You use .600 caliber balls in 20 gauge guns, either smooth bore, or rifled. Some use a .595, and some go as high as .610" for their cast balls. It all depends on the bore diameter of the gun, the groove diameter in rifled .62 cal. guns, and what level of accuracy you are trying to achieve. If your 20 ga. smoothie has a true .620 caliber bore, than a .600 ball with a .015" patch, will allos the patch to be compressed .005" on both sides In a .62 cal. firle, with groove depths of .008-.010, you would probably have to use a thicker patch to fill the grooves properly, probably a .020" patch or even larger. It depends on whether the rifling is square, or round- bottomed.