The bore on my 53 is.222 the diameter of the jacketed bullets. When I tried cast bullets in the Jet I found it to be pretty much a waste of time but I tried .223 to be the most reliable. You really can't push them to much faster than 22 Mag speeds with any accuracy. Or so it was with my 53. Luckily I still have a few boxes of .222 bullets by Hornady and when I find them I buy .223 size 40 and 45 gr jacketed bullets(usually Sierra for some reason)made for the older Hornet bbls. Even have a couple boxes of Sisk .223 bullets. I happen to have a Savage 219 with that smaller bore. I really don't load the Jet any longer as I have about 10 boxes loaded. Probably will not shoot all of them up.