,40cal/10mm RIFLE cartridge "thumper"
A full powered rifle cartridge in .40/10mm bore diameter, capable of launching 200 gr slugs to about 2250 fps. Think of it as a 10mm magnum, or a souped up " .40 carbine".
Think I'd base it on the .308 case, to take advantage of existing mags, bolt face dia's, etc. Don't know if the case is wide enough to offer a shoulder for headspace.
The rifle would be a short barreled semi-auto, based on Coopers "thumper" concept. NOt sure of platform but rugged and reliable.