Disassemble, inspect, lube. Do a function check, check if safety works, feed a couple of magazines, (either do this outside or remove firing pin if using live ammo) clean and lube magazines. Look for the obvious cracks ect. When chambering toggle should lock completely down. When shooting start with one round in the chamber. Fire, if it fires and ejects then put three rounds in the magazine, chamber, and fire. If there is a problem then you know its the magazine.