Use a inside caliper to measure the bore diameter of your barrel. Its critical to choosing the correct diameter of Round ball for it. If you don't have a caliper, borrow one, or take the barrel to a local machine shop and ask them to measure it for you. Nominally, a 20 gauge barrel should be .615-.620". However, your barrel may be smaller , or larger in diameter. A .620" diameter bore can handle a .600" diameter RB, with a .020" lubed patch. Some guns can do well with a slightly smaller, .018" patch, and others will do just fine with a .015" patch. Mostly, the .015" patch is used during hunts to speed loading time, for a second, finishing shot. It doesn't give quite as good accuracy as the tighter patch will, but its close enough for a follow up shot. With a PRB, these smoothbores are good to from 50-70 yards. If you want to shoot these caliber RBs at longer ranges with accuracy, you need a rifled barrel.