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).

[montana]

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).

This is what happened to my M1A1. I got double taps thinking it was a defective trigger. After careful testing, it was the light trigger getting bumped during recoil.

[mel]

Glad to hear it. With regard to high primers; I've heard mixed reviews on that topic. While some say a high primer can induce a slam-fire, others (such as the people on the phone at Hornady), say it's actually the opposite, and excessively crushed primers, are what lead to slam fires. I don't know, there's a case to be made either way. Also, sometimes experiences and lore from M1-Garand/M14 days (where trigger pull fired OOB is a real thing), get assumed to apply to AR15's, not always accurately.

For a slam-fire to occur that doesn't result in a kaboom: that means the bolt had to fully close and rotate, and then detonate. That's a curious thing - because the face of the bolt has fully engaged and slammed against that primer in that bottomed out round, before the bolt starts to rotate. No discharge (or you have an OOB Kaboom, which is extremely rare with an AR15). The firing pin is stopped by its tail on the firing pin retaining pin; long before it can touch the actual primer, and is knocked back. As the bolt rotates closed the carrier continues forward, the pin's flange rebounds off the retaining pin going the other way, and so then goes forward again. This time the bolt has rotated and moved back in the BCG enough, that there's enough clearance in the retaining pin channel in the back of the pin, to allow the tip to weakly impact and bounce the back of the primer lightly, and hence the little ding we all know and love (or at least that's how I've heard it explained). Interestingly, this means a clean and slick firing pin and pin channel will be impacting that primer with more vigor, than a dirty fouled pin and channel. Was the gun freshly cleaned and lubed? (Also, on a side note, don't do this a lot - military and police have learned that cycling the same round in and out of a gun 365 times per year (i.e. shift-change load/unload), can do strange things to that primer - including make it a dude at the worst possible moment); so don't cycle the same round more than a few times, if you plan on relying on it).

If the primer is a high primer, that usually means the anvil isn't bottomed out so well. Indeed, one has elevated chance of a misfire from a high primer for this very reason. You can tell if the round goes off with a second strike. Indeed, this is one reason I prefer DA/SA guns to just SA guns (e.g. A P99 vs a Glock 19); because you can pull the trigger again on a "click-nothing", and have it discharge on the second strike. I've done that with a pistol (never with a rifle though - rifle firing pins hit hard).

Back on topic according to Hornady, if one crushes a primer into the casing, and pre-crushes the primer compound, that actually has a higher chance of a slam-fire. I will say, I've fired a lot of high primer rounds over the years, and think they are right - never had a slam fire. This is why Hornady engineers their progressive press to carefully not over-insert the primer. Sometimes that means the primers are flush or even high if you aren't careful, but they actually do this by design, as they think it's safer to have a high primer, than a crushed primer. I think they're right. What press and what priming method was used on the ammo that slam fired?

On a final note, I've had mixed results with the AR10 firing pin and channel designs with high pressure 6.5CM. In some batches with some primers at some pressures, they will blow a bunt of copper right out of the primer and down into your firing pin channel in your bolt. There's no impact crater, there's a hole of missing metal. When that happens, the bunts will build up and start binding up inside the firing pin channel, and wreck all kinds of havock on operations, and on your firing pin retaining pin. Have you been experiencing this? Same primers in lower pressure 30-06 ammo run just fine; it's an artifact of the high pressure 6.5CM and the fact some primer manufactures metalurgy didn't design for a smaller diameter firing pin giving essentially a sharper holed punch.


Just sharing my own experiences and conversations.

gun was not just cleaned and lubed
im using a rcbs hand primer , i dont use the prime arm on the press , the press in an rcbs rock chucker supreme

i had issues with the bcg i had puncturing primers at well below max load , so thats what i bought the brownells bolt , ill save my other bcg and rebuild it with a new bolt and firing pin for my other creedmoor
i loaded up a bunch of test rounds for it today 90g speer tnts ontop of 48g of r-17 leaving the barrel at 3,520 fps , no issues at all with slamfire or anything funny ,

[Kswhitetails]

There is a technical aspect to this I don't claim to fully understand. The tolerances, protrusions, lug engagement, bolt rotation, BCG travel, battery station, spring rates, cycle speeds, etc are variables that play nicely with each other until they don't.

If I had to guess, I'd wager that you're seeing the result of a new bolt and BCG. There was likely a burr or other production issue that has since either worked/worn itself out/in, or something wound up magically between the pin and primer on the initial bolt closure.

Can you reproduce the conditions and effect? I'd spend some time trying to re-induce the slam fire before I fully trusted the setup, there are as mentioned lots of potential causes of the malf.

[mel]

There is a technical aspect to this I don't claim to fully understand. The tolerances, protrusions, lug engagement, bolt rotation, BCG travel, battery station, spring rates, cycle speeds, etc are variables that play nicely with each other until they don't.

If I had to guess, I'd wager that you're seeing the result of a new bolt and BCG. There was likely a burr or other production issue that has since either worked/worn itself out/in, or something wound up magically between the pin and primer on the initial bolt closure.

Can you reproduce the conditions and effect? I'd spend some time trying to re-induce the slam fire before I fully trusted the setup, there are as mentioned lots of potential causes of the malf.

ive been very careful with the rifle since , though ive already put a ton of rounds down range with it and havent had an issue since , ill put some more range down range with it today and might even take it out for coyotes tonight , though ill prolly grab my grendel for that 11 pounds ontop of the tripod is much nicer then 23/24 pounds, but yeah i still have a lil trouble trusting it everytime i drop the bolt lol