More information is needed.... Is he planning on staying with .22 cal?.... What will be the FPE level of the rifle, as that determines the slug weight.... The slug length depends on that, and the twist rate depends on the length....As an alternative to purchasing a 3/4" OD barrel and turning it down, you could also get one of 5/8" diameter and sleeve the OD up with carbon fiber tubing to as large as you can accommodate.... Ideally, the diameter that recesses into the breech should be the maximum, of course....Bob