I have a Glock 17 1st gen that I've owned for close to 35 years. I shot it some at first but haven't had it out for a good while. I would be surprised that I have put 900-1000 rounds though the pistol. On both mags the plastic is cracked down the back. I defiantly wouldn't use them for anything serious. Is this something that normally happens to these plastic mags or did I get some duds?