That would be frustrating, for sure. Happened to me with my first Sharps.

The .303 BP ctgs. would have to have all full diameter bullet measurements inside the case. With black powder ctgs. throat fouling buildup will prevent too large a bullet from entering. Also, if the case does not seal the chamber area (obturate to seal) the fouling back-flow can impinge on the case chambering as well.

I am not aware of any available cast bullet designs for the .303 specifically designed for black powder. Black powder designed bullet moulds have mulitiple square bottomed grooves to hold as much lube as possible, or are paper patched designs meant to use a lube cookie underneath. The lack of lubrication to keep the fouling soft for subsequent loads, prevents repeated firing without wiping/cleaning the chamber and bore out, sometimes every shot- depending on the load and bullet.

The round/ctg.(.303) may have been designed as a black powder cartridge originally, but for black powder as loaded today, it is a lousy design. Better designs have longer necks to contain long grooved bullets & lots of lube - as in some of the English BP rounds, 450/400 for instance, or even the .43 or 11mm Mauser ctg. A lube cookie helps with paper patched bullets, as well as a "moist burning" powder, quite unlike the available black powder today.