Smyth Busters: AR-15 Dents Primers - Is It Broken?
The quest to separate fact from fiction never ends for Brownells Gun Techs™ Steve and Caleb, and today they tackle a worry that's been circulating since the AR-15 / M16 platform was brand-new. Scenario: a new AR-15 owner unloads their rifle and ejects an unfired round from the chamber, only to discover.... the primer on the unfired cartridge has been dented by the firing pin. That can't be right. Surely the firing pin should not touch the cartridge until you actually fire a shot? There MUST be something wrong with the rifle! Relax, say Caleb and Steve. The AR-15 has a "floating" firing pin, meaning it's free to move around inside the bolt. When the bolt slams into battery on a fresh cartridge in the chamber, the inertia pushes the firing pin forward enough to bump the primer and leave a mark. BUT without the force of the hammer behind it, the firing pin alone doesn't have nearly enough mass to ignite the primer just from the bolt closing. This is actually normal; the AR-15 was designed this way. Any gun with a floating firing pin does this. Don't believe it? Caleb has his AR-15, a primed cartridge case (no powder or bullet), and some safety glasses. See what happens!