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Bart B. · October 26, 2012, 07:37 PM

Mr. Guffey, my comments on pressure in a primer pocket were made discussing what happens when an empty primed case is fired. Not with a live round. Go back to post 30 and read the third paragraph. But they can back out with live ammo; please read on.....

Some years ago, I loaded 10 rounds of .308 Win. ammo with new cases primed with Fed. 210's, 165-gr. bullets with IMR4064 having decreasing charge weights in each case starting with 44, then 43, 42, 41,40, 39, 38, 37, 36 and 35 grains. I measured case headspace with each one and marked the case with charge weight and case headspace. With the 44 down through 41 grain loads, fired case had .001" or .002" increase in case headspace and its primers were all flush with the case head. When the 40 grain charge one was fired; its primer stuck out .001" And that fired case headspace was .001" shorter than when new. Charges 39 through 35 had successively more primer protrusion. Fired case headspace also got increasingly shorter than when new by about .001" per load. The case with 35 grains ended up with its primer sticking out about .006" All this, to me, indicates the case shoulder gets set back from firing pin impact and pushed out of the case as pressure gets higher. If there's not enough powder in the case to make peak pressure high enough to push the case body back so the case head stops against the bolt face, the primer will end up sticking out of its pocket and the fired case will usually have less headspace than when it was new.

Hooligan1, that ridge sticking up around the primer's dimple is typically caused by one of three problems. . . . .

One is the hole in the bolt face is too big for the firing pin's tip diameter. Had one that way but a 'smith hard chrome plate the firing pin tip making it .010" larger in diameter; no more cratered primers.

Another situation is a insufficient pin protrusion from the bolt face when it's at its stop inside the bolt or cocking piece. Primer cup metal gets pushed up around the shallow dimple cratering it. Firing pins should protrude a bit more than their tip diameter so its rounded tip fully indents the primer crushing the priming compound fully against the anvil inside the primer cup. .055" to .065" is typical with most centerfire rifle firing pins. If this is the cause, a new firing pin is needed that's going to let its tip stick out of the bolt face enough. Some firing pins can have their stop shoulder ground back a bit to solve this problem.

And finally, a weak firing pin spring may well fire the primer but it will get pushed back by normal peak pressure inside the case which also presses against the primer cup pushing the pin back and cratering the cup metal around a shallow dimple. This doesn't happen very often.

Of course if the peak pressure's way too high, that can cause primer cratering when all else is perfect.

October 26, 2012, 07:37 PM	#35
Bart B. Senior Member Join Date: February 15, 2009 Posts: 8,927	Mr. Guffey, my comments on pressure in a primer pocket were made discussing what happens when an empty primed case is fired. Not with a live round. Go back to post 30 and read the third paragraph. But they can back out with live ammo; please read on..... Some years ago, I loaded 10 rounds of .308 Win. ammo with new cases primed with Fed. 210's, 165-gr. bullets with IMR4064 having decreasing charge weights in each case starting with 44, then 43, 42, 41,40, 39, 38, 37, 36 and 35 grains. I measured case headspace with each one and marked the case with charge weight and case headspace. With the 44 down through 41 grain loads, fired case had .001" or .002" increase in case headspace and its primers were all flush with the case head. When the 40 grain charge one was fired; its primer stuck out .001" And that fired case headspace was .001" shorter than when new. Charges 39 through 35 had successively more primer protrusion. Fired case headspace also got increasingly shorter than when new by about .001" per load. The case with 35 grains ended up with its primer sticking out about .006" All this, to me, indicates the case shoulder gets set back from firing pin impact and pushed out of the case as pressure gets higher. If there's not enough powder in the case to make peak pressure high enough to push the case body back so the case head stops against the bolt face, the primer will end up sticking out of its pocket and the fired case will usually have less headspace than when it was new. Hooligan1, that ridge sticking up around the primer's dimple is typically caused by one of three problems. . . . . One is the hole in the bolt face is too big for the firing pin's tip diameter. Had one that way but a 'smith hard chrome plate the firing pin tip making it .010" larger in diameter; no more cratered primers. Another situation is a insufficient pin protrusion from the bolt face when it's at its stop inside the bolt or cocking piece. Primer cup metal gets pushed up around the shallow dimple cratering it. Firing pins should protrude a bit more than their tip diameter so its rounded tip fully indents the primer crushing the priming compound fully against the anvil inside the primer cup. .055" to .065" is typical with most centerfire rifle firing pins. If this is the cause, a new firing pin is needed that's going to let its tip stick out of the bolt face enough. Some firing pins can have their stop shoulder ground back a bit to solve this problem. And finally, a weak firing pin spring may well fire the primer but it will get pushed back by normal peak pressure inside the case which also presses against the primer cup pushing the pin back and cratering the cup metal around a shallow dimple. This doesn't happen very often. Of course if the peak pressure's way too high, that can cause primer cratering when all else is perfect. Last edited by Bart B.; October 28, 2012 at 05:48 AM.