New Army "Caliber Configuration Study"

[Tony Williams]

Polymer cases have better thermal efficiency than brass?

Yes - in a brass-cased cartridge a small but significant percentage of the energy developed by the propellant is used to heat up the brass case and, through that, the chamber walls. Polymer is an excellent insulator (it is reportedly possible to stick your finger in the chamber of an MG after a long burst of fire without getting burnt) so this particular element of energy loss does not occur.

I saw a very interesting statement recently from someone who is fairly well-known in the alternate caliber market (.458 SOCOM). Basically, the guys working on propellant technology are light years ahead of what is commonly available. The claim was made that the 5.7x28 case was lengthened and out-performed .223 Remington using these new propellants.

That would be fascinating and a real game-changer, but I'll reserve judgment for now. Similar claims have been made in the past, the most recent a few years ago with a propellant based on rocket fuel. Apparently it sent the chamber pressures through the roof...

Having said that, my own (non-technical, and probably unrealistic) thought on gun propellant is that the form (if not chemistry) of a typical rocket motor might have advantages: that is, a solid fuel block with a central axial channel which ignites first and then burns outwards until it reaches the case walls. If feasible, this would mean that as the propellant burned outwards so the area exposed for burning would progressively increase, generating more gas and keeping the pressure curve much flatter. It would also minimise the heat transfer to the case and chamber because the burn would not reach the case walls until right at the end. The burning rate could be adjusted by the varying the shape of the axial channel (in rocket motors, the cross-section of the channel is often star-shaped to maximise the initial surface area to provide a boost phase). I suspect that a new shape of cartridge case might be needed for this to work well, though.