Looking for insight on blown primers

[C-130 Dude]

I think someone needs to start a thread on the difference between rimmed, rimless and rebated.

Yeah, I goofed. I said rebated. Should have been rimmed. Sorry.

 

[martyj]

Could be machine gun ammo.
Hirtenberger ammo is hot like that in 7.62x51.

 

[dragon798]

Here are the measurements requested:

Cartridge base to shoulder distance (I cannot gaurantee the new ammo is from the same lot as the fired ammo):
Cartridge base diameter:
-IMI new: 0.466
-IMI fired: 0.471
-FGMM new: 0.468
-FGMM fired: 0.470

Overall case length:
-IMI fired: 2.012
-FGMM fired: 2.020

Groove diameter:
IMI fired: 0.406
-FGMM fired: 0.405

Rim diameter:
IMI fired: 0.470
-FGMM fired: 0.469

Unfortunately I only have a set of calipers, so I don't believe I have the tools to measure the distance from the shoulder to the base accurately. Also I was unsure if you were looking for diameter of the rim and groove, or length of the rim and groove. I listed diameters because that's the measurements I could accurately take.
For what it's worth, when I place the brass side by side on a level surface there is no perceptable difference between the two in terms of where the shoulder starts.

You definitely have some differences between the 2 brands. IMI is growing g a lot at the base. Check the actual thickness of the rim if possible and I would suggest firing a few rounds in another gun. Maybe even a bolt gun where you can feel for lock-up

 

[emp1953]

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.

The Semi-Auto on the IMI box suggests that the OP is using it as intended. It is also labeled as Match grade ammo so I'm seriously doubting the machine gun possibility. I have never had a primer exit a case. I've seen where a reloader has used european primers, then switched back to american primers (in the same cases) and have the primers start to back out. But never exit the case. Perhaps there is a new dynamic involved with the semi-auto action that adds to pressure since my experience is with bolt and single shot rifles. I agree that the ripped case head is due to the extraction starting before the case is done expanding. What if carbon buildup is holding the case a microsecond longer thus the case head damage.

 

[emp1953]

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

Did you buy this IMI ammo in one large batch, like 1000 or 5000 bulk? I'm wondering if its all from the same lot number or not.
Which cases did the primers exit from, the IMI or the Federal? The cratering of the primer could just be the firing pin hole in your bolt is a little oversized. The primers in both cases are not flattened. Nice roll left out at the edges. Last question, you are sure this is not remanufactured ammo?

 

[mtwarych]

Adding weight to the buffer doesn't fix the problem of overgassing.

The AR-10 commercially built rifles are commonly overgassed because the industry knows people will try to shoot the cheapest junk ammuniton through the rifle so the barrel makers drill huge gasports to ensure cycling.

The proper way to fix this rifle is to order a new, undrilled AR-10 barrel, take the whole rifle to a gunsmith and have the gunsmith start with the smallest gasport and see if the rifle fully functions. If the rifle won't function, remove the barrel and drill the next smallest gasport. If that size of gasport doesn't function fully, remove the barrel, drill the next smallest gasport and continue the process. It's a major waste of time and money and ensures that you are stuck shooting that 1 type of round the barrel was specifically tuned to or you will start seeing the same problems again and as your gasport is erroded, overgassing problems will slowley re-appear.

OR, you can add and subtract weight from your recoil system until the rifle functions correctly. This is the choice I made when my brand new DPMS GII Hunter was so overgassed that I had ripped rims, bent brass and my firing pin retaining pin to bend in half due to the speed the bolt is travelling. I was shooting FGMM and getting the same problems the OP is seeing with his IMI.

OP, the IMI ammunition is completely safe in your firearm. If I find some IMI around here, I would be happy to purchase some, fire it out of my rifle with and without a heavy buffer and then post pictures of the cases. The IMI is not overloaded, your bolt is opening under pressure and causing what you are seeing.