One thing I learned the hard way was to check the barrel for bulges. I got a good deal on a Gen1 G17. I gave it a quick look over and bought it. Took it to the Marietta Oh GSSF match to get updated. The armorer found it had a bulged barrel. Now I know to field strip the pistol and drop the barrel back thru the muzzle end of the slide to check for bulges. Mine in particular was bulged right past the chamber, it didn't show on a normal cycle of the slide. Glock offered to replace the barrel, but I would have to send in the bulged barrel. In the end I bought a used Glock barrel and kept the original barrel.