would require rifles to be too heavy, and wouldn't be controllable on full auto.  The first point is arguable, but the last two were certainly true.  Still, the US military, having determined that the Belgium-designed FN FAL was a better rifle then the domestic M14 (a modified M1 Garand), chose the M14 anyway.  Such is politics.

The M14 program was a political minefield and during the early 1960s, minor US involvement as "advisors" in the southeast Asian country called Vietnam was beginning to escalate.  It didn't take long before the Vietnam expansion, coupled with manufacturing problems with some M14 contractors, resulted in too many soldiers and too few M14s.  The military initially pulled WWII M1 Garands out of storage and pressed them back into service, but these long, heavy rifles were poorly suited to the jungle environment of Vietnam.  During this time, Eugene Stoner of ArmaLite, the armament division of Fairchild Aircraft, had designed a rifle called the ArmaLite Model 10, or AR-10, which was chambered in the current NATO round of 7.62×51mm.  Though the AR-10 was produced too late to enter the M14 competition, ArmaLite hoped to sell the AR-10 to foreign militaries.

Meanwhile, there was a faction of the US Military and the Congress which supported the idea of a lightweight, select-fire rifle firing a mid-power, small-caliber, high-velocity (SCHV) cartridge.  After seeing the ArmaLite AR-10, they discussed their desire for a scaled-down model.  ArmaLite engineers Jim Sullivan and Bob Fremont scaled down the AR-10 to fit the hot varmint cartridge of the day, the .222 Remington.  During some preliminary military testing, it was decided that the .222 Rem wasn't quite powerful enough.  Though the .222 Remington Magnum existed and had the power they were looking for, the severe shoulder angle would have prevented positive feeding in a semi-auto, and so it was decided that the best solution was to lengthen the .222 Rem case.  The result was the 5.56×45mm cartridge, designed by G. A. Gustafson, which Remington released commercially as the .223 Remington.  This cartridge has virtually identical ballistics as the .222 Mag and, over time, the wide availability of .223 guns and ammo has lead to the demise of the .222 and .222 Mag cartridges.

The AR15 was initially adopted by the Air Force, but the need for rifles for soldiers heading to Vietnam gave the "medium-power cartridge" supporters an opening and the AR15 rifle was hastily procured, initially as a one-time purchase.  Continued problems with the M14 program lead to the official adoption of the AR15, which was given the US military designation "M16."

Fact: The national average engagement range for police 'snipers' has, for the past 20 years, been 78 yards.  The FBI Hostage Rescue Team (HRT) snipers are limited to engagement ranges of 200 yards.  The longest recorded shot taken by a police marksman in the US is 97 yards.  (There are some reports that indicate some longer shots, including one alleged 300 yard shot in 1982 by the U.S. Park Police in response to a bombing threat at the Washington Monument- but these are very rare and not confirmed). The FBI's uniform crime report indicates that the average engagement range in a handgun incident is between 7 and 10 feet.

For a more detailed history of the M1 Garand see: The Complete Guide to the M1 Garand and the M1 Carbine, by Bruce N. Canfield.

For a more detailed history of the M16, see: The Black Rifle: M16 Retrospective, by R. Blake Stevens.

For more details about the history of the 5.56x45mm round itself, see A 5.56×45mm "Timeline" by Daniel E. Watters on Dean Spear's The Gun Zone.

Q. What is the difference between 5.56×45mm and .223 Remington ammo?

In the 1950's, the US military adopted the metric system of measurement and uses metric measurements to describe ammo.  However, the US commercial ammo market typically used the English "caliber" measurements when describing ammo.  "Caliber" is a shorthand way of saying "hundredths (or thousandths) of an inch."  For example, a fifty caliber projectile is approximately fifty one-hundredths (.50) of an inch and a 357 caliber projectile is approximately three-hundred and fifty-seven thousandths (.357) of an inch.  Dimensionally, 5.56 and .223 ammo are identical, though military 5.56 ammo is typically loaded to higher pressures and velocities than commercial ammo and may, in guns with extremely tight "match" .223 chambers, be unsafe to fire.

The chambers for .223 and 5.56 weapons are not the same either.  Though the AR15 design provides an extremely strong action, high pressure signs on the brass and primers, extraction failures and cycling problems may be seen when firing hot 5.56 ammo in .223-chambered rifles.  Military M16s and AR15s from Colt, Bushmaster, FN, DPMS, and some others, have the M16-spec chamber and should have no trouble firing hot 5.56 ammunition.

Military M16s have slightly more headspace and have a longer throat area, compared to the SAAMI .223 chamber spec, which was originally designed for bolt-action rifles.  Commercial SAAMI-specification .223 chambers have a much shorter throat or leade and less freebore than the military chamber.  Shooting 5.56 Mil-Spec ammo in a SAAMI-specification chamber can increase pressure dramatically, up to an additional 15,000 psi or more.

The military chamber is often referred to as a "5.56 NATO" chamber, as that is what is usually stamped on military barrels.  Some commercial AR manufacturers use the tighter ".223" (i.e., SAAMI-spec and often labeled ".223" or ".223 Remington") chamber, which provides for increased accuracy but, in self-loading rifles, less cycling reliability, especially with hot-loaded military ammo.  A few AR manufacturers use an in-between chamber spec, such as the Wylde chamber.  Many mis-mark their barrels too, which further complicates things.  You can generally tell what sort of chamber you are dealing with by the markings, if any, on the barrel, but always check with the manufacturer to be sure.

Typical Colt Mil-Spec-type markings: C MP 5.56 NATO 1/7

Typical Bushmaster markings: B MP 5.56 NATO 1/9 HBAR

DPMS marks their barrels ".223", though they actually have 5.56 chambers.

Olympic Arms marks their barrels with "556", with some additionally marked "SS" or "SUM."  This marking is used on all barrels, even older barrels that used .223 chambers and current target models that also use .223 chambers.  Non-target barrels made since 2001 should have 5.56 chambers.

Armalite typically doesn't mark their barrels. A2 and A4 models had .223 chambers until mid-2001, and have used 5.56 chambers since.  The (t) models use .223 match chambers.

Rock River Arms uses the Wylde chamber specs on most rifles, and does not mark their barrels.

Most other AR manufacturers' barrels are unmarked, and chamber dimensions are unknown.

Opinion: In general it is a bad idea to attempt to fire 5.56 rounds (e.g., M193, M855) in .223 chambers, particularly with older rifles.

Fact: SAAMI specifically warns against the use of 5.56mm ammo in .223 chambers.  The .223 SAAMI specification was originally made with bolt rifles in mind.

For more see the SAAMI website ammo warning.

5.56 v. .223 Remington specification.

Fact: The different manufacturer's chamber types are listed at length and in great detail at: The Maryland AR15 Shooters Site.

Q. Which should I be looking for in an AR15, a 5.56 NATO or .223 Remington chamber?

This is really a matter of the role for which you plan to use your AR.  .223 Remington chambers will give you slightly better accuracy, which is important for a match or varmint rifle.  Any loss of feeding and cycling reliability and the restriction against shooting military ammo isn't as important as the accuracy gains for a rifle used in these roles, because for these rifles, accuracy is everything.  People who just want to plink or who plan to shoot military ammo (such as most of the cheap surplus ammo available), and especially those who may use their AR as a weapon, should choose 5.56 chambers.

Q. What is the circle-cross stamp on some of my ammo?

The circle-cross Å is the NATO symbol.  It indicates that the ammo was loaded in a NATO-approved facility and meets the NATO specifications for that round.  Note that NATO specifications are not the same as US military specifications and that many NATO-approved rounds do not meet US military specs.  US military specs (such as M193 and M855) have additional requirements, such as minimum velocities, that the NATO specs (like SS-109) don't have.

Q. How can I tell if a round is SAAMI, US military, or 5.56 NATO Mil-Spec?

Generally if the round is an M193, M855, M196, M856, or SS-109 round it is Mil-Spec.  This FAQ will help you determine the differences between these specs.  Often Mil-Spec rounds sold commercially have similar model numbers, like XM-193.  Another good clue (but not definitive evidence) is the presence of the NATO cross in a circle on the headstamp.  Ammo that has a painted tip (Green for M855) is generally always military ammo.

Generally you don't have to worry unless you're using a .223-chambered rifle, but it's a good idea to check regardless.  Of course, if you have a Mil-Spec chamber, you needn't bother.

NOTE: All bets are off if the ammo in question has been "remanufactured" or "reloaded."  There's no way to know what you've got with reloads, other than the reputation of the reloader.

FMJ is "Full Metal Jacket" and is used to describe rounds that are entirely encased (except for the bullet base, typically) in a metal jacket, usually copper alloy called gilding metal.  FMJ rounds are also sometimes referred to as "ball" (meaning "standard") ammunition by the military.  Generally these rounds are designed with little to no expansion in mind.  They are comparatively inexpensive to produce, feed well, give good penetration in most materials.  The jacketed nose prevents bullet expansion and typically leaves the bullet intact after striking flesh (the 5.56 round is a notable exception).

JSP is "Jacketed Soft Point" and is used to describe rounds that are encased in a metal jacket, again, usually gilding metal, but leave the soft lead core exposed at the tip of the bullet.  The soft nose deforms upon striking dense mediums, and these rounds are generally designed to expand rapidly at the nose and mushroom, ensuring that the center of gravity stays in front, and causing the bullet to continue traveling forward through the target.  The larger frontal surface area causes more tissue disruption compared to most non-expanding bullets.

JHP is "Jacketed Hollow Point" and is used to describe rounds that are encased in a metal jacket, gilding metal again, but have a small cavity in the nose along with a round opening in the jacket in the nose.  JHP rounds are also designed for expansion but tend to have faster "mushrooming" effects because the hollow point is filled with high-pressure material when the bullet impacts, often peeling back the jacket and making a "mushroom" shaped projectile.

BT stands for "Boat Tail" and refers to the base of the bullet.  A "Boat Tail" is a sloping end which narrows gently at the base of the bullet, so that the cross-section resembles the shape of a boat's hull.  The boat tail shape reduces drag on a bullet, helping it to retain velocity and resist deflection from crosswinds, but causes the bullet to take longer to "settle" after leaving the barrel compared to a standard "flat-base" bullet.  Boat tail bullets are usually selected for long-range shooting, while the flat-base bullet shape tends to be more accurate at short ranges.  A "HPBT" bullet is a "Hollow Point Boat Tail" bullet.

"Ballistic Tip" is actually a trademark of Nosler, who first started making plastic tipped bullets in 1985.

Though originally designed to prevent damage to the bullet nose when feeding (while the nose of a soft tip or hollow point might deform due during feeding to the soft lead content in the nose, a plastic tip bullet will maintain a consistent nose shape) today the primary advantage of a polymer tipped bullets is a high ballistic coefficient. The design also allows the center of gravity to be moved back, increasing in flight stability. This is the same design theory that gives hollow point match bullets better accuracy properties.

In terminal performance, ballistic tips are designed to work like wedges, mashed into the hollow point and inside the jacket on impact, initiating expansion theoretically, quickly and reliably.

The military has used the following ammo types in 5.56mm (excluding blanks and specialty rounds):

Left to Right: M193, M855, M856, Sierra MatchKing HPBT.

Component view of M995 Armor Piercing 5.56mm.

Fact: The specifications for the various rounds are:

M193: Defined by: Mil-C-9963F
55 grain bullet (q 2 grains) at a muzzle velocity of 3,165 (q 40 fps) from a 20" barrel @ 78 feet from the muzzle.  Accuracy: maximum of a two inch mean radius at 200 yards from ten 10 shot groups (~3 MOA).  "Statistically average" M193 ranges from 1.2 to 1.6 inches mean radius, which is equivalent to 1.8 to 2.4 MOA.  Velocity runs about 3,200 fps due to gas loss through the port.  Accuracy is typically around 2 to 2+ MOA from an M16A1 rifle at ranges of 100 to 300 yards.  M193 ammunition should have 1:12 twist or faster.  M193 is barely stabilized with 1:14 at ambient temperatures and will not stabilize at all when the air temperature drops below freezing.

M855: Defined in MIL-C-63989
NATO specifications for M855 Ball require a 61.7 grain (q 1.5 grains) with a hardened steel penetrator at a velocity of 3,000 fps (q 40 fps) from a 20" barrel @ 78 feet from the muzzle.  Typical velocity 15 feet from the M16A2's muzzle is 3,100 fps.  Accuracy: maximum of approximately four MOA over the 100 to 600 yard range.  Typical accuracy of average lots in an M16A2 is about 2+ MOA.  This round must also penetrate a nominal 10 gauge SAE 1010 or 1020 steel test plate at a range of at least 570 meters (623 yards).  The M193 round will penetrate this same plate reliably at 400 yards and about half the time at 500 yards.  The 5.56mm and 7.62mm NATO rounds will penetrate it reliably out to 700 yards or more.  Because the steel penetrator increases the length and changes the weight distribution of the SS-109 bullet, it is suitable for use only in barrels with a twist of one turn in nine inches or faster.  1:10 twist will barely stabilize this round and not below zero degrees F.

Reloaders: Both M855 and M193 in the US generally use Olin Ball WC844 propellant.  Apparently H335 is roughly equivalent to WC844.

Cross sections of various rounds.

Q. What is SS-109?  Is it the same as M855?

SS-109 is Fabrique Nationale's (FN's) name for their 61.5 grain bullet with the steel penetrator in the nose and what they call rounds loaded with this bullet.  (FN calls M193-type ammo "SS-92.")  The US military's M855 round is loaded with the SS-109 bullet, though the US military has additional specifications that ammo must meet before it can be called M855.  So, while all M855 is loaded with SS-109 bullets, all "SS-109 ammo" will not meet the M855 specs.  For example, the British purposely underloaded some lots of their ammo in an effort to get their L85A1 (SA80) rifles to cycle properly.  The ammo is still loaded with SS-109 bullets and labeled as SS-109, but it is nowhere near the M855 velocity specifications.

Q. What type of ammo is current issue for US Military forces?

All front-line forces are armed with M16A2s and M4s and are issued M855 as standard-issue ammo.  A few remaining Reserve and National Guard units, as well as some Air Force units, still carry M16A1s (you've probably seen them in the airports lately) and are issued M193 Ball (if they are issued any ammo at all) because of the difference in twist of the barrel.

Some special forces units, particularly in Afghanistan and Iraq, are using Mk262 and Mk262 Mod1 ammo.  These are rounds loaded with heavy (up to 77 grain) JHP match bullets, in response to some issues with M855 terminal performance.  This continues a recent trend towards heavier rounds (69 grains and over) for improved terminal ballistic performance.

Q. What about Mk262 or Mk262 Mod1?

Due to the poor performance of M855 ammunition, particularly in short-barreled carbines of 10.5-14.5" in length, Navy SEALs, and eventually other SOCOM units, began experimenting with using loads originally designed for marksmanship units for combat.  It was soon discovered that while these loads were both very accurate and had excellent terminal ballistics even from short barrels, the loads weren't quite ideal for combat.  The target bullets had no cannelure, and the bullets weren't crimped in place, which could allow bullet set-back during feeding and raise chamber pressures to dangerous levels.  Further, most loads were of somewhat mild velocities, as the load was chosen with accuracy, not terminal ballistics, in mind.

Sierra was asked to produce a bullet cannelured version, but they intially refused.

Nosler did not have any problems putting a cannelure on their 77 gr bullet. Black Hills Ammunition was approached to make a slightly modified version of these loads for combat use.  A cannelure was specified, the bullets were to be crimped, and the load was to be up to military chamber pressures, with maximum safe velocity being desired.  The primers were to be crimped and sealed, and of course, overall length had allow for loading in standard magazines.

The Marines (in conjunction with a large Federal LE agency) did extensive testing of this large experimental batch of BH loaded Nosler 77 gr cannelured OTM's in the Fall of 2002. It offered outstanding terminal performance out to the maximum test distance of 300 yards. They then ordered 1.1 million rounds of cannelured 77 gr OTM's via the existing Mk262 SOCOM contract (which did not specify a manufacturer) administered through Crane. The cannelured 77 gr load was designated Mk262 Mod 1, and the orginal Mk262 was re-designated Mk262 Mod 0.

According to one observer: "At this point bureaucracy, nepostism, and capitalism converged. Sierra realized they were about to lose a VERY LARGE contract and suddenly they agreed to make the 77 gr SMK with a cannelure. Crane pushed for Sierra to get the contract over Nosler, although the Nosler offered better terminal performance. On the other hand, in all fairness, the Sierra bullet was slightly more accurate out of government test barrels than the Nosler--both shoot nearly the same out of real rifles, such as the by then type classified Mk12 SPR."

Therefore, while a few hundred-thousand rounds of 77 gr Nosler OTM was manufactured and used primarily for testing, the cannelured 77 gr SMK was used in the the multi-million round contract for the Mk262 Mod 1.

Recently, Sierra agreed to add a minimal crimp to their bullet, and this has since replaced the Nosler bullet in the current versions of Mk262 Mod1.  As of April 2004, Mk 262 Mod1 has seen extensive use in Afghanistan and Iraq, in carbines with barrels as short as 10.5", and has proven to be very effective at ranges that M855 is woefully inadequate from the same weapons.  It is also commonly used in the Army's "Special Purpose Rifles" (SPRs), which  are accurized 18"-barreled rifles used by soldiers with additional combat marksmanship training in a squad sharp-shooter role.

Q. Why did the US Military adopt M855 for the M16?

M855 and M856 are newer rounds developed in the late 1970s by Fabrique Nationale (FN) of Belgium.  FN was developing a new 5.56mm belt-fed machine gun they called the "Minimi" (Mini-Machinegun) for entry into the US military's Squad Automatic Weapon (SAW) program.  The SAW was to augment, and in many cases replace, the 7.62×51mm M60 made by Saco Defense (now part of the General Dynamics Armament Division).  Because there was a lot of resistance to giving up larger, longer-range round of the M60, FN focused on making the SAW perform better at longer ranges than existing 5.56 platforms (i.e., the M16).  They did this primarily by developing new bullets: the SS-109 "ball" round and the L-110 tracer.

The SS-109 bullet uses a "compound" core, with a lead base topped by a steel penetrator, all covered in a gilding-metal (copper alloy) jacket.  The L-110 tracer bullet has a copper-plated steel jacket and like all tracer bullets, is hollowed out at the base and filled with tracing compound.  Both bullets are much longer in length than the earlier 55gr bullets, especially the L-110 tracer, which was designed to trace out to 800m, verses 450m for the older M196 tracer round.  Due to their increased length, these bullets require a faster rifling twist to be properly stabilized.  The military settled on a twist rate of 1:7, which is a compromise between the 1:9 twist ideal for SS-109 bullets and the 1:6 twist ideal for L-110 tracers.

Remember, the goal of these new bullets was improving long range performance.  For example, the SS-109 bullet was proven to have better penetration of the then-current-issue steel helmet at 600m than the M80 "ball" ammo fired by the M60.  The M80 ammo was not able to penetrate both sides of the helmet at that distance; the SS-109 bullet could.  The L-110 tracers provided a visible trace out to 800m, which was seen as the maximum effective range of the SAW.  These improvements in long-range performance satisfied the military and the US ultimately adopted the Minimi as the M249 SAW.  They also adopted the new FN bullets and the US specs for the loaded rounds are called M855 and M856.

About the time the SAW was adopted, the M16 "A2 revision" program was underway and it was decided to adopt the new SAW ammo (and its rifling twist) for the M16A2.  As older M16A1 1:12 twist barrels were not able to stabilize the longer bullets, the new bullets had to be marked (in countries with older 1:12 rifles) in order to make sure that the new ammo wasn't used in the older rifles.  M855 received green painted tips and M856 received orange.  M193 is plain and M196 is red.

Take a look at:

Fabrique Nationale (FN)

The Minimi from FN--precursor to the SAW.

The M249 Squad Automatic Weapon (SAW).

Q. So why don't all US military units carry M855?

The original ammo for the M16 was M193, with a 55gr copper-jacketed lead-core bullet.  The rifling twist on the first M16s was 1 turn in 14 inches, or 1:14.  This twist rate was selected simply because it was the twist rate commonly used in the .222 Remington-chambered varmint rifles that the .223 round was based on.  During tests of the M16 in arctic regions, it was found that the slow 1:14 twist wasn't fast enough to stabilize the 55gr bullet in the denser air.  To correct this problem, the twist was tightened to 1:12 and all future M16s and M16A1s came with 1:12 barrels.

The M855 round and particularly the M856 tracer round, are very long bullets and require a faster twist rate in order to be stabilized in air.  Firing M855 from a 1:12-twist rifle would result in an understabilized bullet that would only fly straight for about 90 yards, then veer off as much as 30° in a random direction.  In order to prevent soldiers from accidentally firing M855 in 1:12-twist rifles, M855/SS-109 was given a green-painted bullet tip.  This allows M855/SS-109 to be differentiated from plain-tipped M193.  M16A2s, A3s, A4s, M4s and M4A1s all have a 1:7 twist and can stabilize both M855 and M193.

Fact:  Stabilization is a factor of caliber (bullet diameter), velocity, and bullet length, not bullet weight.

Q. Is all SS-109/M855 ammo marked with green bullet tips?

No.

Countries that previously issued 5.56mm rifles with a 1:12 barrel twist will mark their SS-109/M855 ammo with (usually) green bullet tips, to prevent the ammo from being accidentally fired in the older 1:12 rifles.  Also, countries that regularly supply other countries with older 1:12 rifles usually mark their bullets for the same reason.  Countries that didn't adopt 5.56mm rifles until the NATO SS-109 standard was adopted usually don't mark their ammo with green tips, as they don't have any old 1:12 rifles to be concerned with.  Note that many other countries that now use 5.56 weapons were still using 7.62mm rifles until recently and never used any other ammunition than the SS-109/M855 and L-110/M856, so they don't mark their bullets with green or orange paint unless they intend to sell it to countries who require these markings (the US, Germany, and Belgium, primarily).  They also typically refer to their rounds by the FN bullet name.

Performance of .223 and 5.56 Ammunition.
 

Q. I have my rifle zeroed with M855 ammo.  Will any 62gr ammo shoot the same?

No.

While M855 uses a 62gr bullet, it is a longer bullet due to the steel penetrator in the front of the bullet core.  Steel is less dense than lead, so more volume of steel is needed to end up with the same weight (mass).  There is also a small air cavity in front of the penetrator, unlike a bullet with a solid lead core.  Any non-M855/SS-109 62 grain ammo (such as Wolf and Federal's American Eagle 62gr FMJ offerings) will have a solid lead core, and the resulting bullet will be significantly shorter than an SS-109 bullet.  That means you can expect trajectory and penetration performance to differ as well.

Q. Do M193 and M855 shoot to the same point of impact?

No...

...but within 300 yards, they're generally close enough (for combat use) that rezeroing isn't necessary.  Obviously, you wouldn't want to switch from one to the other for a match without rezeroing.  Consider the graphs below with battle zeros for each round.  (250m zero for M193, 300m zero for M855).

Even out to 300 meters there is a mere 4.5 inch difference between the paths of the rounds.

The big differences in bullet path are past 400 meters.

A: M16A1 firing both M193 and M855, zeroed for M193.
B: M16A2 firing both M855 and M193, zeroed for M855.
 

Fact: The Scoop from the Army's Ammunition Information Notice (61-01) "INTERCHANGEABILITY OF 5.56MM BALL, TRACER AND BLANK AMMUNITION."

Do not zero M16A2, M16A3 rifles or M4 and M4A1 carbines with M193 and then fire M855/M856 as performance will be affected.

Fact: Generally M193 is zeroed out to 250 meters for the flattest trajectory.  Using that "battle zero" the round is never more than 4 inches from the point of aim until almost 300 meters.

By contrast M855 is usually "battle zeroed" to 300 meters.  With this zero the M855 round is never more than 6 inches from the point of aim until 325 meters.

Comparing the bullet paths with these zeros out to 300 meters, we find that M855 is about 5 inches higher than M193 at 300 meters.

M855 (left) and M193 (right).

Q. OK, what is all this stuff about rifle twists and different ammo?

Rounds in flight spin for stability because of the rifling on the inside of the barrel.  Depending on how much they spin, they are more or less stable in their flight and therefore more or less accurate.  The earliest AR15s from the early 1960s had a twist rate of 1 complete twist every 14", or 1:14.  This was increased to a twist rate of 1 turn in 12" for the M16, XM16E1, M16A1, and later rifles and carbines.  The current M16A2s and up and the M4 carbines have a much faster twist rate, 1 turn in 7".  The reason for the 1:7 twist is mainly to stabilize the M856 tracer bullet, which is much longer than other bullets.  You will recall from above that the M856 was designed to provide 800 meters of trace out of the SAW.

While the slow 1 in 12" twist is adequate to stabilize the 55 grain M193, it will not stabilize the 62 grain M855.  As a result, the newer M855 ammo will group 1-2 feet at 100 yards, with bullets flying through the air sideways, instead of shooting to about 2" at 100 yards, like military ammo should.

All this has some ramifications for ammunition selection depending on your rifle's rate of twist.

You can also overspin projectiles and cause overstability. This results in the not-so-desirable condition that keeps the nose of the round pointed high, as illustrated below:

You can also spin them so hard they fly apart.  That's rare, but it happens if you are dealing with very tight twists and very high velocities.  When fired at 3200 fps in a 1-in-7 twist rifle, a round is rotating at over 300,000 rpm when it leaves the muzzle.  Light, thin-jacketed varmint bullets (i.e., 40gr Hornady TNT or Federal Blitz bullets) often can't take that much spin and will pull themselves apart.

Fact: Generally you want a gyroscopic stability factor (Sg) of 1.3 or greater in a given round, about the low end for normal shooting.  You get this on the larger M855 round with a 1 in 9" twist.  By comparison a 1 in 10" twist will keep that M855 round down to about 1.2- not enough if it starts to get cold.  Really you want stability to be between 1.5 and 2.0- a 1 in 8" twist on a M855 round.  In actuality a 9" twist is a bit better for accuracy as it doesn't spin up non-balanced bullets too fast causing them to wobble in flight.  If you have match rounds, well balanced and tested, you don't really have to worry about overtwisting until you hit 5.0 or so.

The result of unstabilized bullets:
A 1 in 12" FN-FNC firing M855 at 100 yards.
(Note the profiles cut out of the target).

Math and Physics: A spin-stabilized projectile is said to be gyroscopically stable, if, in the presence of a yaw angle, it responds to an external wind force with the general motion of nutation and precession.  In this case the longitudinal axis of the bullet moves into a direction perpendicular to the direction of the wind force.

It can be shown by a mathematical treatment that this condition is fulfilled, if the gyroscopic stability factor (SG) exceeds unity.  This demand is called the gyroscopic stability condition.  A bullet can be made gyroscopically stable by sufficiently spinning it.

As the spin rate decreases more slowly than the velocity, the gyroscopic stability factor, at least close to the muzzle, continuously increases.  Thus, if a bullet is gyroscopically stable at the muzzle, it will be gyroscopically stable for the rest of its flight.

Q. OK, that's complex.  Simple question: Can I fire M193 ammo in my 1:7 or 1:9 twist barrel?

Yes.

M193 is essentially a "universal" round; able to be stabilized by barrels with twists between 1:14 and 1:7.  Point of impact will change slightly compared to an M855 zero, so rezeroing is recommended.

It is acceptable to use M193 and M196 ammunition in training in M16A2, M16A3 rifles and M4 and M4A1 carbines (16 percent range reduction).  Substituting between types of ammunition during firing is not recommended.

Q. Can I fire M855/SS-109 in my 1:12 twist barrel?

Yes, but...

...it won't be stabilized properly and after 90-95 yards, it will typically veer off in a random direction.  You often won't hit paper at 100 yards.  Though it won't hurt your rifle to fire this ammo, it is not recommended.  Military manuals warn that it should only be fired in 1:12 twist barrels in a "combat emergency."

"Cartridges M855 and M856 ammunition are extremely inaccurate when fired in the M16 and M16A1.  The M16 and M16A1, with their 1:12 twist, do not impart enough spin on the heavier M855/M856 projectile to stabilize it in flight causing erratic performance and resulting inaccuracy.

Therefore, while safe to fire in M16 and M16A1 they should only be used in an combat emergency and then only for close ranges 91.4 meters (100 yards) or less."

Q. Will M193 be accurate in a 1:7 or 1:9 twist barrel?

It may be marginally less accurate due to the fast twist rate, particularly in 1:7 twist barrels.  Unless you're trying to use these rounds for benchrest shooting, though, it shouldn't be enough to matter.

A bullet's flight is disrupted slightly as it leaves the barrel and after traveling some distance, will "settle down" into an even spiral, similar to a thrown football.  The faster a bullet is spinning, the longer it takes to settle down.  The most accurate twist rate for any length of bullet will be just a bit faster than what is required to stabilize it for its entire flight path (1.3 SG).  But note that bullet quality plays a much bigger part in this equation.  A uniform bullet will spin true; a non-uniform bullet will wobble and be inaccurate.  As a general matter when shooting M193 or M855 (as opposed to match ammo) its better to err on the side of a faster twist rate.  Regardless, both 1:9 and 1:7 twists seem to shoot M193 and M855 very well.

Q. What twist rate do I want for my rifle?

Probably 1:9, but it depends on what kind of bullets you intend to shoot.

Special purpose rifles often have uncommon twist rates.  For example, if you are building a varmint rifle and want to shoot the short 35 grain, 40 grain, and 50 grain bullets, a 1:12, or even 1:14 twist would be best.  On the other hand, long range High Power shooters often select 1:8, 1:7.7, 1:7, or 1:6.5-twist barrels to stabilize the long 77, 80 and even 90 grain bullets used for 1,000 yard competition.  Additionally, new testing of heavier rounds (68-77 grains) seems to show that they perform very well in simulated tissue and may be a better defensive choice than 55 grain or 62 grain rounds. The majority of shooters, though, typically shoot bullets of 50 to 69 grains in weight (note that the 62gr SS-109/M855 bullet is as long as a 71 grain lead core bullet) and should select 1:9 twist barrels.  At typical .223 velocities, a 1:9 twist will stabilize bullet lengths equivalent to lead-core bullets of 40 to 73 grains in weight.

1:12 twist rifles cannot stabilize SS-109/M855 bullets and 1:7 twist rifles are slightly less accurate with lighter bullets and will often blow apart the thin jackets of lightweight varmint bullets.  The 1:7 twist is used by the military to stabilize the super-long L-110/M856 tracer bullet out to 800 yards, but unless your plans include shooting a significant amount of M856, the 1:9 twist rate is better suited for general use.

There is, of course, an exception: if you want to use loads utilizing the heavier, 75-77 grain match bullets currently used by Spec-Ops troops and other selected shooters, you'll want a 1:7 twist barrel.  Although military loadings using these bullets are expensive and hard to get, some persistent folks have managed to obtain a supply, and will need the proper barrel twist to use them.  Anyone who foresees a need to shoot this ammo should consider a 1:7 twist barrel.

Opinions (Pro and Con):

1:9 is best.
Why?  Flexibility.  It doesn't seem to have any problems throwing M856 tracers around, unless it gets really cold, it wears better than 1:7 and it stabilizes more rounds than 1:12.  Additionally, 1:9 rifles, even Mil-Spec chrome chambered and barreled, can attain 1.0-2.0 MOA out to 300+ meters.

No, 1:7 and 1:8 are the best.
Why?  Accuracy.  For heavier and longer rounds during competition shooting, 1:8 and 1:7 twists are the best for heavy 77-80 grain rounds that I use to shoot competitively at 500-1000 meters.  Who needs to shoot tracers anyhow? More importantly, heavier rounds are showing very good results in terminal testing and are proving to be much better defensive rounds.

Q. What about XM193 from Federal and Q3131A? I have heard that some of these don't meet military specifications, particularly with respect to sealant on the necks and primers?

Most manufacturers who market "Mil-Spec" M193 like ammo are either taking "factory seconds" that would otherwise go to the military and packaging them for civilian sale or reducing the second inspection of rounds before distribution. Ammo destined for the government is tested in lot batches and the entire lot is rejected if the batches fail spec tests.  Generally, this ammo is still excellent for both plinking and defensive use.  XM193 and Q3131a in particular are exceptional rounds for all around civilian use and still show very reliable function in AR15s.

Some AR15ers have noticed, however, that sealant or other small details are sometimes lacking for some lots of these rounds.  Sealant in particular is not a critical component to average civilian sales and therefore if sealant problems develop in M193 rounds destined for military contracts (and therefore out-of-spec rounds) they are usually sold as civilian versions of M193 (e.g. XM193).  It should be pointed out that this is no reason whatsoever to avoid these rounds.

If you are really concerned about sealant or intend to use the rounds for long-term storage or quasi-military use where they are likely to see harsh and moist conditions do some testing on random samples of your lot for sealant.  See also the testing done below in the Ammo Oracle.

Yep.

Q3131A and XM193 are Mil-Spec M193.  They mean business.  They are loaded hotter than most commercial loads and you're likely to notice that as soon as you fire them--especially out of a 16" post-ban barrel without a flash hider, you are going to get quite a bit of blast and a fireball.  Some M193 may have flash retardant, but it's just no match for a short barrel without a hider.  Prepare yourself for surgery on your ears if you have one of those short barrels and a muzzle brake.  Always wear eye and ear protection when shooting!

Q. But military ammo has flash retardant, right?

Well, not exactly. This had a lot of us fooled too.

Back in the early Vietnam period M193 called for flash retardant components to be included in the round.  Despite this no current specification any of us are aware of calls for flash retardant in M855 or other military small arms rounds.  After some references from former procurement officers and contractors it's pretty clear that current standards don't call for it.  It's a toss up if there is any flash retardant in your rounds.  Surplus M193 might contain retardant, but fresh M193 probably does not.  The best way to find out is to test it.

"It seems that Picatinny Arsenal feels that flash suppressant might eventually cause a build up-in the gas tubes of M16 type weapons and cause the weapons to malfunction."

He goes on to note sardonically:

"...of course this will not happen since all the soldiers will be dead before this theoretical fouling problem occurs..."

M193 should give you around 3200-3250 fps from the muzzle of a 20" weapon and around 3150 from a 16" weapon.

M855 should give you around 3050-3100 from a 20" weapon and around 2950-3000 from a 16" weapon.

Here's some Q3131A (M193) ballistic data, of course your mileage may vary:

Altitude 1480 feet 70 degrees 50% humidity. 55gr FMJBT Average cD over velocities: .267