Basically, there is a "standard" bore size for each caliber rifle, or (2) possibly with 7.62x39mm. If you are shooting a .45 Long Colt for example, nominal bore size for this caliber is .452" while many guns will have slightly more or slightly less. If you grease the barrel and force a fishing weight through the barrel, you can use a micrometer or caliper to determine the actual land bore diameter. In the case of the example colt, lets say it was actually .4524". Thus you would purchase lead bullets sized to .453" diameter or if leading still occurs or accuracy is still off, then try .454". This allows pressures to push the bullet base thus expanding it and it seals tightly without leakage or gas bypassing the bullet. 8) Hope this helps some.