Parts guns with mismatched springs may set off a round. The ejector plunger/spring should soften the blow from the closing of the bolt in an AR15, M16. If the buffer spring is to heavy, this can make the speed of the closing bolt to high. This makes it possible for the firing pin to set off the round. Always load a round from the magazine. Other so called "slam fires" are caused by light hammer/sear engagement. (trigger to light)