Do you know who made the barrel?
But, keep in mind, in this game there are few exact standards. What might be referred to as a '44 caliber' or '45 caliber' might actually be a .42 caliber barrel. A soft lead ball can be loaded safely, albiet with effort. Your bore needs to be measured accurately. Either using a micrometer or with a lead slug. Get a brass rod (e.g. 3/8"), put in barrel and force an oversized lead ball about two inches down the bore. Then turn the rifle muzzle down and bounce the rod up and down until it drives the slug out. Once out measure carefully with a micrometer. That will tell you what you have caliber wise and give you a starting point for ball size selection. Tell us what you find out.
But, keep in mind, in this game there are few exact standards. What might be referred to as a '44 caliber' or '45 caliber' might actually be a .42 caliber barrel. A soft lead ball can be loaded safely, albiet with effort. Your bore needs to be measured accurately. Either using a micrometer or with a lead slug. Get a brass rod (e.g. 3/8"), put in barrel and force an oversized lead ball about two inches down the bore. Then turn the rifle muzzle down and bounce the rod up and down until it drives the slug out. Once out measure carefully with a micrometer. That will tell you what you have caliber wise and give you a starting point for ball size selection. Tell us what you find out.