The 6.5×40 Cartridge

Wednesday, April 16th, 2014

Mitch Shoffner decided to design a replacement for the venerable 5.56×45mm cartridge used in M16 rifles and M4 carbines, the 6.5×40mm, which would have the range of the much bigger 7.62×51mm without being too big to fit into the AR-15 platform:

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 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.

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.

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

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.

So, what are the downsides?

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.

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