I shoot my M-640 predominantly with full power loads. This is not a plinker, fun gun. It is a for serious defensive gun and I count on it to bring my fat old butt home safely. So every round I shoot is a full power defensive load. Hell, I've even shot some Cor-Bon 200 gr. hunting ammo in it! (That was no fun I assure you.)
I don't keep track of my ammo expendature as much as I probably should, but I would guess that I've run maybe 3,500 to 4,000 rounds through my M-640 with no appreciable wear.
I will go D. Moon even further. Shooting a strict diet of .38 SPL's, your grandson will not wear out a M-640.
Just in case I've not made myself clear, I am a TREMENDOUS fan of the S&W M-640. It is my Always gun.
Doc Hudson