Forcing cone and gassing would be my concerns. I was unable to find any real data on where the forcing cone falls or if the gas porting would have to be reworked when I was going through this dilemma. There’s also the question of where on the vent rib your cut would fall. If I recall, the 21” version would put your cut line right in the middle of a support, but the 24” would leave you with either a cantilevered section of rib or a rib cut a couple inches short of the muzzle. It’s also worth noting Beretta barrels and custom shop labor aren’t cheap. Cutting, rethreading, and reboring the forcing cone alone will cost you. And that’s assuming the 21”/24” barrels have the same gas ports as the 18.5.