9mm AR primer oddities.

Chainsaw. · May 16, 2017, 09:54 PM

Hey guys, just finishing up a little AR pistol in 9mm and have an oddity to throw at ya. First a few specs, 7"barrel, cheapy bolt/carrier. Ammo is my reloads, well with in spec per the powder manufacturer. What Im getting is buldging primers, as in where the firing pin punches in gets pushed back out a hair. The edges of the primers are not flattened as Ive seen plenty of times in my magnum handgun loads, just the pin hole. CCI 500 SP primers. My only guess is the hole for the firing pin is to generous. Thoughts?

ShootistPRS · May 17, 2017, 03:41 AM

Are the primers cratering or just pushed out?
A firing pin hole that has clearance (too much) around the pin will allow the cup to extrude into the hole around the pin. If that is not happening then it is likely a chamber that is long for your brass, allowing the case to be driven forward upon firing and being held against the chamber walls allowing the primer to push out into a dome shape.
I would look at the depth of the chamber and your cartridge length to see if the case is going too deep. You can find the SAAMI acceptable range of cartridge length and measure your cases against that measurement. Check your fired cartridges against the tollerances. If you are using reloads you may want to allow the case to lengthen to get a better fit. If you are using new brass I would suggest "Blazer Brass" ammo because in my experience the cases are closer to maximum SAAMI specs.
You should also check that the cartridges pass the "plunk test" where the loaded round chambers without hitting the rifling and without by-passing the end of the chamber. If the chamber is cut too deep and you reload you can load cases a few times to see if they will lengthen to fit the chamber. Then you can set up your case trimmer to keep the cases within a couple of thousandths of the chamber length.

Let us know what you find and if it is corrected.

Chainsaw. · May 17, 2017, 09:47 PM

"A firing pin hole that has clearance (too much) around the pin will allow the cup to extrude into the hole around the pin."

This is pretty much what is happening. I think my cheapy bolt just has to generous of a firing pin hole. No other signs of over pressure, and like I said all my ammo is kept within spec. Guess Ill run her but keep an eye on things.

**44 AMP** · May 17, 2017, 10:46 PM

Do you have a firing pin protrusion gauge?? You should if you are building.

Not that it is the likely cause in this case, but with a gauge, its one thing you can eliminate.

Quite possible the bolt hole is slightly too large. Compare it with other bolts, if you can. Also remember that a "too large" hole might be the result of a "too small" (dia.) firing pin.

Chainsaw. · May 17, 2017, 10:52 PM

I can use my calipers protrusion stem to check the firing pin. Good thought. Cant recall whats acceptable, a hunting I will go...

**44 AMP** · May 18, 2017, 08:02 PM

see if you can find one of the GI gauges. Simple rectangle with a min notch on one end, max on the other.

Assuming, of course that the proper amount for a 9mm and 5.56mm are the same...

Chainsaw. · May 18, 2017, 10:23 PM

Being they are both small primers Im sure they would be real close.

May 16, 2017, 09:54 PM	#1
Chainsaw. Senior Member Join Date: September 12, 2015 Location: Issaquah WA. Its a dry rain. Posts: 1,774	9mm AR primer oddities. Hey guys, just finishing up a little AR pistol in 9mm and have an oddity to throw at ya. First a few specs, 7"barrel, cheapy bolt/carrier. Ammo is my reloads, well with in spec per the powder manufacturer. What Im getting is buldging primers, as in where the firing pin punches in gets pushed back out a hair. The edges of the primers are not flattened as Ive seen plenty of times in my magnum handgun loads, just the pin hole. CCI 500 SP primers. My only guess is the hole for the firing pin is to generous. Thoughts? __________________ Just shoot the damn thing.

May 17, 2017, 09:47 PM	#3
Chainsaw. Senior Member Join Date: September 12, 2015 Location: Issaquah WA. Its a dry rain. Posts: 1,774	"A firing pin hole that has clearance (too much) around the pin will allow the cup to extrude into the hole around the pin." This is pretty much what is happening. I think my cheapy bolt just has to generous of a firing pin hole. No other signs of over pressure, and like I said all my ammo is kept within spec. Guess Ill run her but keep an eye on things. __________________ Just shoot the damn thing.

May 17, 2017, 10:46 PM	#4
44 AMP Staff Join Date: March 11, 2006 Location: Upper US Posts: 28,804	Do you have a firing pin protrusion gauge?? You should if you are building. Not that it is the likely cause in this case, but with a gauge, its one thing you can eliminate. Quite possible the bolt hole is slightly too large. Compare it with other bolts, if you can. Also remember that a "too large" hole might be the result of a "too small" (dia.) firing pin. __________________ All else being equal (and it almost never is) bigger bullets tend to work better.

May 17, 2017, 10:52 PM	#5
Chainsaw. Senior Member Join Date: September 12, 2015 Location: Issaquah WA. Its a dry rain. Posts: 1,774	I can use my calipers protrusion stem to check the firing pin. Good thought. Cant recall whats acceptable, a hunting I will go... __________________ Just shoot the damn thing.

May 18, 2017, 08:02 PM	#6
44 AMP Staff Join Date: March 11, 2006 Location: Upper US Posts: 28,804	see if you can find one of the GI gauges. Simple rectangle with a min notch on one end, max on the other. Assuming, of course that the proper amount for a 9mm and 5.56mm are the same... __________________ All else being equal (and it almost never is) bigger bullets tend to work better.

May 17, 2017, 03:41 AM	#2
ShootistPRS Senior Member Join Date: January 3, 2017 Posts: 1,583	Are the primers cratering or just pushed out? A firing pin hole that has clearance (too much) around the pin will allow the cup to extrude into the hole around the pin. If that is not happening then it is likely a chamber that is long for your brass, allowing the case to be driven forward upon firing and being held against the chamber walls allowing the primer to push out into a dome shape. I would look at the depth of the chamber and your cartridge length to see if the case is going too deep. You can find the SAAMI acceptable range of cartridge length and measure your cases against that measurement. Check your fired cartridges against the tollerances. If you are using reloads you may want to allow the case to lengthen to get a better fit. If you are using new brass I would suggest "Blazer Brass" ammo because in my experience the cases are closer to maximum SAAMI specs. You should also check that the cartridges pass the "plunk test" where the loaded round chambers without hitting the rifling and without by-passing the end of the chamber. If the chamber is cut too deep and you reload you can load cases a few times to see if they will lengthen to fit the chamber. Then you can set up your case trimmer to keep the cases within a couple of thousandths of the chamber length. Let us know what you find and if it is corrected.

May 18, 2017, 10:23 PM	#7
Chainsaw. Senior Member Join Date: September 12, 2015 Location: Issaquah WA. Its a dry rain. Posts: 1,774	Being they are both small primers Im sure they would be real close. __________________ Just shoot the damn thing.

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