Call me stupid (most do...) but why not just chamber it in whatever you like?
Im sure if you did the math, the .308 is going to lose less per inch than any other chambering, but even if it does, it's still not going to be as fast as a .300rum. More efficient, maybe, burning all its powder, yeah, why not. But a faster caliber in a 24" barrel is still going to be a faster caliber in a 16" barrel. IF speed is a major concern in the first place, that is. They can both use the same bullet & with monos, we all know, speed is essential. Yes a silly rum-like chambering might fart out some powder from the muzzle, but it's going to be getting a hell of a lot more burnt than the .308 before then. Calisafe bullets being what they are, i'd rather start one out at 3200fps than 2750.