OT: Barret's new new 6.8 x 43mm conversion

A very interesting article- the conversion kit looks great, especially with the rail adapter interface. Wachenroder’s linked website/forum (in his signature) had a thread on the 6.8 accuracy that showed great precision for a group of 5 shots. Will it displace 5.56mm for US forces? Will it cause other countries, namely Russia (5.45mm) and China (5.8mm) to rethink their calibers?

I apologize for this off topic message- I realize it has very little to do with aviation, unless you use a modded M4 to shoot down a plane, but the whole caliber debate is quite interesting.

Date Posted: 02-Apr-2004

INTERNATIONAL DEFENSE REVIEW – MAY 01, 2004

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Barrett’s M468 special-purpose carbine
CHARLES Q CUTSHAW

Military cartridges in the 0.280in (approximately 7mm) range have long been considered more advantageous for combat than ‘full-power’ cartridges such as the 0.30-06 or 0.308 Winchester, save for some who insist on having a military cartridge capable of engaging targets beyond 500m. At closer ranges, smaller diameter bullets are just as effective and enable the soldier to carry more ammunition while maintaining lethality at realistic combat ranges. Indeed, in infantry training at Fort Benning, Georgia, in the early 1960s, marksmanship training emphasized that in combat, targets were fleeting in nature and rarely were at distances beyond 300m. Given this premise, the 7.62x51mm caliber used by NATO at the time represented overkill.

In the early 1950s, the UK had developed a 0.280 caliber cartridge that would have changed history had it been adopted by the US. The UK candidate for the NATO rifle was the EM-2, a bullpup-configured firearm that fired a 7mm bullet weighing 8.4g or 9g at a little over 762m/sec. Original FAL rifles were also chambered for this cartridge. The US Army, however, insisted on a ‘full-power’ cartridge embodied in the 7.62x51mm, whose ballistics are very similar to those of the 0.30-06, although propellant technology allowed the case and overall cartridge length to be shortened. After persuading NATO to adopt its ‘full-power’ cartridge, the US essentially convinced NATO to adopt the 5.56x45mm a few years later. If the 7.62mm was overpowered, the 5.56 was distinctly underpowered for military use, being essentially a small game (‘varmint’) cartridge. In fact the 5.56x45mm is illegal for deer hunting in many areas of the US.

The original M16 rifle, designed to fire a 3.5g bullet from a 508mm barrel at velocities in excess of 910m/sec, was essentially different from the one now in use. Two changes have transformed the M16 small arms family into something that Eugene Stoner (its inventor) never envisioned. The M16A2 rifle is closer in concept to the M14 than to the original M16. The M16A2 fires a 4g bullet at a slightly lower velocity than the original. Unlike the M16, the M16A2 can engage targets out to 800m. The second transformation came with the standardization of the M4/M4A1 family of carbines in the US Special Operations community. When used with the 4g SS109 (M855) cartridge, the M4 launches a bullet at only 792m/sec as opposed to some 884m/sec from an M16A2. The M16 achieves its lethality when the bullet strikes its target at over 731m/sec. At this velocity, as the bullet slows it begins to turn base forward. As the bullet turns, the spin causes it to come apart at the cannelure, causing fragments to fly in a miniature explosion that enhances the size of the wound cavity. When an SS109 bullet is fired from a standard M16A2, the threshold for enhanced wound effects comes at about 200m as the bullet velocity falls below 731m/sec. When the SS109 is fired from an M4 or M4A1, this threshold occurs at less than 100m. This is why the SS109 has proven to be a poor ‘people-stopper’ in Afghanistan and Iraq. Many after-action reports state that the wounds inflicted by the M4 are essentially the same as if the enemy had been shot with a 0.22 Magnum. Since almost all M4 carbines in the US military are in the hands of special forces organizations, this problem has not been observed to as great an extent in conventional organizations that use the standard M16.

The immediate reaction of the US special operations community to the M4’s lack of lethality was to order a new 5.56mm cartridge with improved terminal ballistics. This is the Mark 262, Mod 0 and Mod 1 cartridge, which fires a 5g Sierra Match King bullet produced by Black Hills Ammunition for the US military. Some elements of Special Operations Command, however, did not believe that any 5.56mm cartridge would satisfy special forces requirements and so quietly began exploring alternatives. There were several criteria in developing the new cartridge, not least that the new round could be used in existing weapons. Hence the military would not need to buy new weapons but could convert existing M4 carbines to the new cartridge by simply adding a new upper receiver. Specific requirements for the new cartridge were as follows:

* increased lethality versus the SS109;

* increased range and energy versus the SS109;

* increased cover destruction versus the SS109; and

* increased efficiency and bore life versus the SS109.

When the special forces set out to design a new cartridge, the developers realized that they could not simply change barrels and bolts and go off to war with their new carbine and ammunition.

Genesis of the M468

The program actually began in early 2002, when initial reports of the SS109’s failures started to trickle back from Afghanistan. The ammunition was designed by Remington, in conjunction with Army Special Forces and the US Army Marksmanship Unit at Fort Benning, Georgia, while the new carbine upper receiver was designed by Barrett Firearms, famous for its 0.50-caliber rifles.

The developers reviewed the entire history of intermediate class cartridges, beginning with the 7.92mm German Kurz from the Second World War. The 7.62x39mm was discarded as a candidate, partly because there are no known magazines that will reliably feed the cartridge from a standard M4 lower receiver and also because the cartridge profile is such that an entirely new magazine design would be required.

Cartridges of 6mm, 6.5mm, 6.8mm, 7mm and 7.62mm were considered and tested, but it was ultimately decided that the 6.8x43mm Special Purpose Cartridge (SPC) offered the best combination of accuracy, lethality and reliability.

The conversion units will be made by Barrett and other manufacturers are already tooling up to produce rifles and carbines in this caliber, as it is not only a very effective anti-personnel round, but should prove to be a very effective cartridge for medium-sized game. Thus, any organization using M16 or M4-type rifle or carbines can convert to 6.8x43mm by purchasing Barrett upper receivers and placing them on existing lower receivers.

Of course, 6.8mm magazines will also be required (see below). Barrett will offer both conversion uppers and full carbines. Barrels are produced with hard chrome lining and a 1:254mm twist rate. Modifications to the upper receiver are limited to the barrel and barrel extension, extractor and bolt face.

There are a number of other changes to the rifle’s external configuration. All Barrett M468s will come with the Arms SIR System, a rail adapter interface that allows mounting optics and accessories. The SIR allows the barrel to be fully free-floated along its entire length in addition to the enhanced versatility over conventional handguard systems. (The M468 designation is a combination of the year in which the carbine is being introduced, 2004; the 68 refers to the 6.8 caliber.)

Because the 6.8mm cartidge’s bore diameter is significantly larger than the original 5.56mm, the barrel’s external diameter has been increased and the gas block modified. Instead of machining grooves into the barrel, Barrett chose to add mounting bosses for the gas block, which also acts as the front sight base and suppressor interface.

The gas block is mounted using roll pins rather than the tapered pins of the M16. This ensures that the gas block stays in place and that the interface grooves do not adversely affect barrel wall thickness. Since the M468 is designed to be used with optical sights, the open sights fold flat out of the line of sight.

The front sight, which is used to adjust for elevation with a small tool that is provided with the rifle, is a Barrett design that folds flat and locks vertically. The rear sight is an Arms design specifically intended for use with the SIR System. It has two apertures, one long range and the other for close-range use.

The suppressor interface is unusual and is designed around Barrett’s proprietary suppressor. The interface is part of the gas block. To install the suppressor, a knurled protective ring is removed and the suppressor simply screwed down into place. A spring-loaded detent locks the suppressor into place and holds it until the operator depresses a small button to disengage it.

The bolt carrier is standard AR, but the bolt has been significantly modified, not only to accommodate the larger 6.8mm SPC head, but for reliability, as well. The locking lugs have been modified and the extractor slightly enlarged with two stronger springs as opposed to the original single spring that occasionally causes extraction problems.

The 6.8x43mm or 6.8mm SPC is derived from the obsolete 0.30 Remington cartridge. It is significantly shorter in length, but the case head and rim diameter are identical to the earlier cartridge. The only current load is a 115 grain match cartridge, but full metal jacket and soft point hunting ammunition will soon be in production.

We should note that the 6.8mm SPC’s ballistic performance is almost identical to that of the 50-year-old British 0.280 cartridge. This is not to imply that Remington copied the older design – only to point out that the old concept worked and that modern research, development and testing have validated the original 0.280 concept.

Part of the 6.8 SPC’s development was ballistic testing in standard ordnance gelatin. During this testing, the 6.8mm SPC was fired from barrels as short as 368mm. The loss of muzzle velocity was only 15m/sec and the weapon’s lethality proved superior to the 5.56mm SS109, regardless of the barrel length from which the latter was fired. Tests show that the 6.8mm bullet is an excellent close-quarter battle (CQB) round for military or law enforcement use when fired from an M4-type carbine.

Even in barrels as short as 305mm, velocities are approximately 731m/sec with muzzle energy of 1,996J, actually greater than that of a standard 7.62x39mm whose muzzle energy is only 1,975J. Fired from a standard 406mm barrel at a nominal velocity of 792m/sec, the 6.8 delivers an energy level of 2,341J. In US government testing at 100m, the average penetration of 45 rounds was 399mm in 10% ordnance gelatin.

After defeating NIJ Level IIIA body armor, the 6.8mm bullet penetrated an average of 178mm. The bullet typically expanded to 11.7mm with approximately 40% weight retention over a 45-round test average.

IDR informally fired short-barreled and select-fire M468s at Barrett’s Murfreesboro, Tennessee facility on full auto and suppressed. There is no subsonic ammunition available for the carbine, so CQB distance targets behind the factory were fired at.

The M468 was easy to control on full auto and IDR was able to keep all of its bursts within a 10in circle at 50m using the suppressed carbine. The M468 was not fired unsuppressed at the Barrett facility due to noise considerations. The suppressor enabled shooting without ear protection. Of course, the rifle evaluated for this article is semi-automatic, although it is otherwise identical to fixed-stock military versions. Folding stock versions are available.

The M468’s magazine is externally almost identical to standard M16 magazines, although it is slightly longer and holds only 28 rounds. The magazine is internally different, however. The stiffening grooves on the magazine are shallower than those on M16 magazines to accommodate the larger 6.8mm cartridges.

IDR tried inserting 6.8mm cartridges into several 30-round M16 magazines and was able to load fewer than 10 cartridges in each. Also, the feeding was impaired.

Options are available

IDR set up its M468 with a few options that most organizations using the weapon are likely to employ. First, one of EO Tech’s superb Holographic Weapon Sights (HWS) was mounted. US Special Forces soldiers have been purchasing these sights using their own funds for some years, because they provide the fastest target acquisition of any such device. The US Army finally recognized the efficacy of the HWS and recently purchased thousands for issue as CQB sights.

Second, a Surefire M900 vertical foregrip light was added. This is also in widespread military use. IDR installed a BattleGrip, available from Buffer Technologies or Brownell’s in the US. This ingenious accessory provides storage for spare batteries used in both the HWS and M900 in the rifle’s pistol grip.

To forestall battery failure at the worst possible moment, the prudent ‘operator’ will have some spares always at hand, even if he may have installed fresh batteries at the beginning of the operation.

Finally, IDR added an Ambi-Catch, a device recently adopted by the Canadian military. The Ambi-Catch puts a mirror-image magazine catch on the left of the magazine well, enabling the shooter to use either hand to disengage the magazine.

Shooting the M468 brought no surprises. The match-type trigger broke like the proverbial ‘glass rod’ at an average of 4 lb. There was no perceptible creep or overtravel. The M468’s operation is exceptionally smooth. Recoil is mild and the carbine easily controllable due to the excellent muzzle brake. Functioning during the 200-plus rounds expended was flawless; there was not a single stoppage of any kind.

Considering that both the rifle and its ammunition are still in the final stages of development, this performance was exceptional. One potential drawback is that the magazines did not drop free when the release was pressed, but it must be recalled that the guns and ammunition evaluated for this article were pre-production units that can reasonably be expected to still have a few ‘bugs’. Accuracy from the carbine we tested was sufficient for the rifle to eventually be considered for precision shooting.

The Barrett M468 functions flawlessly and delivers a significantly greater punch than a 5.56mm-caliber weapon. It is reliable and accurate and provides military users with a highly flexible carbine, CQB weapon and possibly even a precision tactical rifle in a caliber that should prove much more effective than the 5.56mm cartridges in use.

There are, however, unresolved issues. The 6.8x43mm round has not yet been formally adopted by the US Army, although there is strong pressure from operational elements for it to do so. Logistically, the 6.8mm could be problematic, although it would be a simple matter to convert back to 5.56mm weapons should the need arise.

Also there is the question of NATO standardization. It would not be the first time that the US marched to the tune of its own drum, pulling NATO along with it.

The ultimate irony, of course, would be the adoption by the US military of a cartridge that, ballistically, is virtually identical to the UK one that it rejected over 50 years ago. Regardless, the 6.8x43mm offers the users of M16 or M4 rifles and carbines a significant upgrade at reasonable cost.

SPECIFICATIONS: BARRETT M468*
Caliber 6.8x43mm SPC
Operation Gas, semi-automatic, select fire available
Feed Detachable box magazine: 5,10 or 28 rounds
Barrel length 406mm (other lengths on request)
Twist rate 1 turn in 254mm
Overall length 889mm
Empty weight 3.9kg
* SPC carbine (as tested). Other variants available, including select-fire and short barrels

SHOOTING TEST RESULTS: BARRETT M468 6.8X43MM SPC
Ammunition* High velocity Low velocity Average velocity Extreme spread Standard deviation Average group size
Lot no. 6177 791m/sec 776m/sec 780m/sec 14.6 13 27mm**
Lot no. 6214 804m/sec 787m/sec 796m/sec 17 15 38mm**
Testing conducted at Florence, Alabama Police Range. Temperature 12ºC. Velocities measured with Competitive Edge Dynamics Millennium Pro Chronograph. Group size average of three five-round groups fired at 100m.
* All ammunition manufactured by Remington. These were pre-production developmental lots.
** Group size excludes called flyers.

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