The 6.5x40 Cartridge: Longer Reach for the M4 & M16

The 6.5×40 Cartridge: Longer Reach for the M4 & M16


Feedback from Afghanistan was clear – much small-arms combat was taking place at far greater distances than expected, a lot more than the U.S. Army’s standard carbine and light machine gun were designed for.  The Army was being outgunned, and had to rush older, more powerful weapons to the dismounted troops.  These did the job, at the cost of significantly greater weight and recoil.  Mitch Shoffner has come up with a different solution – the 6.5×40, a new cartridge designed to fit into modified versions of the carbine and LMG that will allow them to reach out much further.

Two of the constants that have featured in the field of U.S. military small arms for the last half-century are the AR-15 firearms family, in the form of the M16 rifle and M4 carbine, and the 5.56×45 cartridge.  Ever since the M16 and 5.56mm combination first entered service during the Vietnam War it has been a source of controversy.  It had a poor start mainly due to a change in the ammunition propellant in the process of moving from prototype to production status.  This seriously affected reliability in combat, leading to some bad headlines about guns jamming in the heat of battle.  These initial problems were soon addressed but the gun and ammunition have had their critics ever since, focused partly on the direct gas impingement operating system of the gun but mainly on the small size and power of the cartridge.  Criticism of the cartridge applies just as much to the Army’s light machine gun, the M249, which uses the same ammunition.

Both rifle and cartridge have seen many modifications and, given the conclusion in 2013 of the Army’s Individual Carbine competition without a winner, it looks as if versions of the M4 will remain in service for a long time to come.  The Army has so far shown no interest in changing the cartridge other than in modifying the bullet and loading; the original M193 gave way to the NATO-standard M855 over 30 years ago, but continued complaints about lack of effective range and erratic terminal effectiveness, combined with the Army’s wish to standardize on lead-free ammunition, led to the 2010 introduction of the M855A1 EPR (Enhanced Performance Round).  Other forces make use of different loadings including the heavy-bullet MK262 for SOCOM and the MK318 for the USMC.

Danny Mathers on the test bench.

In addition to these official developments, designers and manufacturers keep coming up with proposals for further improvements to the gun, including modifying it to use larger cartridges.  The 6.5×40 is the latest attempt to introduce a new round for the M4 platform and offers some specific benefits mainly focused on achieving extended range for military operations.

One of the strengths of the AR-15 is its versatility, making it easy to adapt the design to accommodate larger cartridges.  The main constraint is the overall length of the complete round, which must remain within the 2.26 inches (57.4 mm) of the 5.56×45 in order to avoid fundamental and costly changes to receiver/design and construction.  There are also limitations on the width of the cartridge case linked to both the width of the magazine well and the maximum chamber pressure developed by the cartridge: within the standard AR-15 external barrel dimensions, a wider cartridge means thinner chamber walls and (other things being equal) the bolt thrust will also be higher, putting more stress on the action.  Within these limitations, a very wide range of larger-caliber cartridges has been developed for the AR-15 platform, up to a maximum bullet diameter of 12.7mm in the .50 Beowulf.

The simplest type of modification is to retain the basic 5.56×45 case (adjusted in length as required), but increase the caliber to anything up to a maximum of 8.6mm as used in the straight-cased version of the .338 Whisper.  This approach has the advantages of requiring the minimum change to the gun and retaining the full capacity of the magazine.  The disadvantage is that there is no room for more propellant, in fact there may be less if the case has to be shortened to accommodate a longer bullet, so performance improvements are limited.  As a result, such cartridges are usually intended for special purposes, particularly subsonic loadings for use in suppressed weapons.  The 7.62mm caliber is by far the most popular for this purpose; the original .300 Whisper having been joined by variations on the theme, most notably the .300 AAC Blackout (300 BLK), which is virtually identical to the Whisper but SAAMI registered.  Both subsonic and supersonic loadings of these cartridges are available, but the effective range even of the supersonic loads is limited by the combination of the short, light bullets and a modest muzzle velocity.

To achieve a significant performance improvement, it is necessary to increase the diameter of the cartridge case from the 0.38 in (9.6mm) of the 5.56×45.  This is most easily achieved by adapting existing cases, so there are several steps based on available production cases.  The first step up in common use is the case for the 6.8mm Remington Special Purpose Cartridge (SPC), which measures 0.42 in (10.7mm) in diameter and was itself developed from the old .30 Remington.  Next up is the 0.44 in (11.2mm) case of the Russian 7.62×39 AK round that has since been used for a number of different cartridges, most significantly the 6.5mm Grendel.  The largest of the common case diameters is the 0.47 in (12.0mm) as used by the 7.62×51 NATO, which can trace its direct ancestry back over a century to the U.S. military .30 cartridge and has since spawned countless variations.

Left to right: 7.62mm 175gr OTM, 7.62mm 147gr FMJBT, 5.56mm 62gr FMJBT, 6.5mm Berger VLD 140gr OTM, 6.5mm Lapua 144gr FMJBT, 6.5mm Norma 120gr FMJBT, 6.8mm 115gr FMJBT.

The Origins of the 6.5×40
Mitch Shoffner of Rockingham, North Carolina, is a combat veteran who was assigned to the 82nd Airborne Division 1/508 PIR in December of 1967, served with Recon in 1968 in Vietnam with the same unit, and subsequently became a weapons specialist with A Company, 6th Special Forces Group, before leaving the army and training as a teacher.  From his own experience he was aware of the limitations of the 5.56mm cartridge, and his opinion was reinforced by complaints about its combat performance in the 1990s.  After 9/11 he decided to begin work on a new cartridge that could replace the 5.56mm, a project that was given even greater relevance by more recent events in the Middle East.

The lengthy conflicts in Iraq and Afghanistan have both featured extensive use of small arms, with contrasting results.  The M4 carbine proved well suited to the mainly urban, short-range fighting in Iraq, but circumstances changed dramatically when the Taliban began to make their come-back in Afghanistan.  Noting the limited effective range of 5.56mm weapons, they have often chosen to attack dismounted ISAF patrols in open country from distances of 500-900 meters, using SVD rifles and PKM LMGs firing the old but powerful 7.62x54R Russian round.

The response of the U.S. Army to this has been to update and reissue to the infantry squad the old M14 rifles in 7.62×51 caliber, along with the M240 7.62mm machine guns that had previously been held back as support weapons.  The main problems these brought with them were those that had caused them to be replaced by 5.56mm weapons and ammunition in the first place: gun and ammunition weight.  These problems were exacerbated by the heavy loads that modern infantry carry, especially troublesome when operating in the rugged terrain and at the high altitudes common in parts of Afghanistan.

Mitch Shoffner’s project is focused on greatly improving the long-range performance and effectiveness of the existing 5.56mm weapons by developing a new cartridge that could fit into them, with the M4 requiring only a new barrel, bolt and magazine.  He has not been the first to try to improve the performance of the AR-15 family in this way.  The two most significant attempts from the point of view of their military potential have been the 6.8×43 Remington SPC and the 6.5mm Grendel already mentioned.

The Barnes 126.5 grain lead-free hunting bullet that performs so well in the 6.5x40. Mitch comments that a variation with a steel or tungsten rather than polymer tip could meet military requirements.

The 6.8mm SPC was the result of a joint effort between Remington and members of the U.S. Special Operations Command, working in conjunction with the U.S. Army Marksmanship Unit at Fort Benning, Georgia.  The project took place before the problem of the long-range engagements in Afghanistan emerged, so the priority was to develop a round that would deliver more reliable terminal effectiveness than 5.56mm at normal combat ranges.  This it does very well by all accounts but, as Mitch discovered after experimenting with the round, the relatively short, stubby bullets blunt the long-range performance.  Using the finely-pointed long-nosed bullets needed to achieve the high ballistic coefficients required would make the 6.8mm cartridge too long to fit into the AR-15 action.

In contrast, Alexander Arms designed the 6.5mm Grendel around the use of long, low-drag bullets.  To provide enough space for their long noses without exceeding the overall length limit, the case length has been held back to 1.52 inches (38.7 mm).  To compensate for its shortness the case has been made wider than the 6.8mm’s in order to hold enough propellant.  This round can provide excellent long-range performance when bullets of around 120 grains are used, although commonly quoted muzzle velocities are usually from long (24 inch) barrels.  Furthermore, the short, wide case, with little taper and a sharp shoulder, has prompted some debate about its suitability for military use in belt-fed machine guns as well as about the potential for increased stress on the M4’s action, as described earlier.

Design and Performance of the 6.5×40
The approach that Mitch Shoffner has taken with his 6.5×40 is to design a compact, long-range military cartridge that would not experience any functioning problems in magazine or belt-fed automatic weapons.  He accordingly adopted the same 0.42 inch case diameter as the 6.8mm SPC together with a case taper and a shoulder angle similar to those of the 7.62×51.  He chose the 6.5mm caliber and a case length of 1.57 inches to allow the use of long, low drag bullets within the M4 platform.

It is worth mentioning that a 6.5mm version of the Remington case was explored during the development of the SPC but 6.8mm was preferred as it was found to have superior terminal effectiveness.  However, long range was not a priority in the development of that cartridge, and the 43 mm case length meant that the 6.5mm version could only use relatively short, light bullets.  As always in cartridge design, compromises are necessary; if you emphasize one characteristic there will be penalties elsewhere.

Comparison data is shown here, all from the 14.5 inch barrel length of the M4 carbine except for the 7.62x39 of the AKM, which is from a 16 inch barrel. The JBM Trajectory ballistic calculator has been used for all of these comparisons. The 6.5mm muzzle velocities have been measured by MagnetoSpeed chronograph. The chamber pressures of the 6.5x40 are stated to be comparable with those of the Grendel and SPC.

The use of low-drag bullets means that the initial velocity penalty compared with the equivalent 7.62×51 loads gradually reduces as the range increases.  The lightest and least aerodynamic military-pattern bullet recommended for the 6.5×40 is the 120 grain Norma FMJBT.  This loading develops 97% of the velocity of the 7.62mm M80 at the muzzle and 100% at 1,000 meters when both are fired from 14.5 inch carbine barrels.  The 144 grain Lapua FMJBT performs even better at long range, with the velocity of only 90% at the muzzle rising to 106% at 1,000 m, at which distance it also retains more energy than the M80.  The optimum bullet for long-range performance in the 6.5×40 is the 140 grain Berger VLD, a match-grade target bullet, which remains supersonic to 1,000 meters even from the 14.5 inch carbine barrel – an impressive statistic given the modest initial velocity.

Mitch has sought the views of other combat veterans concerning the characteristics of his cartridge.  MSG Danny Mathers is a ten-year veteran with the Combat Materials Evaluation Element (CMEE) and a master sniper with experience of all types of weapons and combat, and was part of the early development of the M4 with the CAR15 platform. He comments: “I was immediately impressed with the power and the accuracy at 400 meters with his carbine chambered in a 6.5mm cartridge.  I later fired several of his weapons at a police range on full auto [and] I could not believe the natural control of the weapon with a standard flash suppressor.  I have no doubt that the 6.5×40 is the most logical improvement to the assault rifle.  The conversion is simply the most cost effective improvement available.”

SFC Charles Williams was a U.S. Army Special Forces sniper with the 5th in Iraq, and as a very experienced hunter has been testing the 6.5×40 in the field to evaluate its effectiveness.  He has used rifles with barrels ranging from 14.5 to 24 inches and has found the 6.5mm to be accurate and highly effective, bringing down over 20 deer and coyote.  He has tested a wide variety of bullets and found the Barnes 126.5gr LRX to be most effective.  He comments: “The round has performed extremely well in every situation.  I documented every harvested animal with wind, weather, distance, round impact site, internal organ damage, and exit site.  No animal required more than one shot to bring down.  The damage performed by the round was the same at 375 yards as it was at 3 yards.”

Mitch Shoffner and his daughter Stephanie with two of his 6.5x40 rifles. Both rifles use an ADC (Arnold Dew Custom) adjustable carbine length piston gas system, which Mitch says is a low maintenance system. Both also use Noveske HBAR Taper barrels with a SureFire adapter/flash hider. The rifle held by Mitch has a 14.5-inch barrel with a 1:7 twist. Stephi’s has a 12.5-inch barrel with a 1:8.5 twist. The stocks on both are Magpul CTR. Mitch’s rifle is fitted with a TA11 3.5x35 ACOG sight while Stephi’s has an Aimpoint M2 red dot sight. The lower and upper receivers are all Mil-Spec and Mitch has used various brands – Colt, Tri Star, DPMS, Rock River, Sun Devil, etc. Any 6.8mm Rem SPC magazines can be used; Mitch finds that PRI are the best.

What all of this means is that the 6.5×40 remains an effective cartridge out to ranges at least equal to those of the equivalent 7.62×51 loadings, while fitting into appropriately modified versions of 5.56mm weapons and saving around 30% of the 7.62mm’s ammunition weight.  It is not intended to replace the 7.62mm cartridge, but as a replacement for the 5.56mm it could make it unnecessary for dismounted soldiers to carry the 7.62mm weapons that would be retained for the support role.

The 6.5×40 clearly has some very desirable characteristics, but given that all cartridges are compromises, what are the downsides in this equation?

The most obvious one is that the ammunition is some 30% heavier than 5.56mm.  It also has a greater recoil impulse although, as comparative testing has revealed with other cartridges of this power such as the 6.8mm Remington, the perceived recoil is much closer to the 5.56mm than it is to the 7.62mm and controllability is not seriously affected.  Another inevitable downside is that the lower initial velocity means a steeper trajectory at medium ranges compared with the 5.56mm.  At 300 meters, the 5.56mm M855A1 drops around 16 inches when zeroed at 100 meters, while the 6.5mm 120 grain Norma drops 20.5 inches.  These comparisons are however only relevant for distances that are within the relatively short effective range of 5.56mm weapons.  If troops ever need to engage at longer ranges, the only valid comparators are the 7.62mm systems, which have trajectories not very different from the 6.5×40.

A potential issue is that of terminal effectiveness against unprotected personnel, which is affected by the bullet’s yaw characteristics.  If the bullet does not yaw shortly after impact, it may just punch a neat hole through the body.  This may not matter so much at longer ranges when the primary problem is hitting the target at all, but the shorter the range, the more vital it becomes to incapacitate the enemy as quickly as possible before he can kill you.

With all small arms ammunition, bullet placement is the most important factor: immediate incapacitation will only occur if the central nervous system is hit.  However, this is a small target, and with hits elsewhere on the body, the larger the wound channel created, the faster incapacitation is likely to be.  Choosing or designing bullets that demonstrate rapid and reliable yaw characteristics to maximize the size of the wound channel are therefore important.

The 6.5x40 loaded with a Lapua 144 grain FMJBT compared with other rounds.

If a yaw performance similar to that of the 5.56mm can be achieved then the long 6.5mm bullets, twice the weight of the 5.56mm, should deliver more reliable terminal effectiveness.  They should also penetrate intermediate barriers more easily, given their high sectional density.  By comparison, the 7.62mm M80’s yaw performance is not particularly good and the sectional density is worse than the 6.5mm, but its sheer size and power provide some compensation for that.

Another potential downside is that the 6.5×40 cartridge really requires the use of heavy, lead-cored bullets to deliver the full performance potential at long range.  Replacement of lead with less dense materials such as copper or steel entails a reduction in bullet weight and therefore ballistic coefficient and long-range performance.  The alternative approach of lengthening the bullet to maintain the weight isn’t really feasible since that would leave too little room for propellant in the case thereby reducing the velocity at all ranges, and the heavier bullets could also become too long to stabilize properly.  These problems could be resolved by using tungsten cores, which would also deliver effective armour penetration but at considerable material cost.  Tungsten is also a controversial material to have accumulating in the environment of practice ranges.

Having said that, it is worth noting the performance of the Barnes 126.5 grain LRX in the table above, which is a solid copper bullet with a polymer tip.  An efficient nose shape helps this to match the long-range performance of the lead-cored 120 grain Norma FMJBT.

Finally, there is the issue of suitability for use with polymer/metal hybrid cases, which are steadily being developed towards military acceptance.  These reduce ammunition weight by 23-32% depending on design and materials, so are likely to be adopted very quickly as soon as they prove satisfactory.  However, they have thicker walls and will therefore reduce the propellant capacity to some degree.  In compensation, there is some evidence that they may be thermodynamically more efficient since polymer is such a good insulator that very little of the energy generated is lost in heating up the case or the chamber, but that remains to be tested for each cartridge.

To sum up, the 6.5×40 offers an intriguing mix of characteristics that could well make it attractive for those armed forces who need a much better long-range performance than 5.56mm can offer, don’t want to burden dismounted infantry with carrying the extra weight of 7.62mm guns and ammunition, do want to retain their light and handy 5.56mm weapons and, for optimum performance, aren’t too concerned about lead-free bullets.  If these characteristics appeal, then the 6.5×40 is worth careful study.