How deep should a primer pocket be?

[RegionRat]

Is the depth you seat the primer at going to make any of this 50,000 PSI go away? I am not sure but setting it deeper may be worse, wiggle room may let the prime move more than it should.

Not going to change the gross pressure issue one way or the other. However, that isn't the issue.

Primers are designed to have the anvil in contact, plus+, Not slack.
Even when seated compressed, that the cup will slide back in the pocket up to the bolt face, and then the head moves there as well to bring the cup to flush, and this is normal.

I do not recommend seating primers exactly flush to the head or zero depth to avoid the movement of the primer cup since the firing pin strike will also move it, but in this condition the erratic performance will not make you happy.

ETA: If you are not convinced, go ahead and run a special test. Not every system is fine enough to resolve the issues, but many are. Take velocity stats and see what happens with the primer at zero depth, and then compare to samples that have the anvils touching to compressed about 0.000" to 0.002" and compare. I wouldn't ever recommend a zero depth seating for a semi auto for safety reasons, but a special test in a bolt gun while being very careful should be low risk.

 

[M-61]

Even when seated compressed, that the cup will slide back in the pocket up to the bolt face, and then the head moves there as well to bring the cup to flush, and this is normal.

Oh yes!

 

[ebb]

Rat if I even had a clue what you were talking about I may have a response. Zero depth ?? off of bolt face or primer pocket bottom? Anvil in contact with what? The anvil is seated during manufacture to be deeper than the open end of the cup, the cup will touch the bottom of the primer pocket and the anvil is up several thousands deeper than the open end of the cup. I haven't measured it but I can see it with a magnifying glass so it is a pretty good bit. Why would they make a bottom to the primer pocket if they didn't want the primer seated there?

 

[RegionRat]

Rat if I even had a clue what you were talking about I may have a response. Zero depth ?? off of bolt face or primer pocket bottom? Anvil in contact with what? The anvil is seated during manufacture to be deeper than the open end of the cup, the cup will touch the bottom of the primer pocket and the anvil is up several thousands deeper than the open end of the cup. I haven't measured it but I can see it with a magnifying glass so it is a pretty good bit. Why would they make a bottom to the primer pocket if they didn't want the primer seated there?

Anvils are inserted but still higher than the cup and the "crush" is that process of seating to the stack height and then pushing in a few mils deeper. The anvil will "sensitize" the compound between the cup and the apex of the anvil. Here is a section side view. Notice the "legs" stick above the cup.



Setting them slightly "crushed" by at least 0.002" to 0.006" is the intent of the designs. Folks will do all sorts of other things beyond the OEM design recommendations but doing so will not give better results.

I still recommend you run around 0.003" - 0.004" crush if you just want to assemble ammo, or run tests based on letting the anvils just touch and then no deeper than about 0.008"

You will probably see little if any difference from 0.002 to 0.006" of crush, and then with careful work you will see degradation in your stats when exploring beyond. Try not to leave slack between the anvil and the pocket, and there isn't any benefit to excessive crush. Excessive crush can even cause reliability issues.

I have seen too many primer tests to pull your leg, so go give it a try with a good rifle some time.

Sorry if the previous reply was a little unclear, I was being distracted at the time. When I said flush with the head, that means zero depth from the head such that the primer and case are flush.

Determining the "crush" requires you to inspect the primer pocket depth, and the primer height. A straight subtraction of the primer height from the pocket depth would be how deep the primer should be inserted to get the anvil feet to just touch the bottom of the pocket. Crush would be the amount you go beyond this.

 

[Scrumbag]

This thread has taken a really interesting turn. When I asked the question about depth, it was because I looked at some of my brass that has been fired and wondered if the pocket was too deep to give consistent ignition...

 

[Coyotefurharvester]

After looking at primers, I thought the doomed shape of the anvil was designed to help the anvil resist movement (flattening) during the firing pin strike. Because ignition is the result of pressure on the priming compound between the cup and anvil??

 

[JFrank]

 

[jackieschmidt]

I am in a minority, but I do nothing to primer pockets. I seat my primers with a hand tool that I have been using for years. I seat it, turn the case 180 degrees, then do it again with the same pressure.

I have a Sinclair tool that I bought way back in the ‘90’s. I just use it to swipe the carbon out after each firing.

As a note, I don’t use anything but Lapua Brass.