I believe if there is already a rifle out which already has a...

I believe if there is already a rifle out which already has a strong enough breech and barrel? then this patent may provide the solution already, the weapon would have to be certified for firing this type of ammunition though, it is the same patent they used for the mach 5/50 stuff just a scaled down version of it that's all, they show that type of round in some of the drawings, it is the one with the ejection ring at the base of the cartridge.

7707941 Cartridge assembly for multiple projectiles

www.freepatentsonline.com/7707941.html