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excessive primer pocket depth?

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Old 06-07-2012, 10:50 AM
Bronze Member
Join Date: Sep 2010
Posts: 48
excessive primer pocket depth?

What would be a good rule of thumb in regards to primer pocket depth?
I am reloading "once fired" .280 brass and 4x reloaded .300wm.
In both lots of brass i am seeing some pocket depths exceeding .0133, some up towards .0144.
I am trying to determine at what depth to start throwing brass out?
I am guessing ~.0132 would a good limit....not sure though
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Old 06-08-2012, 08:08 PM
Platinum Member
Join Date: Aug 2003
Location: NC, oceanfront
Posts: 3,262
Re: excessive primer pocket depth?

You mean .133-.144"
The Sinclair large primer pocket uniformers go to .133"

You need them all the same depth. I would pick a depth I could uniform.
So IMO, 0.133 max
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Old 06-08-2012, 11:11 PM
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Join Date: Nov 2008
Location: oregon
Posts: 750
Re: excessive primer pocket depth?

pretty sure its .128 to .132... thats what I have always herd and thats what I use for all my rifles from 25wssm to 338 lapua mag.. they say best seating is with primer .001 to .002 from top of pocket.. my pockets are at .130.. and I get great results and very consistant velocities so I must be doing something right lol
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Old 06-10-2012, 06:30 AM
Silver Member
Join Date: Nov 2009
Posts: 298
Re: excessive primer pocket depth?

You will find out how deep is too deep when you pull the trigger and get a light primer strike. Instead of "Pow" you will just hear "Click". Time to throw that one out. I try and keep my primers seated just below flush to the bottom of the case. .002" - .004" works for me.
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Old 06-10-2012, 08:05 AM
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Join Date: Aug 2003
Location: NC, oceanfront
Posts: 3,262
Re: excessive primer pocket depth?

A while back I was handed an opportunity to see that primer striking is way bigger than talked about.

My son's Cooper started throwing big fliers. This was a solid 3/8moa gun that seemed to drift off into 3/4moa. And it was a gradual, but abstract change.
I chased my tail on it for over a month, swapping scopes, checking bedding & action screw torques, checking max OgvOAL. I changed bags, tried different barrel cleaning & fouling. I disassembled the bolt a couple times and found no issue, and went into the trigger, no problem.
The gun just stopped shooting well, as it was triple grouping. That is, if I fired 10 or 20 shots at a single bull, I would end up with 3 relatively normal groups separated from each other, and they didn't walk there in order but randomly landed into one area or another & back.
Also, ES was higher than I'd ever seen from the load, and I knew there was no aspect of my ammo that had any issue. This put primer issue in the back of my mind(bad lot?) Couldn't prove it..

One day I got lucky as hell; a misfire. About 30 shots later, another misfire.
Back at the bench I measured the fired firing pin protrusion, cycle, dry-fire, remeasure(bunch of times). Found that my pin was intermittently slipping through it's set-screw in the cocking piece, resulting in several protrusion distances.
I had pulled on the pin earlier while checking the bolt, but my scrawny little pull check didn't get it..
Start over, where should the pin be set?

Back to the range with a bunch of tools and my standard load. Tweak the pin setting 5thou protrusion at a time & firing groups. Took a while to home in on it, but I ended up with one setting that was better than all others -with that primer -and my standard seating(3thou crush in uniformed pockets).
With this info I went back to the bench and dimpled the pin to put it on the magic setting, and double set-screwed it -with locktite!
Now my kid's Cooper shoots solid 1/4moa and no more mystery fliers.

If I had never seen the gun shoot better,, if it had come to me shooting with this problem,, I never would have had any luck with the gun. I would have assumed it needed a better barrel. Then when that didn't work, I would have separated the gun into several dumpsters and been done with it.
Now I have a lot of things to test and learn about primers & striking of them. I should be out there doing it today!
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Old 06-10-2012, 10:56 AM
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Join Date: Nov 2009
Posts: 298
Re: excessive primer pocket depth?

That's an interesting post Mike. Did you notice differences in the primer strike indentations on the primer itself?
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Old 06-10-2012, 03:49 PM
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Join Date: Aug 2003
Location: NC, oceanfront
Posts: 3,262
Re: excessive primer pocket depth?

No, the indentations were exactly the same, even with misfires.
There are at least two relative aspects/approaches to primer ignition; Speed and/or Power

You can crush primers flat in a vice, slowly, without setting them off(but not always).
As mentioned if the pockets are too deep, and primers not seated in contact with pocket bottom, you get misfires. This, even with normal indention.
I believe the primer compound lead styphnate requires more shock, than force, to set it off.

How a primary explosive can vary in it's ignited performance is totally beyond me.
I used to think that once ignited, even if 'barely', it's output would be the same.
Now I know this isn't true at all.
But I don't understand why.
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