Both the Ar15 and Garand/M14 have firing pin blocks to keep the FOP from running forward full tilt.


If I was a betting man, would dare to bet the the Free gap of the disco is way too wide, which is not holding the hammer back on the disco as the bolt slams closed, and this is causing the double fire instead.

Where is there a firing pin block in an AR15?


CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

I have seen a couple of early M16's that had the large heavy mass firing pin, that had enough force to slam fire upon loading, and slam fire when cycling causing bursts.  This was due to too much mass of firing pin.




While teaching an Armorer course at a Law Enforcement Academy, the State Game & Fish Officers brought a couple of LMT 308 rifles to class that were slam firing upon loading, and firing bursts when shooting semi-auto.  Upon inspection, I didn't see anything out of the norm.  After playing around with these rifles, I determined that the slam fires were most likely due to the extremely long heavy duty action spring (Buffer Spring) that was inside the stock system.  What we figured out is that the heavy resistance spring was slowing the action during the rearward cycles of fire, but the heavy spring was also speeding up the bolt carrier going forward, and this extra force going forward was causing the free floating firing pin to impact the primer on the ammunition with enough force to make it fire.  We put standard action springs in the rifles, along with heavier buffers from HeavyBuffers.com, and the rifles ran flawless.      


CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

Firing pin protrusion blocks until the Bolt is almost locked up, then the FP's can protrude out the bolt face on final lock up.

So on the AR, the FP slams the back of the carrier until the bolt is almost locked up and the then the bolt face will protrude out the back of the bolt to allow full protrusion final.

On the Garand and the M-14, slot in the receiver that does the same thing, and its not until the Bolt is locking if finally caming over to lock up, that the back L part of the bolt can clear the receiver slot to allow the FP to protrude out the bolt face.

So even with a full sling shot of the bolt/B/C, the cam over needed part is enough to slow the FP down to prevent a slam fire with standard primers.

Now if we have thin jacket pistol type primers in play with the loads, then that is a game changer, and then the FP may have enough inertia on the final cam over to ignite such primers (what the FBI did to prove that you could have FA fire with bolt riding the carrier home on a AR rifle).