Originally Posted by
Bigslug
That, and the .303 British example (& Eduard Rubin's experiments that led to it) are possibly something of a guide. I guess we'd have to lay some ground rules as to determining a "practical" maximum, because on the large extreme, most of us don't have a turbine-powered battleship to carry our hunting rifle for us, and on the small extreme a highly overbore cartridge pushing a light .22 at what would still be well shy of smokeless speeds wouldn't be suitable for medium/large game or military purposes. For anybody wanting to experiment, I guess it should be something you'd be comfortable taking a deer with.
As part of the U.S.S. Iowa's museum displays, they have an inert shell set in front of a stack of inert powder bags. Any red-blooded American kid is going to look at that propellant to projectile ratio and immediately see a scaled-up .30-06 or .50 BMG. The very short-lived era of compressed black powder behind a jacketed bullet showed us an overbore bottlenecked case with a highly compressed charge. This was also in the era of longer barrel = longer bayonet platform, so you'd have that working for you.
A blown-out, straight-walled, sharp-shouldered .30-06 Ackley case behind a long barrel and a slow twist would probably be about the most effective way to do this, provided some thought were put into the best way to compress the charge in a bottlenecked case, but any of the 29" barreled service rifles of the pre-WWI era would be valid test beds. If you have a Long Lee, uncut Krag, Gew '98, Swede '96, or comparable 7x57 and an inquisitive mind, you could figure this out easily enough.