Looking for insight on blown primers

[fmuguira]

Blown primers = most likely, too hot/pressure, firing pin issues, headspace issues.

 

[Rosebud]

308 and 7.62x51 are rimless not rebated

Correct, my bad.

308 and 7.62x51 are rimless not reb

 

[Mike Matteson]

!. It clearly show that the powder load is to hot. 2. Not all primers are not flatten like I would feel they should be for overpressures, but they are sure creatored. 3. With all the other problem he is having I wouldn't use that type of ammo in that rifle.

 

[tmoxley]

To answer a couple questions:
- My typing of 7.62x54mm was in error and I was referring to 7.62x51mm.
-I would assume the powder burn rate is appropriate for a semi-auto due to the fact these are sold by IMI specifically as "Semi-Auto Match"
-I checked the headspace of the rifle using Clymer .308 go/no-go gauges and the bolt locked on the go and did not close on the no-go.
-I have used a chronograph on the rifle using the IMI ammo and came out with an average velocity of 2573 fps at the muzzle, which seems appropriate for a 20 inch barrel.

 

[tmoxley]

One other issue I had with the rifle is that last month while shooting the IMI ammo the rifle jammed with the bolt being stuck halfway into the buffer tube. I disassembled the rifle and discovered that the firing pin retaining pin was partially out of the bolt and the head of the pin had become wedged between the bolt and the upper reciever wall. There was only superficial damage to the bolt and the reciever wall. I have since replaced the pin. I'm assuming that this supports the theory that the IMI ammo is causing overpressure. I searched multiple and found no other reports of this occurring to a Savage MSR 10 so I'm assuming the issue is not with the gun itself.

The only reason I have continued to use this ammo is due to the fact it groups tightly and I have several hundred rounds of it left. Based upon the responses here I will stop using it and switch to the FGMM.

Thanks to everyone who responded! Your extensive knowledge and willingness to help is always appreciated!

 

[tmoxley]

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.

Sorry, I forgot to mention. I do not see any distinguishable ejector marks on either the IMI or FGMM shells.

 

[MagnumManiac]

It is your gas block and lock time causing your issue…

Cheers.

 

[dragon798]

To answer a couple questions:
- My typing of 7.62x54mm was in error and I was referring to 7.62x51mm.
-I would assume the powder burn rate is appropriate for a semi-auto due to the fact these are sold by IMI specifically as "Semi-Auto Match"
-I checked the headspace of the rifle using Clymer .308 go/no-go gauges and the bolt locked on the go and did not close on the no-go.
-I have used a chronograph on the rifle using the IMI ammo and came out with an average velocity of 2573 fps at the muzzle, which seems appropriate for a 20 inch barrel.

I am very curious as to the actual cause of your issues with the IMI ammo. I wonder if you have the ability to accurately take some measurements of the IMI and FGMM brass. I would like to know if there is a difference, particularly in cartridge base to shoulder distance. The "headspace" measurement of both cartridges. I'm also curious about measurements at the rim, the groove and the base of the cartridge. I feel that you may find a difference in the IMI ammo. It is possible that the IMI is slightly out of spec. Maybe the rim is just large enough that the extractor isn't completely snapping over, holding the bolt back a couple thousandths and creating a headspace issue. Maybe the cartridge base to datum line is short, again creating a headspace issue. You eliminated any headspace issue with the gun, what about the ammo? SAAMI dimensions are easy to find and you have a cartridge that doesn't have issues.

 

[joelpend]

Generally gas guns will not take the same pressures as bolt rifles without issues. While you may be able to add weight to bolt and or buffer and reduce gas and keep it from blowing primers I personally would just shoot ammo that works. AR-10 platform gas guns can be frustrating at times.

 

[marksman1941]

Sell off the IMI and run higher quality ammo. Usually that stuff works fine but you either got a bad lot, or your rifle is sensitive. If you have a buddy with a similar rifle it would be worth testing that ammo in something else. But overall, just swap to better ammo and save your rifle from more problems.

 

[T3ninja]

Im doubting your rifle has a high pressure bolt from the looks of the primer cratering… so I'd go with:

1.High pressure bolt, 2.heaviest buffer you can get ahold of and a 3.heavier recoil spring. Then fire a couple rounds. If you still have problems, stick with the FGMM, and you'll have a soft shooting rifle anyway.

 

[Doom2]

There is absolutely no question that you are seeing pressure issues. One thing not mentioned is ammunition condition, in this case temperature. If left in the sun on a hot day some powders with go haywire. The FGMM will likely not do that as it's a mildly loaded cartridge. The IMI is simply loaded but the powder may be the issue.

 

[milo-2]

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

What? Looks like the Federal blew out also.

 

[milo-2]

I'm going to go against the grain here and say your chamber is oversized at the case base. Simple check with a caliper of new brass to fired to determine fired brass expansion may solve this.