Primer deformation... What am I seeing here?

Twmaster · July 14, 2012, 02:23 PM

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.

Thanks.

--
Mike

If you do not stand for something you will fall for anything.

mete · July 14, 2012, 02:37 PM

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.

PA-Joe · July 14, 2012, 03:09 PM

Is this like a rectangular shape? Common with some Eu guns.

Twmaster · July 14, 2012, 03:55 PM

Typical round pin shape

wncchester · July 14, 2012, 06:56 PM

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.

Crusty Deary Ol'Coot · July 14, 2012, 10:45 PM

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!

Crusty Deary Ol'Coot

FrankenMauser · July 14, 2012, 11:20 PM

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.

Twmaster · July 15, 2012, 10:41 PM

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.

--
Mike

12 ga Shotguns. Helping to keep kids off your lawn for more than 100 years.

July 14, 2012, 02:23 PM	#1
Twmaster Member Join Date: April 23, 2012 Location: Dallas, TX Posts: 35	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. Thanks. -- Mike If you do not stand for something you will fall for anything.

July 14, 2012, 02:37 PM	#2
mete Senior Member Join Date: June 14, 2004 Location: NY State Posts: 6,575	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. __________________ And Watson , bring your revolver !

July 14, 2012, 03:55 PM	#4
Twmaster Member Join Date: April 23, 2012 Location: Dallas, TX Posts: 35	Typical round pin shape Last edited by Twmaster; July 14, 2012 at 04:03 PM.

July 14, 2012, 11:20 PM	#7
FrankenMauser Senior Member Join Date: August 25, 2008 Location: In the valley above the plain Posts: 13,424	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. __________________ Don't even try it. It's even worse than the internet would lead you to believe.

July 14, 2012, 03:09 PM	#3
PA-Joe Senior Member Join Date: June 9, 2010 Location: NEPA Posts: 909	Is this like a rectangular shape? Common with some Eu guns.

July 14, 2012, 06:56 PM	#5
wncchester Senior Member Join Date: December 1, 2002 Posts: 2,832	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.

July 14, 2012, 10:45 PM	#6
Crusty Deary Ol'Coot Senior Member Join Date: November 19, 2009 Location: Deary, Ideeeeeeho Posts: 531	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! Crusty Deary Ol'Coot

July 15, 2012, 10:41 PM	#8
Twmaster Member Join Date: April 23, 2012 Location: Dallas, TX Posts: 35	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. -- Mike 12 ga Shotguns. Helping to keep kids off your lawn for more than 100 years.