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) From CCI - NO. 34 an NO. 41 Primers. Military-style semi-auto rifles seldom have firing pin retraction springs. If care is not used in assembling ammunition, a “slam-fire” can occur before the bolt locks (M1 & M14). The military arsenals accomplish this using different techniques and components—including different primer sensitivity specifications—from their commercial counterparts. CCI makes rifle primers for commercial sale that matches military sensitivity specs that reduce the chance of a slam-fire when other factors go out of control*. If you’re reloading for a military semi-auto, look to CCI Military primers. *Effective slam-fire prevention requires more than special primers. Headspace, chamber condition, firing pin shape and protrusion, bolt velocity, cartridge case condition, and other factors can affect slam-fire potential.