When you guys start buying slugs, for the Western Arms .357, since it is a TJ barrel, TJ has disclosed that the land diameter is 350, and the groove diameter is ,357,,,,,,,,,,so you would want a slug that is sized down to somewhere around .306 maybe .307 but going just a tad less than the groove depth produces a little less friction and likely somewhat more velocity. I will be testing mine soon and will let you know what I find out as far as what works accurately. What I have noticed is that the grease or oil used to lubricate it at factory seems to freeze up in temperatures like 15-20 degrees. I had mine magazine not rotate properly and thought the barrel was in too close but it was not the case. Airguns of Arizona probally tests these guns for function and since it is Arizona the probably do not get function tested in 5 degree weather like we have here often in Nebraska winters. So I will disassemble mine and remove all lubricants and use only a lubricant that will not freeze up or gel under arctic conditions. The magazine is much thicker than the Huben so it can take a much longer slug or it can take a much shorter pellet. I will be testing over the weekend unless the Artic vortex comes back and its 4 degrees. Its not a lot of fun calling in coyotes in 4 degree weather. Then have your gun not rotate the mag after the first shot. Simple fix with the right lubricant