BLUE72CAMARO Wrote:I have an Ar in 204 ruger that I bought used online quite a few years ago. It has an older Giessele trigger that everything is adjustable installed. My first pull of the trigger ran a three round burst before I could get off the trigger. Turned out whoever had installed the trigger did not put a drop of locktite on the adjustments after setup and the disconnect adjustment vibrated out of adjustment enough that it was non functional.

CWOZ Wrote:Out of curiosity, do the bolt face depths measure the same? Do you use the same firing pin for both bolts?
I am no expert on any of this, nor do I claim to have any expert advise, but I seem to recall different bolt face depths and different length firing pins...

Lastrites Wrote:I recall AR10 cm supposedly recommended to have a slightly thinner firing pin tip and bolt face pin hole compared to a 308, hadn't heard of a length issue like in a grendel.

mel Wrote:Thanks guys i put some more rounds down range today and even ejected a few chambered rounds to look at primers and it all looked good. Im thinking i had a primer on that one round that wasnt fully seated

lazyengineer Wrote: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.

montana Wrote: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.

lazyengineer Wrote: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).

lazyengineer Wrote: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.

Kswhitetails Wrote: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.