For the amount of dry practice that anyone of us will do with any of our weapons, NO harm should come to the parts where you would get metal to metal contact. If we were Dry Practicing 500 hours a week for three months, than one can see the distinct possiblilty for considerable damage to metal parts. You would simply wear the gun out..!! These alledged cracks etal are probably the result of some other activity. As far as snap caps are concerned...I am not in favor of their use and from the comments here on this forum, you can find a wide divergence of opinion on that subject.
Dry practicing with a Glock should be a "Slamdunk"....!! Simply place your trigger finger on the trigger, take the slack out pause at mechanical resistence, start a SLOW steady pressure to the rear(Your focus is on the front sight) and let the shot "break". HOLD the trigger to the rear(Important). Reset the slide by moving it slowly to the rear about 1/2"...this will reset the trigger and striker. SLOWLY ease the trigger forward to reset and you should hear a distinct click. Hold the trigger there, get back on the front sight and slowly repeat the process....... Noto problemos....Good luck