I have a cape gun with a rifle barrel in caliber 11,2x51R Kropatschek-Heissig, bottlenecked case based on the .43 Mauser.
Around 20 gram was a usual weight for 11,15mm hunting bullets.
I shoot 295 grain lead bullets cast from an original mold , and 300 grain jacketed bullets for the .44 Magnum.
I suggest to slug the barrel because bore diameters may vary.