Cause of slam-fire in my 6.5CM AR10?

[lazyengineer]

You are correct, unless the manufacturer product is insanely out of spec. It takes very little for bolt bounce in full auto, to induce a light strike malfunction. The Stoner AR is a very well designed and safe operating system. Excellent description and explanation, thanks for sharing.

Indeed. Also, with the M1 Garand/M14 design, there is a bridge in the receiver which is designed to engage the tail of the bent Garand firing pin, to knock it back right as the bolt is closing. Due to tolernaces, that bridge and gap are sometimes not quite right; or there's enough clearances to clear on only a partially closed bolt. On a reweld Garand receiver (of a demilled receiver that someone welded back together), it's almost a certainty that bridge isn't right. Also, they rely on the bent firing pin being of the right demensions, and it's not always necessarily a strong component that doesn't get bent. the result is it's not unheard of at all for the system in a Garand to allow the firing pin to hit the primer before the bolt is fully locked. M14's are built the same way. So OOB Kabooms with those guns were a real thing that wasn't that unheard of. Those guns are far more likely than an AR to light off from firing pin inertia upon closing (one failure event), and from a firing pin sneaking through via trigger pull, on an unlocked bolt (a separate failure event). Beware of insufficiently resizing your brass with those guns - better to overdo that than underdo it.

This is far less likely with the AR system, due to how the firing pin and rotating bolt system is designed. So long as the retaining pin is actually in the gun, it's Very unlikely (almost impossible) to pull the trigger and light off a primer on an unlocked AR Bolt. Even if the retaining pin breaks, it'll still be wedged enough across that gap to stop the back flange of the firing pin tail from continuing to the bolt. And the firing pin is at least 1/4" behind the exit hole, while the bolt is still unlocked. But because OOB was a real thing with M14's and Garand, for the longest time every time an AR Kaboomed on a trigger pull, invariably someone would say it's an OOB almost right away. With an AR, the only way you can have an OOB Kaboom is if it slam-fire Kaboom's on bolt drop. THEN maybe it happened. Otherwise, no - it's not an OOB Kaboom in an AR. I personally suspect about half of the slam-fires that occur, are actually trigger discharges for one reason (it got bumped) or another (tolerances/excessively light trigger). Or trash in the bolt face (e.g. bunts from blown out creedmoor primers). Or sometimes, maybe someone really did just have a super sensitive primer and the bounce was enough to light it off I guess.

One item of note - since there are no standard dimensions to AR10's, it is possible to get different dimension firing pins - which would indeed be a problem. Either too long and sticking out, or the flanges location and the retaining pin location in the BCG in relation to the pin-hole at the end, may not be the same from supplier to supplier (or even within the same supplier).