Concave primer cups

Hello folks...I've got a 308 rifle that, every now and then, does crank out primer cups with an odd concave surface (see the picture herewith). No matter if reloading or factory ammo. Any thought about? Thanks

 

Probably just the way some pieces and the metal makeup contract or expand from heat and pressure of firing.
Don't think I would worry about it, since no signs of high pressure are there. Just a thought.

 

Looks like you need a new firing pin spring or some work on your bolt. What action is this on?

 

Thanks Dusty, the action is a Nesika K 1.470" dia.
What kind of work on the bolt do you mean? btw: of course bolt face is perfectly squared

 

Looks to me like a combination of firing pin clearance and weak spring, possibly some drag inside the bolt. The firing pin is bouncing back after a delay

 

...incidentally this specific rifle has never shot screaming groups; on the other hand we've deeply checked it and found nothing wrong but sometime this odd primer cup shaping

 

How many times has it been sold? Most times thats a good indicator

 

The rifle is brand new (our own build): it's an F/TR rig with Nesika K 1.470" action, 30" Krieger Heavy Palma 1:9.5" barrel, McM XIT stock and Jewell BR trigger

 

Wouldn't that more likely be tight firing pin/excessive spring tension and possibly a low pressure load?

Not enough pressure for the primer cup to push the pin back, so it forms around it?

Checking that would be as simple as firing a case with primer only and seeing if it was crushed the same way or possibly worse.

 

Firing pin bounce. The 2nd contact makes a concave primer?

https://thefiringline.com/forums/showpost.php?p=5417594&postcount=17
I like this one. 30" barrel, powder stopped burning. Pressure dropped. VACUMM.
A 30" 22 lr would loose velocity.


Spring weight @ http://benchrest.com/showthread.php?102626-Kelbly-Actions-What-not-to-do-to-improve-accuracy

Interesting.

 

We've checked for firing pin/spring drag inside the bolt...they're fine. About ammo pressure: we've got the same issue even with factory match ammo, not only with reloading ( in the picture above there are factory Lapua Scenar spent brass)

 

Pocket pressure is estimated to be around 2700 psi (if i remember correctly?) when fired with no powder in the case. From search here https://discover.dtic.mil

Before firing a primer only, no powder, measure head to datum. Is shoulder set back after primer firing?

30-06 ammo when fired with no flash hole (spit hole) has deformed the shoulder.

 

The Nesika's are notorious for weak ignition.

The first thing to do is check the amount of firing pin fall. -Al

 

When I looked at the pictures in post #1 the first thing that popped into my head was vacuum.

Then I read post #10

I'm scared, really scared...….

And I don't want to hear anything about how great minds think alike.

 

Do your primers look like this every shot? Or every now and then? Will a no-go gauge chamber in your rifle? Have you checked for excessive chamber depth?

 

I've this shape every now and then, regardless of pressure level (i.e. factory ammo and reloading alike)
Headspace checked: +0.0015" from Go-Gauge.... it's a no issue

 

thanks Al.... what in your opinion should be a correct (minimum) firing pin fall distance for Nesika?

 

This is repeatable and if you think about old school revolvers, these might look familiar. The only difference between the two is the chrome primer is a Rem 1 1/2 and the brass is a 6 1/2. Both loads around 13k psi.

Not all that uncommon with old school firing pins and hammers.

Same action and 1 1/2 primer, different barrel and cartridge. Probably around 40k psi.


My "low pressure" comment would be relavent to cup thickness/hardness and what would considered normal cartridge pressure. These were shot in an action with a fixed firing pin and why I think the firing pin in question may be hanging up occasionally.

 

The net result of the firing pin fall and spring pressure discussions is the amount of crush on the priming compound pellet between the cup and anvil. A simple test rolls both the fall and spring pressure components into one result which is the depth of the firing pin dent in the primer. Take a correctly sized empty primed case and fire the primer. A depth mic can measure the difference between the pin resting against the unfired primer and against the dent in the fired primer. It should be .020 or more to assure good ignition. Remington spent tons of $$ testing this very thing years ago, and the Department of Defense spent a pile doing the same thing. Their conclusion was it should be .020 to .025 penetration for everything to be kosher.

 

Sorry, don't relieve there is