The US Army Marksmanship Unit's .264 USA

Header image: The .264 USA on a display block, flanked by 7.62x51mm (left) and 5.56x45mm NATO (right). Modeled and rendered by the author in SolidWorks. The .264 USA was designed by members of the Army Marksmanship Unit as a possible next-generation military rifle cartridge.

It’s been an interesting decade for military cartridge development. It’s reasonable to guess that at no other time in this country’s history have there been so many home-grown proposals for military rifle and machine gun ammunition as in the ten plus years following the invasion of Iraq. Even though my own opinions on the subject are pretty conservative, it’s great to see so much discussion and attention paid to the subject of my interest.

One of the schools of thought that has gained some ground within the Army itself is the idea of a “general purpose round”, a proposal to replace the current two-caliber system of 5.56mm and 7.62mm ammunition with a single round, usually somewhere in-between those two calibers in size and weight.

It’s a theory I am highly critical of, but that does not mean it is without merit. The paradigm has definite benefits, and should be investigated even if there is some certainty that, at the moment, it’s not practical or desirable to implement such a scheme. So it’s right that the Army Marksmanship Unit has endeavored to produce and test a round that could fulfill such a role, called the .264 USA. The round is a 6.5mm caliber round, based on lengthened 7.62×39 brass, with very little case taper (0.26 degrees, almost half that of 5.56’s 0.5 degrees, and still considerably less than 7.62x51mm’s 0.35 degrees) but a very slender 17.5 degree shoulder. At 2.6″ long, the .264 USA is closer to 7.62mm than 5.56mm in size and weight, but it nonetheless would afford slightly smaller and lighter weapons and magazines. Interestingly, due to a request for proposal specifying a certain number of conventional-layout polymer cased rounds in this caliber be produced, there has been some speculation that the .264 USA’s unusual case taper and shoulder angle are indicative that it was designed from the outset for polymer cartridge cases. If this proves true, it would make the .264 USA the first caliber to my knowledge to be purpose designed for conventional layout polymer cases.

The RFP linked above specifies that the .264 USA be capable of producing 2,875 ft/s with a 107gr lead-cored Sierra HPBT from a 16.7″ barrel, or 2,657 ft/s with a 123gr Sierra from the same length. Also mentioned is a .277 USA, producing 2,527 ft/s with a 135 gr Sierra, but much less is known about this cartridge. This level of performance is common to many different general purpose cartridge proposals, including that of Anthony G. Williams, so it appears the AMU is following suit with the .264 USA. The cartridge itself is very similar in size and weight to the design created for my evaluation in “The Case Against…” linked above, which confirms my suspicions about the considerable weight penalty of the GPC concept in general.

The .264 USA follows the 7x46mm UIAC, designed by Cris Murray (who was once also of the USAMU), very closely in performance. Further, it will be very close to the 7x46mm in size and weight, and therefore closer to 7.62mm in both respects than to 5.56mm. Because of this, while the .264 USA will afford somewhat lighter and more manageable weapons and ammunition than might be possible with the 7.62x51mm alone, it will negatively impact these characteristics relative to 5.56mm weapons. Whether this is deemed acceptable given the augmented ballistics remains to be seen.