I have an original 44 1/2 and ended up with 4 barrels for it: a .219 Donaldson Wasp that came on it, a 6mm/30-30 that someone advertised here, a 25-35, and a 32-20. The last two are BRC choked barrels that I ordered for match shooting with cast bullets. The 6mm may be a standard Bullberry but I have only shot light jacketed loads or cast loads in it. Some of Harvey's Wasp loads were fairly high pressure and the 25 Krag was put on the 44 1/2 also.

So it sounds like if I work up to it I should have extraction problems before I get to dangerous pressure levels

Guessing, the switch to bolt actions in military rifles late 19th century had a lot to do with reliable extraction. Bolts also allowed higher pressures, receiver covering the cartridge head. 

Another from memory, did a lot of 25 Kraig research before ordering a barrel for my CPA. Cartridge was originally developed on a 44 1/2 action.

Working up loads another thing to consider is gas escape routes should a case let go. Example the hole left side of 1903 Springfield receiver. Better example CPA action has a gas escape hole bottom of the block. Original 44 1/2 blocks don’t.

Boats

I don't have much to offer in the way of suitable cartridges other than I have a 5 barreled set with an original 44 1/2 action that included 219 Zipper and 40-70 barrels. I have not shot either. One thing I would caution is that a 44 1/2 CPA rifle is a modern rifle and much more conducive to modern cartridges than originals so keep that in mind when someone has a CPA chambered in X caliber. Just my opinion and other will have their own. I'm sure if you wanted the straight facts on what CPA thinks their action (and maybe recommendations on original) can handle you could call Gail at CPA. She is always great to deal with (they all are). Regards, Chris.

Kind of surprised no one has mentioned 257 Roberts with with a .472” base.