264 USA (6.5x48)

[andrewctillman]

I have an unusual request you may or may not be able to help with.

I would like to calculate likely velocity, chamber pressure, and suitable powders for the 264 USA. As you may know this is a developmental military cartridge for possible adoption by US armed forces.

OCL is 2.6 inches. Cartridge base is identical to 762x39 case head. Body taper and shoulder taper are modest for use in automatic rifles and light machineguns. I can supply case dimensions if required. The International Cartridge Collectors association says it has a 42 grain H20 capacity. Bullets used in development are the 108 grain Scenar open tip match bullets.

I am curious about this bullet because the claimed velocities for it are so high in very short barrels. According to the RFP:

2,875 ft/s for the 108 grain from a 16.7″ barrel, and 2,650 fps from an 11.5 inch barrel. 2,750 fps from 11.5 inch hoped for. 14.5 inch will also be tested. .

This is intermediate in size between a Grendal and Creedmoor!

It just sounds very ambitious to me with what is likely to be a 35-37 grain useful powder charge.

thanks for any comments.

Andy

 

[Nimrod84]

Andy,

I've read that in the initial request for proposal (RFP) they also asked for 123 grain Sierra projectile at 2657 fps out of the 16.7" barrel. Additionally, internet scuttlebutt says that the round is designed for a polymer case. If implemented with a polymer case, then most bets are off as to pressure limits and usable case capacity; most articles / opinions assume lower case capacity and or a lower pressure limit with polymer.

You might also want to take a look at the 6.5x40mm round; 16" barrel with 120 grain bullet = mid 2400 fps range. Not sure on case capacity vs. the 264 USA round, but 6.5x40mm might help give a functional baseline for reality...

My gut says that the 264 USA RFP numbers are high, but hey a RFP is something to shoot for and push the industry with...

 

[andrewctillman]

Namo has the contract for the hybrid brass polymer case. The polymer is press-fit with ribs into the brass case head. Case capacity is reduced slightly. The 6.5 x 40 has about 37 grains of water capacity I think. So about 4 gr8 less than the USA.
The 108 grain will probably be used for rifle and 123 grain for LMG and/or sniper.
I think QuickLoad should let us know how optimistic the velocity is, esp w short barrels.

 

[1Shot]

I don't really see a problem reaching 2875 fps out of a 16" barrel with the 108 gr bullet with a ball park 40 grs of powder. You can reach 2650 fps with 107 Sierra in a 16" 6.5 Grendel pretty easy. As a somewhat comparison I have a 30x47. It is a 300 Savage case with the shoulder pushed back 10 thousands inch. It will hold 42 grs IMR 4895 and pushes a 125 Nosler BT 2850 fps but from a 24" barrel. I have a 30-30AI 14 inch Contender pistol barrel that does 2670 fps with 33 grs IMR 4198 and a 125 Nosler BT. I also have what is called a 6mm-225 Winchester 14" Contender Pistol barrel and with 30 grs Varget it will push a 80 gr Sierra SSP bullet 2850 fps.