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Old January 21, 2022, 04:15 PM   #1
603Country
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Full auto runaway

I’ve been loading a specific bullet for my 223 AR, using a medium powder charge, and using CCI BR4 primers. Never had a problem till today. I wanted to check the scope to see if the POI is unchanged. I had a 10 round mag full of the rounds with the BR4’s. The first round went Ok, but the second trigger pull ran off about 5 rounds, full auto. All those rounds had the BR4 primers. All rounds with the CCI 41’s were Ok. When I cleared the action, the unfired round that had been in the chamber (CCI 41 primer) had a lightly dimpled primer.

Why would rounds with the BR4 primer now be a problem, when they never were before? They might have a softer primer cup than the 41’s, which I knew, but again, it has never been a problem before. Only diff is that today is colder than my normal shooting days. Otherwise, all else is unchanged (brass, bullets, powder). The action isn’t very dirty.

All new loads for that rifle, now and for the future, are with the CCI 41 primers.

What do you knowledgeable guys think?
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Old January 21, 2022, 04:24 PM   #2
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My guess is the bolt/firing pin have some issue (cold might = gooed-up lubricant?) and the softer primers were more susceptible. I'd be checking the BCG to see if you have a firing pin that's stuck 'out' enough to act as an open bolt on the softer primers.

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Old January 21, 2022, 04:24 PM   #3
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Sear's probably not resetting the hammer on the trigger shelf properly would be my guess. Check the entire trigger group carefully and the well in the lower receiver for anything loose or debris (I've seen a blown primer lodged under the rear of the trigger once cause this). Kinda cool though, but highly illegal if you don't get it fixed asap.
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Old January 21, 2022, 04:31 PM   #4
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Yeah, I'd look at your trigger group before I blamed primers.
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Old January 21, 2022, 04:38 PM   #5
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That will wake you up!!

A critical piece of information. Did the hammer fall?

It certainly could be a primer issue...maybe.

But it could be worn fire control parts. I have experienced two or three rounds at a time when trying to slowly squeeze off a shot. The slow squeeze has a way of finding worn fire control parts. A new Giselle trigger is one cure.(In my case, it was worn parts,and it was immediately fixed)

If the Mil-Spec primers solve the problem.....I guess you have your answer.

And that is possible.
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Old January 21, 2022, 05:49 PM   #6
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Yeah, I'd look at your trigger group before I blamed primers.
You can check, but as described, I'd blame the primer sensitivity. First off, if its a mechanical problem with the gun (hammer, sear, trigger group) its NOT going to only do with one primer type and not another.

the AR has a floating firing pin. It will bounce off the primer when the bolt shuts. There are several other designs that also do that, and every one of them is fine with the mil-spec primers it was designed to use. Change that, and things can change. Cold MIGHT have had something to do with it if it made the firing pin "stick" but is not likely. Clean the gun and run the proper lube for temp and its not an issue.

My guess would be that the particular batch of BR4 primers is just slightly more sensitive than previous batches and THIS time it was enough to let them fire when the action shut.

BR4 are bench rest primers, and not specifically designed to withstand the rigors of a semi auto loading cycle. Add in a floating firing pin and I'm actually surprised you haven't had an AD before now.

My advice is to save the BR primers for bolt guns or other manual actions and run your AR with other primers.
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Old January 21, 2022, 09:15 PM   #7
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The specific load was originally worked up for my 223 bolt gun, but was found to be very accurate in the AR, so became a good load for both rifles. Recently I decided to switch that load to the CCI 41 military primers because I was also using it in the AR. I had maybe 25 rounds left with the BR4 primers.

I’ll give the rifle a good cleaning. It isn’t an old worn AR, but is relatively new, and I run it pretty wet with SLIP 2000 lube.
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Old January 21, 2022, 10:26 PM   #8
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What is the trigger group--standard mil spec--or some adjustable or light triggerpull drop-in custom trigger group? Is the AR a build or off-the-shelf factory gun? If your gun is somehow slam-firing the BR primers--It's probably going to do it with something else as well sooner or later IMO. I've never done it--but I've heard that it's possible to simulate a bump fire if you let the rifle move back enough under fire while holding the trigger and not allow reset and the hammer may still fall.
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Old January 22, 2022, 02:52 AM   #9
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Ammo?

Not discussed is a potential ammo problem....high primer? A worn seating tool could yield a run of loads more susceptible to slam fire. Too, might a headspace/resizing issue, combined with a high primer, further lend to the problem?

Might not be the rifle, could be that run of ammo.
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Old January 22, 2022, 10:37 AM   #10
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The trigger group is a LaRue and the primers are well seated. And, I had a firm grip on the rifle. It isn’t an ammo problem.

Upon further reflection, I might have used some CCI 400 primers instead of the BR4’s that I said I used. The 400’s have a thinner metal primer cup than the BR4’s and the 41’s. The 400’s are not recommended for use in an AR, but since I originally loaded that batch for the bolt gun, I wouldn’t have been paying much attention to whether or not they were 400’s. I’ll have a look when I go out to the workshop.
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Old January 22, 2022, 11:04 AM   #11
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Problem with slathering the bolt and carrier up as is often recommended is that it invites the formation of carbon cement on some parts. I find the possibility of your firing pin somehow being fouled up far more likely than "weak-walled" primers as the cause of slam-fires (if that's in fact what's happening). You should be able to easily able to determine that by observing where the tip of the firing pin protrudes when the bolt is both fully forward and fully back in the carrier. I have a few of the LaRue triggers--if it's that relatively inexpensive 2 stage model you're talking about--and they are OK but I'm not as enamored of them as others are, but that's just my personal taste of the feel. But they have been totally reliable and I have had no issues. That doesn't mean drop-ins are 100% immune from timing problems, especially since it's hard to resist modifying them for some people (and I'm one of them, occasionally). It doesn't take a whole lot to alter the proper functioning of the trigger sequence.
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Old January 22, 2022, 11:40 AM   #12
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If anything caused a stuck/fixed/protruding firing pin.....like a Sten or M-3 Grease gun,
Ignition would occur approximately when the cartridge case shoulder stopped against the chamber shoulder.

The bolt carrier would not have traveled far enough forward to translate the forward linear motion to rotary motion engaging the bolt locking lugs.

I realize lighting the fire takes a few microseconds, but "fixed firing pin" ignition will be,to some extent, ignition without full locking lug engagement.

I'm skeptical the violence of extraction would accommodate full auto operation.

I have experienced 2 to 3 round bursts in an Armalite AR-10 T ,stock trigger, wih a very rough estimate of 4000 rounds downrange.
This was not my rifle. The Armalite AR-10 is unique in that it has a firing pin retraction spring in the bolt. My assumption is this spring makes inertial slam firing less likely.
Primers were almost certainly Win WLR s . Thats the primer the owner prefers as it runs smoothest in his Dillon1050 process.

I was shooting on the bench using a gradual trigger pull . Initially, it had something to do with my trigger pull. It only doubled/tripled with me.

But soon,it was not just me. There was a trigger group problem.

A new Gisselle fully cured the problem. QED for worn trigger parts in this case. YMMV
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Old January 22, 2022, 11:55 AM   #13
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I think I have identified the problem. The slam fire rounds were originally loaded for the bolt gun, and I used the last of my CCI 400 primers. They aren’t recommended for use in an AR. They have a thinner cup than the CCI BR4 and 41 primers. So, probably the primers and the reloader guy (me) are at fault. This afternoon I’ll do some cleaning and shooting with BR4 and 41 primer ammo, and not a full mag.
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Old January 22, 2022, 01:00 PM   #14
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Quote:
and not a full mag.
Use a 100 round drum magazine with every 5th round a tracer. Videos too.

Just kidding.
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Old January 22, 2022, 03:11 PM   #15
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I called cci a while back to adk about primers gor ars. Per cci they reccomend the cci #41military primers or cci 450 small rifle magnums in the ar platform. They are both magnum primers with thicker cups, and the #41 has an anvil adjustment to reduce sensitivity. They said you may be able to get by with regular small rifle primers in an individual firearm bases, but it is not recommended in ARs.
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Old January 22, 2022, 04:39 PM   #16
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Quote:
(cold might = gooed-up lubricant?)
(More) likely ^^^ This ^^^

FWIW, the firing pin & pin channel should be absolutely dry.

.
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Old January 22, 2022, 05:34 PM   #17
603Country
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I cleaned it. And the BCG was a bit dirtier than I expected, but the firing pin was not gunked up too much and moved freely. And I shot it with the rounds loaded with the CCI 41’s, and no problem.

I’m out of CCI 400’s, so I can’t mess that up again. The BR4’s have the same primer cup metal thickness as the 41’s, so I should be Ok if I mistakenly use them in the future. And the BR4’s are what I used prior to the 41’s, except for my short term use of CCI 400’s. Never had a problem in 2 years with the BR4’s.

To answer an earlier question, the rifle is an off the shelf item by Black Rain Ordnance, though I replaced the trigger with the LaRue and replaced the mil spec (not so accurate barrel) with a Seekins precision. Danged accurate rifle now.
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Old January 22, 2022, 05:52 PM   #18
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Glad you found the problem. I've seen the 400s, and some Feds that would do that on a pretty regular basis in ARs that have certain combinations of things...overgassed, heavy buffers, worn buffer springs.
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Old January 22, 2022, 06:08 PM   #19
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Good news, happy shooting.
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Old January 22, 2022, 09:18 PM   #20
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I think you need to do some measuring on your BCG and fire control group. Blaming this on the primer is sidestepping logic, provided your primers are seated to the correct depth. You can have the softest primer in the world and the firing pin should never touch it to set it off unless you pull the trigger. You have something out of spec. It’s possible that your firing pin is too long, protrudes too far out of the bolt, for a number of reasons or your trigger is malfunctioning. Get on YouTube and watch a bunch of episodes of School Of The American Rifle and you’ll soon gain an appreciation of how many things can be out of spec on any AR.
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Old January 22, 2022, 09:39 PM   #21
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Originally Posted by Generic View Post
I think you need to do some measuring on your BCG and fire control group. Blaming this on the primer is sidestepping logic, provided your primers are seated to the correct depth. You can have the softest primer in the world and the firing pin should never touch it to set it off unless you pull the trigger. You have something out of spec. It’s possible that your firing pin is too long, protrudes too far out of the bolt, for a number of reasons or your trigger is malfunctioning. Get on YouTube and watch a bunch of episodes of School Of The American Rifle and you’ll soon gain an appreciation of how many things can be out of spec on any AR.
Ar's have a free floating firing pin. The firing pin touches the primer every time a round is chambered. You can generally see a small dimple in the primer if you chamber a round and clear it. The firing pin was designed to have low enough mass that it should not set off the primer when going into battery. However out of spec parts, or dirty firing pins can cause slam fires like an open bolt machine gun with a fixed firing pin. Due to the free floating firing pin cci reccomeds using 450 or #41s as they have thicker cups. And the 41 has a modified low sensitivity anvil, both of which help prevent slamfires.
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Old January 22, 2022, 09:59 PM   #22
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For a couple of years all I used in the AR were the BR4 primers (thick primer cup), and I had no slam fire problems. Then, when our reloading parts became hard to get, I decided to use up the CCI 400 primers and save the BR4’s for later. The CCI 400’s worked fine in the bolt gun and I already knew that the load (previously loaded with BR4’s) was excellent in the AR. So I moved the new loads with the CCI400 primers over to use in the AR. I shot 6 rounds in the AR with no problem and then had the full auto runaway. All the other similar loads have the CCI41 primer, and I tested them and they were fine.

I am very sure that my problem was the softer primers in the AR. I had read that the softer CCI primers were not recommended (by some) in the AR, but managed to neglect that info - senior citizen memory.

41’s from now on.
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Old January 22, 2022, 10:01 PM   #23
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yeah, I understand that it’s a “free floating” firing pin, but this isn’t a primer problem. Something is out of tolerance, over-gassed, over sprung…something. I use CCI 400s in a lot of my loads with zero problems in 3 different ARs.
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Old January 22, 2022, 10:29 PM   #24
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I have used cci 400, winchester small rifle, and federal small rifle in my ar with no issue. I was told it was a bad idea. I called cci to ask. What they said was what I stated. Reccomend 450 or 41s. They also stated that it can vary from gun to gun and 400s may be ok in some guns. They reccomended taking the gun to the range and, while safely pointed down range, chamber a round and checking the size of the bird peck in the primer after chambeting. If it was pin point you were probably ok. If it was anything more probably a bad idea. But it was a gun by gun kind-of thing, and would not give any guarantees. The gun could be just fine and within allowable tolerances, just not tolerant of soft primers. Could the gun be out if spec? Possibly. Could the gun be tuned to use 400s. Probably. But when #41s seem reasonably available and others are not, why?
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Old January 22, 2022, 11:45 PM   #25
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Ive fired thousands of 223/5.56 with cci 400 through atleast 10diffrent ar15s both rifle and pistol and the only time I had something like that happen it was the gun. My Issue was the pin had walked halfway out.
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