Well, it would have to be a cartridge that is considered a "short-action" round for bolt-action rifles, e.g., .243 Win, .308 Win, etc. Maybe .338 WSM? Open that up to a .375, .400, .450, .475, and/or .500. Just on a .308 case, how far have they opened that up? The biggest I'm aware of is the .358 Win, though the .338 Federal is supposed to develop quite a bit more muzzle energy. I'd like to see something along the same lines as the .300 Whisper based upon the .308 case (the .338 WSM could probably make something REALLY effective!), wherein the case is opened up to basically the width of the body and pushing a bullet of that diameter. In the case of a .308 case, pushing a .454/8 diameter slug of 450 grains at just under the speed of sound...we could get cute and call it a Whisper, but the .450 THUMP sounds better. By law we would be regulated to a .500 diameter bullet in a WSM based case...so a limitation of what...600 grains? It would probably be worth paying a licensing fee to call it something like the ".500 Sante Fe Locomotive". Sorry Senor Terrorist...your Russian issue bullet proof vest doesn't count anymore. Jon