This is not uncommon for heavy loaded ammo in lightweight revolvers to jump the crimp after a few rounds are shot. Thats why you check and/or rotate your ammo. Perhaps the poly coating made the matter worse and I'm not sure why you would need that coating anyways. Its not really of any benefit for a defense use ammo. The only thing I see that caused this is that the crimp is further up the bullet than normal so the round will fit in the shorter cylinder of your Smith revolver. This does not allow for as deep a roll crimp, making the crimp grip on the bullet weak. However, that is the only way you are getting a 300gr hardcast bullet into that shorter cylinder. Not really the ammo makers fault, really. Perhaps it's better to get a 270gr hardcast instead, which will be just as good at a bit more velocity. You can also just rotate your ammo too, which is what many do.