Hi Jim, how you doing? Just a quick thought about that fine S&W of yours. Have you tried it without a mag' in it. Try dry firing. I've got 6 mags with mine and one of them is a right s.o.b. almost need two fingers pulling to index it. When I look closely at it there's no visible difference to the others. Maybe you've got a doggy mag ?
Without a mag in it you'll be able to determine if it's the trigger or something else causing it to be stiff
Good luck, Phil