Vert, I can see your 20k round count as a possibility, I think it's a bit generous, but definetly possible, what parts should be replaced in that time? Other than a recoil spring assembly, perhaps a couple trigger springs just for peace of mind, I can't think what else would be nessecary and all those parts can be swapped in 2 mins. I could see how it's possible that a super worn out recoil spring could contribute to frame cracking, but I'd call that a small contributing factor and not a cause.