What causes a popped primer?

[jelenko]

What causes a popped primer in an ar15?

Does it happen when the bolt first starts to unlock because there's pressure still inside the case?

Thanks

 

[RegionRat]

Lets assume the gun is otherwise fine and has reliable operation with other ammo… then…

Overpressure, headspace, worn brass.

 

[jonbearman]

It could be the case shoulder was set back to far and factory ammo can do it to if it didnt get caught by quality control. Secondly how old and how many rounds through the rifle. Is the firing pin in bad shape or maybe it is the problem. It could also just be a bad primer.

 

[762plinker]

and now with one popped primer a burned firing pin..that makes the next one even easier

 

[RegionRat]

Oh, and there is a distinction between “pierced” and “popped”, I assume OP means the primer came out of the case head and was found in the action where it can jam the fire control group.
I hate when primers get pierced and ruin the firing pin and spray junk into the bolt.

 

[jelenko]

I should clarify.

Nothing has popped yet.

I do have a bunch of primed brass that I've discovered has ~ 1/3 of the primer pockets that the 'no go' side of the primer pocket gauge from 21st Century will go in 1/3 to 1/2 of the way.
The brass is all bumped 2 thousandths from the chamber headspace. The loads I'm using are 52K to 56K [according to GRT].
Before starting to prime this lot I knew the primer pockets were getting looser, but only recently tested a some with the primer pocket gauge. This lot of brass is all primed with Unis Ginex SRP to account for the loosening pockets. But, with the testing I just did [just over 100 unprimed cases], I'm beginning to wonder how many of the primers are going to come out.

Do I have it right that, in an ar15, when the powder is first ignited, the case is forced back against the bolt face, yes? When the bolt starts to open does it retract from the base of the case so that there is room for the primer to be pushed out [assuming there's enough pressure left in the case].?

 

[RegionRat]

Till the bullet gets past the ejection port, the dynamics in the bolt face and case head are very similar to a bolt gun.
Once the pressure and gas flow in the gas tube gets going, then the bolt starts to rotate and then it moves back.
In a “normal” example, there would be something close to 12 ksi at the port when the bullet passes. That pressure and flow drops quickly once the bullet is out the muzzle, but that flow has enough potential energy to operate the cycle.

In and overpressure event, this pressure pulse is higher to the point where the pressure in the case and barrel along with the extra energy in the gas tube and bolt are more violent. This is hard on the rims toward the end of the pulse, plus the peak pressure is harder on the case head. This is what causes primer pockets to open up prematurely.

 

[big5ifty]

The loads I'm using are 52K to 56K [according to GRT

Your primer pockets are loosening because your loads are over pressure.

GRT is just software, use it to get an approximation, then physically test.

Use a 1/10,000 micrometer to measure case head expansion next to the extractor groove from virgin brass.

If you don't have virgin brass, or you're using mixed headstamp range brass, you already know you're at max pressure, so drop the charge 5%.

 

[fkimble@charter.net]

Frank, sounds like this batch should be used for practice. When I feel a too easy primer seating, I take the case and holding it a little off vertical, I rap it hard a couple times on my bench top and then look at the primer. If it has moved any, it goes into the "knock out the good primer pile", other wise continue loading procedure.

The case head is held in the boltface by the extractor until the case exits the chamber at which time the ejector causes the brass to tilt away from the boltface. At this point clearance is sufficient to allow a primer to move back if the pocket is too loose. Couple that with the sudden stop of the bolt at its rear limit and it can come out. Think of it as an inertia primer puller.

Frank

 

[Bill K]

Think you answered your own question, by saying the primers pockets are loose. Primer pocket just loose enough, from previous use, that the new primers going in loose will pop or drop out of the pocket, often upon extraction. Get new brass and solved this issue.

 

[Sandstorm]

Get new brass and solved this issue.

This right here.

 

[jelenko]

Your primer pockets are loosening because your loads are over pressure.

GRT is just software, use it to get an approximation, then physically test.

Use a 1/10,000 micrometer to measure case head expansion next to the extractor groove from virgin brass.

If you don't have virgin brass, or you're using mixed headstamp range brass, you already know you're at max pressure, so drop the charge 5%.

The primers are well rounded, there is no sign of an ejector swipe.

I did forget to mention they've been fired a total of 8 times; 7 times by me. It's only in the last two - i.e., the 7th and 8th firing - that I've noticed primers are seating easier.

Total lot size started at 5,000.

 

[Sandstorm]

You may be fine for most of them however, they will not get any better if they are loose.
Shoot em and find out.

 

[jelenko]

Get new brass and solved this issue.

This right here.

Absolutely. There's two lots of 6000 pieces ready - one of Wolf and one with Starline.

 

[jelenko]

You may be fine for most of them however, they will not get any better if they are loose.
Shoot em and find out.

Yeah.
I've loaded up 100 of them to test. If there's popped primers, I may have to just eat the cost of the ones already primed. There's a 7/8th's full 5 gal pail of them.

 

[jelenko]

All, thank you for your replies!

I do want to apologize for not providing more information about the situation with the brass and the objective of my question.

With my experience in project management, I should know better.

I'll do better next time.

Thanks!

 

[RegionRat]

BTW, is Service Rifle, the common Pet Loads are typically spicy. Many seasoned competitors automatically retire brass at the fourth or fifth cycle in order to avoid the statistical failures that start up if they try to go farther. YMMV

 

[markm87]

and now with one popped primer a burned firing pin..that makes the next one even easier

That's pierced primers. Popped primers don't always etch the FP tip, but I suppose they could damage the pin. We went through a huge rash of popped primers recently with some CCI 41s. I normally use Wolf SRM. Our loads were yielding modest velocities, and still giving swipes and pops.

I'd bet I have the record for popped primers on this forum.

 

[Hoser]

When my primer pockets start getting a touch loose they get marked with a red stripe and go in my "lost brass" ammo can. That way after that reload, they can go in the scrap bucket. But until then, I just press on.

 

[jelenko]

BTW, is Service Rifle, the common Pet Loads are typically spicy. Many seasoned competitors automatically retire brass at the fourth or fifth cycle in order to avoid the statistical failures that start up if they try to go farther. YMMV

Right.
For matches, I use brass that's 3X fired or less. The lot with the loose pockets is currently used for practice only.

I've been culling split necks and incipient case separations.

As the next lot 'ages' to 4X, I'm thinking to start sampling 50 or so to see how the primer pockets are doing.

On the current lot, I had been culling loose primer pockets by trashing cases that felt loose when primed.
I tested a couple samples with the primer pocket gauge so I could calibrate seating feel to cases where the gauge went in more than 1/2 way and culled those.
My short range load has changed from 25.3 of 2520 with RMR 69 grain bullets to 25.3 BLC2 with Nosler's 69 grain bullets - maybe that had some effect.
Scattered in the lot are cases for prone slow - but that's either 23.2 of 8208 with B80.5 or 23.6 8208 with S80's and there aren't a lot of them.

When I'm out of N69's, I plan to use 26.8 of BLC2 with the RMR 69's. In testing, no pressure signs, but it should be pretty spicy - and is consistently accurate. Fill ratio and burn rate are both high 90%.