Looking for insight on blown primers

[tmoxley]

Hi all,
I have a Savage MSR 10 Long Range (AR-10 style platform) chambered in .308 which I have been very pleased with. I am not a handloader but I did decide to try out a wide variety of ammo in the rifle to see what it liked best. I settled on IMI systems 7.62mm Semi-Auto Match 168 gr Razor Core because it seemed to group the best. It was always my understanding that 7.62 x 54 mm was safe to fire in a .308 rifle.

I have been running this ammo for the past few years and have experienced some issues. Some rounds blow the primers out and I find the separated case and primer on the ground after the round has cycled. Some rounds the rifle rips the rim off the cartridge during the extraction and ejection. Neither of these occurances were consistent, but both would occur at least once every range trip. I thought the issue may have been related to overpressure in the gas system so I turned the adjustable gas block down to one notch above "not cycling". This has not fixed the issue.

I ran FGMM 168 gr .308 through the rifle last weekend and did not experience a single issue.

I have attached a picture of the IMI with ripped cartridge rim, and a picture of the FGMM from last weekend. Unfortunately I forgot to save any cartridges with blown primers to post. None of the rounds show any bulging or disfigurement aside from what has been noted.

I am looking for any information that will help me understand what the issue with the IMI ammo is and if it can be mitigated or if I need to stick to other ammo in the rifle.

Thanks for your help!

 

[SSG Graybush]

That sounds like overpressure, sometimes factory ammo can be a bit hot. Maybe a hot lot. The case sticking slightly by overpressure is probably causing the extractor to damage the case rims. It could break the extractor. Try another ammo, if it doesn't happen with other ammo, there you go. It's not your gas system. It should be adjusted to eject to 2 :30 to 3:00. More forward is over gased and rear is under gased. Make sure your cleaning your chamber well, not just the barrel. And AR's are machines, they need lube. The fed looks good, the imi can't tell. another pic of the case head would be good. Can you see a ejector mark on imi and not the fed? That's a clue that the imi is too hot.

 

[mtwarych]

On top of the IMI being a little hot, you are seeing the signs of your bolt unlocking and ripping the case out of the chamber while the brass is still under pressure.

I would suggest trying a heavy buffer to keep your bolt locked up just a smidge longer and let the pressure drop before extraction/ejection. Just so it's known, I had the same exact problems with my DPMS GII.

I have an adjustable gas block, a heavy recoil spring and a heavy buffer. My GII's recoil is about as brutal as a herd of Golden Retreiver puppies.

Slash's Heavy Buffers | Home

Custom Heavy Buffers for AR10 AR15 rifle carbine. Home of the Slash CAR-10 Buffer.

heavybuffers.com

Best of luck out there.

 

[SSG Graybush]

On top of the IMI being a little hot, you are seeing the signs of your bolt unlocking and ripping the case out of the chamber while the brass is still under pressure.

I would suggest trying a heavy buffer to keep your bolt locked up just a smidge longer and let the pressure drop before extraction/ejection. Just so it's known, I had the same exact problems with my DPMS GII.

I have an adjustable gas block, a heavy recoil spring and a heavy buffer. My GII's recoil is about as brutal as a herd of Golden Retreiver puppies.

Slash's Heavy Buffers | Home

Custom Heavy Buffers for AR10 AR15 rifle carbine. Home of the Slash CAR-10 Buffer.

heavybuffers.com

Best of luck out there.

I disagree, the buffer only affects the bolt after it comes out of battery. Am not saying a heavier buffer doesn't help to slow the bolt, soften the bolt bottoming out, smooth cycling out if needed, but it ain't gonna fix this issue. And adding a new buffer just makes diagnosing harder.

 

[ShtrRdy]

The powder IMI uses may not be the right burn rate for an AR-10 rifle. i.e. if the powder has a slower burn rate it might be creating too high a pressure at the point of unlocking and extracting.

 

[Bob Wright]

The powder IMI uses may not be the right burn rate for an AR-10 rifle. i.e. if the powder has a slower burn rate it might be creating too high a pressure at the point of unlocking and extracting.

Even my reloading manual has gas reloading listed separately with narrower charge weight spreads. So unless I know it's for running in a semi auto, I would be cautious.

 

[Evil Otto]

I second the recommendation for adding buffer mass. A heavier buffer will help delay extraction ever-so-slightly, in many cases just long enough to allow the brass to shrink down.

Most .308 semi autos run better with the heaviest buffer possible for a given configuration. Unfortunately most manufacturers insist on putting the cheapest buffer/spring combo into their weapons.

 

[cohunt]

I run a 1 oz buffer, std spring, and titanium bcg in my midlength 16" 308--- I tune with the gas block.

I think the ammo is too hot for the op's gun---- do you have any velocity data on any of the ammo you shot?

 

[Rosebud]

Did I read the part about the 7.62x54 being compatible with a.308 which is 7.62 x51. The 54 mm case is made for a mosin nagant a rimmed case , the the .308 is a rebated rim case. Someone explain what is said here, I'm somewhat confused about this.

 

[cohunt]

Did I read the part about the 7.62x54 being compatible with a.308 which is 7.62 x51. The 54 mm case is made for a mosin nagant a rimmed case , the the .308 is a rebated rim case. Someone explain what is said here, I'm somewhat confused about this.

Pretty sure he meant 7.62 x 51 -- the rimmed case wouldn't even fit in a 308, would it?

The ammo he showed is 7.62x51 and it says its made for semi auto rifles to Nato/mil specs

 

[Evil Otto]

Did I read the part about the 7.62x54 being compatible with a.308 which is 7.62 x51. The 54 mm case is made for a mosin nagant a rimmed case , the the .308 is a rebated rim case. Someone explain what is said here, I'm somewhat confused about this.

I'm almost positive that was a typo. He meant 7.62x51.

 

[dragon798]

Hi all,
I have a Savage MSR 10 Long Range (AR-10 style platform) chambered in .308 which I have been very pleased with. I am not a handloader but I did decide to try out a wide variety of ammo in the rifle to see what it liked best. I settled on IMI systems 7.62mm Semi-Auto Match 168 gr Razor Core because it seemed to group the best. It was always my understanding that 7.62 x 54 mm was safe to fire in a .308 rifle.

I have been running this ammo for the past few years and have experienced some issues. Some rounds blow the primers out and I find the separated case and primer on the ground after the round has cycled. Some rounds the rifle rips the rim off the cartridge during the extraction and ejection. Neither of these occurances were consistent, but both would occur at least once every range trip. I thought the issue may have been related to overpressure in the gas system so I turned the adjustable gas block down to one notch above "not cycling". This has not fixed the issue.

I ran FGMM 168 gr .308 through the rifle last weekend and did not experience a single issue.

I have attached a picture of the IMI with ripped cartridge rim, and a picture of the FGMM from last weekend. Unfortunately I forgot to save any cartridges with blown primers to post. None of the rounds show any bulging or disfigurement aside from what has been noted.

I am looking for any information that will help me understand what the issue with the IMI ammo is and if it can be mitigated or if I need to stick to other ammo in the rifle.

Thanks for your help!View attachment 479547

I would absolutely not use that ammo. It appears you are getting blow by past the primer, indicating a potentially dangerous pressure situation. Contact IMI, and see what they say. Either way, don't use that ammo, a stronger buffer is not going to help. The other possibility is you have excessive headspace, although the FGMM doesn't show any signs. The other option is try the IMI in another gun, and see if you get same signs..

 

[dragon798]

Did I read the part about the 7.62x54 being compatible with a.308 which is 7.62 x51. The 54 mm case is made for a mosin nagant a rimmed case , the the .308 is a rebated rim case. Someone explain what is said here, I'm somewhat confused about this.

308 and 7.62x51 are rimless not rebated

 

[JAYgs8163]

Run the FGMM and done. Feed it what it likes.