The bore diameter of any caliber barrel can vary widely between nations & manufactures.
The only way your going to get good loading & accuracy results is to measure your bore size & depth of rifling,
this info. will enable you to determine the proper patch & ball combo.
As others have mentioned, it's also important to remove excessive roughness in the bore & get the muzzle crowned to make loading easier & obtain consistent grouping.
If the bore has excessive roughness wrap a smaller diameter cleaning jag with 0003 or 00004 grade steel wool & lap the bore until you get good results.
To crown the muzzle;
Take a larger caliber lead ball & screw a drawer-pull & screw the wood screw end into the ball.
Insert the machine threaded end into your hand drill & attach some coarse abrasive cloth to the ball with double-sided carpet tape & apply enough force in-alignment with the bore until you round of sharp edge of the bore.
When done apply fine grit abrasive cloth to the ball to polish the crown.
Happy trails