POTD: The Polymer Cased .264 USA

Nathaniel F
by Nathaniel F
The .264 USA in both brass and polymer composite cased configurations. Image credit: Anthony G. Williams, used with permission.

At the end of last year, we told you about the US Army Marksmanship Unit’s .264 USA round, which has been designed for an intermediate length AR platform rifle larger than an AR-15 but smaller than an AR-10. The round is one of the first to incorporate polymer case composite construction so early on in its development

Tony Williams, co-author of the book Assault Rifle with Maxim Popenker, sends along this image of the .264 USA cartridge in both conventional brass-cased and composite brass-polymer case guises:

The .264 USA in both brass and polymer composite cased configurations. Image credit: Anthony G. Williams, used with permission.

Alex and Patrick of TFBTV recently related their story shooting composite cased PCP ammunition, and the resultant explosion that could have seriously injured one or both of them. Polymer composite cased ammunition is an ambitious idea, and PCP’s incarnation of that concept is seriously unsafe. Not only has the .264 USA possibly been designed specifically for composite cased ammunition, or at least with it in mind, but the composite cased ammunition itself is not being made by the PCP ammunition company, but rather a company called MAC, LLC, out of Mississippi. One of MAC, LLC’s relevant patents is available here.

Readily apparent from the patent image is the greatly increased thickness of the case wall for the MAC, LLC case design. This reduces the capacity of the case, but is necessary given the use of polymer material in the place of brass.

Additional information about MAC, LLC’s .50 caliber polymer cased project can be gleaned from this presentation from 2012.

Nathaniel F
Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. He can be reached via email at nathaniel.f@staff.thefirearmblog.com.

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  • Brent Blanchard Brent Blanchard on Jul 28, 2015

    What's even more important is how far forward the brass portion extends into the chamber. That high-pressure metal gasket function is NOT to be taken lightly! IME, anything forward of the ammo-identification M852 case cannelure can fail without serious threat of gas leakage--that's about a half-inch forward of the casehead/bolt face. A nice flexible metal strong enough to hold the gas must be able to bridge the transition from the portion pressed against the chamber wall, to the more rearward portion which does not expand. The PCP stuff appears to foolishly depend on the polymer to seal that area.

  • Unlucky Eddy Unlucky Eddy on Jul 28, 2015

    Glad I never bought these kinds of rounds.. I had terrible luck getting steel case out of my revolver...