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Old July 14, 2012, 02:23 PM   #1
Join Date: April 23, 2012
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Primer deformation... What am I seeing here?

A friend recently bought an AR57. He's asked me to reload some ammo for him. As such I got him a bag of fired cases. (5.7x28FN)

Some of the primers look like they have, for lack of better language, 'flowed'. Where the firing pin struck the case the resulting divot looks more like an asteroid crater where the edges of the imprint are raised up.

On many of these cases the amount of deformation is enough to prevent the case from sliding into the shell holder on the press.

I see no other indications of over pressure. No soot around the primers, no flattened edges, no signs of impending case separation etc. (I've read the 5.7FN likes to separate at the neck, not head)

I do not know the history of these cases. Whether they are 1x fired factory ammo or if they've been previously reloaded and fired.

I'm curious whether I am seeing signs of over pressure or did somebody use pistol primers in a case loaded to carbine pressures?

The other possibility I can think of is a worn out firing pin hole allowing the primer to flow into that void around the firing pin.

Some of the cases I have are obviously factory ammo with crimped in primers. These appear normal. These primers only have the dimple from the firing pin impact.



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Old July 14, 2012, 02:37 PM   #2
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Many variables !
Crimped cases are usually military. Military primers are also harder and thicker .
Primer hardness , thickness, cartridge pressure , bolt face [chamfered firing pin hole]. Chamber dimensions change pressures especially very long military bullets in a 5.56 chamber may cause excessive pressure fired in a 223 chamber .Yes the two are different.
Checking all those variables will give you the whole story.
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Old July 14, 2012, 03:09 PM   #3
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Is this like a rectangular shape? Common with some Eu guns.
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Old July 14, 2012, 03:55 PM   #4
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Typical round pin shape

Last edited by Twmaster; July 14, 2012 at 04:03 PM.
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Old July 14, 2012, 06:56 PM   #5
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As you suspect, cratering is a very common sign of a poor firing pin to bolt hole fit. In some rare instances the primer makers get soft sheets of brass stock and primers made from that can crater more easily. (It can also be due to a weak firing pin spring in bolt rifles.)

Try a different brand of primers and see if that doesn't stop it.
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Old July 14, 2012, 10:45 PM   #6
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Good to keep an eye on those primers.

Very likely the problem is fit of the firing pin to the firing pin hole, but as stated above, there can also be other factors.

A weeK ago, some of us went out for a fun and informal "shoot out" and one of the fellows allowed me to shoot his older S&W .38/.357.

As I removed the fired cartridges from the cylindar, it was clear that the bushing on this wheel gun was well warn, showing pitting and beginning to elongate into an oval.

He isn't to the point of danger, also firing some .357 mag loads with no extrusion of metal into the fireing pin hole, but clearly this gun will need to have the bushing replaced in the recoil shield.

Keep em coming!

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Old July 14, 2012, 11:20 PM   #7
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What you're describing is generally referred to as "cratering" (as mentioned by wncchester). The actual cause depends on the firearm the cases were fired in.
Cratering can be cause by a sloppy fit of the firing pin to firing pin channel, or by excessive pressures.

With 5.7x28mm, things are a little more tricky. Because the 5-7 brass stretches and pushes the shoulder forward when fired, it can hide other pressure signs. Since the 5-7 runs at rather high pressure, to begin with, you don't have much leeway before serious pressure signs suddenly emerge. (case rupture, blown primers, loose primer pockets, etc.)

Keep an eye on it.
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Old July 15, 2012, 10:41 PM   #8
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Thanks folks. As I have no info about the history of these cases I cannot tell whether an individual gun is the culprit or over pressure.

I do know that decapping most of a lot of 250 cases with cratered primers was misery.

I've loaded some at the starting load point per my Lyman book. We'll see how they look once fired.

I got another ~500 or so cases for my buddy. All of these look good. No signs of deformation of the primers.


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