Extractor Marks and Bent Rims on 308

[DMW1116]

I was trying a load I made right after getting my AR10 and noticed the extractor claw chewing on my brass more than normal and bending the case rims. This load has not shown this before.

The load is CCI #34 primer in 308 brass, 168 grain Amax, and 46 grains of CFE 223. Hornadys max is 47.2. Is shot 2 of the same load but from a different batch and didn’t have bent rims but did get some marks from the extractor.

 

[troy fairweather]

What brass

 

[DMW1116]

Mixture of Winchester and PPU 308 Winchester brass.

 

[Cheesemaker]

I was trying a load I made right after getting my AR10 and noticed the extractor claw chewing on my brass more than normal and bending the case rims. This load has not shown this before.

The load is CCI #34 primer in 308 brass, 168 grain Amax, and 46 grains of CFE 223. Hornadys max is 47.2. Is shot 2 of the same load but from a different batch and didn’t have bent rims but did get some marks from the extractor.

So, new rifle - right?
Some ARs will tear up brass - maybe this is what you are seeing? Chamber clean? Brass clean?

 

[DMW1116]

No this is an old rifle. I’ve shot probably 200 rounds over the last couple years. Mostly my own loads but it really likes the Winchester M80 FMJ.

 

[Blue68f100]

Is the rim being beat in or pulled back? What is your ejection pattern ? 3:00-4:30 Do you have an adjustable gas block.

 

[DMW1116]

Pulled back. I have an adjustable gas block. On the current setting (for my 175 grain SMK load) brass is going at 2 o’clock.

 

[troy fairweather]

Over gassed

 

[The Glockodile]

Over gassed

Prolly...

 

[DMW1116]

So that’s what the adjustable gas block is for I presume? There aren’t any other pressure signs on these loads. The primer faces are slightly flattened but the corners are still rounded (CCI #34) and no ejector smears on the cases.

CFE 223 seems to do that in every load I try with it. It’s my go to powder for Mock 262 with 77 grain SMKs in my 223 rifles. Accuracy is great but I’m sure mine are slower than the real Mk 262 loads.

I was hoping to avoid adjusting the gas block for different loads but there seems to be a big enough difference between the powders to justify it.

 

[dh1633pm]

I took my buffer apart and filled two of the hollow weights with lead. It made things better. An adjustable gas block probably better. Also used a heavier spring.

 

[DMW1116]

If that’s all at issue it’s going back to be fixed. How much heavier do springs get? This one already makes my 223/556 buffer springs look like chicken wire.

 

[AJC1]

Gas guns normally like a certain powder speed. An adjustable gas block let's you play outside that nominal range. Cfe is probly to slow for your non adjustable system.

 

[DMW1116]

I took the rifle to the range yesterday and brought some PMC 147 grain FMJ and Winchester M80 149 grain 7.62 Nato along with the offending hand loads.
I started by firing the factory ammo. These showed some ejector marks on the case head as well as some extractor marks on the rim, but nothing bent. Ejection with these was also in the 2 to 2:30 range, slightly forward. I turned the gas block down until these wouldn't cycle, then turned it back up one click. These looked noticeably better and were ejecting at about 4 o'clock.

On the notion that my loads were a little hotter still, I turned the block back down one click and tried the offending hand load. The marks were mostly gone, just some extractor marks on the rim. No ejector marks and no bent rim. Ejection was about 4 o'clock, but the bolt didn't pick up the next round. I gave it one click more gas on the block and everything worked great. Brass is a little chewed, but not damaged. Ejection is still in the 4 o'clock range, and the gun cycled and locked open on the last round. I probably will still redo this load with a different powder. I'd like to be able to leave the block alone, but that may not be possible. On the other hand, the hand load and Winchester load shot under 0.3" groups at 50 yards. The PMC load does not play well with this gun, and shoots about 2.5 MOA typically. It did better this time but I doubt I could keep it under 1" at 50 yards if I shot a few more.

 

[Slamfire]

Looked at a burn rate chart and CFE223 is ranked slower than IMR 4064. Burn rate charts are crude approximations and don't take into account pressure drop, which is why AA2700 (a ball powder) is suitable for Garands. AA2700 started when Accurate Arms purchased surplus WC852, which was used in loading Garand ammunition. Anyway the old advice for Garands and M1a's was not to use anything faster than IMR 3031 and nothing slower than IMR 4064. No doubt the AR10 gas system is timed to service rifle standards and the action is opening when residual breech pressure is too high.

Whether or not that is the actual cause, cut your loads. Cut your load by a half grain until such time, the problems go away.

 

[Charlie98]

So that’s what the adjustable gas block is for I presume? There aren’t any other pressure signs on these loads.

The adjustment of the gas block has nothing to do with your chamber pressures... by the time the bullet has passed the gas port, and the gas reaches it, everything is well on it's way. What the adjustable gas block does is allow you to better meter the gas that cycles the action.... how fast the bolt unlocks, etc. It can further be tuned by a heavier or lighter buffer; usually an adjustment on the gas block will be enough, but sometimes it's better to change both the gas, and the buffer. It's not really a defect, per se, simply something else that can be tuned. You spend gobs of time tuning your handloads, why not a little time spent on the rifle? I would close the gas valve down a bit, and then work my way back to see what effects it has on ejection and brass... or you can go up to the next heavier buffer and likely call it a day.

 

[thump_rrr]

I took the rifle to the range yesterday and brought some PMC 147 grain FMJ and Winchester M80 149 grain 7.62 Nato along with the offending hand loads.
I started by firing the factory ammo. These showed some ejector marks on the case head as well as some extractor marks on the rim, but nothing bent. Ejection with these was also in the 2 to 2:30 range, slightly forward. I turned the gas block down until these wouldn't cycle, then turned it back up one click. These looked noticeably better and were ejecting at about 4 o'clock.

On the notion that my loads were a little hotter still, I turned the block back down one click and tried the offending hand load. The marks were mostly gone, just some extractor marks on the rim. No ejector marks and no bent rim. Ejection was about 4 o'clock, but the bolt didn't pick up the next round. I gave it one click more gas on the block and everything worked great. Brass is a little chewed, but not damaged. Ejection is still in the 4 o'clock range, and the gun cycled and locked open on the last round. I probably will still redo this load with a different powder. I'd like to be able to leave the block alone, but that may not be possible. On the other hand, the hand load and Winchester load shot under 0.3" groups at 50 yards. The PMC load does not play well with this gun, and shoots about 2.5 MOA typically. It did better this time but I doubt I could keep it under 1" at 50 yards if I shot a few more.

Take a few pieces of Winchester brass and a couple of the PPU brass and weigh them.
Mark the weight on each case.
Fill them with water exactly flush to the top.
Weigh them again and calculate the water weight.

Winchester usually has the highest water capacity.
This means more case volume.
Any military brass will usually have the lowest water capacity lowest volume.

The lower the case volume the higher the pressure when using the same load.

PPU usually has similar case capacity to LC.

 

[DMW1116]

I'll probably abandon this load altogether. I have 100 more Amax bullets, so I have enough to try another. Shooters World AR Plus has data for Amax bullets, so that's likely my next stop.

 

[DMW1116]

The adjustment of the gas block has nothing to do with your chamber pressures... by the time the bullet has passed the gas port, and the gas reaches it, everything is well on it's way. What the adjustable gas block does is allow you to better meter the gas that cycles the action.... how fast the bolt unlocks, etc. It can further be tuned by a heavier or lighter buffer; usually an adjustment on the gas block will be enough, but sometimes it's better to change both the gas, and the buffer. It's not really a defect, per se, simply something else that can be tuned. You spend gobs of time tuning your handloads, why not a little time spent on the rifle? I would close the gas valve down a bit, and then work my way back to see what effects it has on ejection and brass... or you can go up to the next heavier buffer and likely call it a day.

That's basically what I did yesterday was close the gas down until the load in question didn't cycle, then open it up one click. I do need to do a more careful observation of the brass, but the quick look at the range showed no sign of the bent rims or ejector marks found with more gas.

 

[AJC1]

Looked at a burn rate chart and CFE223 is ranked slower than IMR 4064. Burn rate charts are crude approximations and don't take into account pressure drop, which is why AA2700 (a ball powder) is suitable for Garands. AA2700 started when Accurate Arms purchased surplus WC852, which was used in loading Garand ammunition. Anyway the old advice for Garands and M1a's was not to use anything faster than IMR 3031 and nothing slower than IMR 4064. No doubt the AR10 gas system is timed to service rifle standards and the action is opening when residual breech pressure is too high.

Whether or not that is the actual cause, cut your loads. Cut your load by a half grain until such time, the problems go away.

Some powders produce a double pressure peak sometimes with the second peak being higher. That has to be hell dialing in a semiautomatic.

 

[DMW1116]

Yea if this load groups the same or just a little better from a bolt gun, it would be a real winner. Time to look for one I guess.

 

[DMW1116]

FWIW the Winchester M80 is marked LC.

After a more careful observation of the brass with a flashlight and my spectacles, it appears turning the gas block down remedied the issues. There are some marks from the extractor but they’re tough to identify without good light.

There are slight ejector smears on some of the factory ammo but none were evident in the hand loads. The faces of the primers were flattened on the Winchester factory loads and the hand loads but not on the PMC loads. The reloaded brass was head-stamped PPU, Winchester, and Hornady.

 

[Kaldor]

FWIW the Winchester M80 is marked LC.

After a more careful observation of the brass with a flashlight and my spectacles, it appears turning the gas block down remedied the issues. There are some marks from the extractor but they’re tough to identify without good light.

There are slight ejector smears on some of the factory ammo but none were evident in the hand loads. The faces of the primers were flattened on the Winchester factory loads and the hand loads but not on the PMC loads. The reloaded brass was head-stamped PPU, Winchester, and Hornady.

Gas block tuning on an AR-10 is by far the most effective way of keeping your brass from being chewed up. Ive fixed a few guns for people just by swapping in an adjustable gas block and getting it tuned correctly. Heavier springs and buffers help, but are truly only a bandaid, as they increase felt recoil, and have minimal effect on loads with higher port pressure. A little less port pressure slows the gun slightly, allowing for the action to remain closed longer, and have less violent extraction which will bend rims, and put extractor marks on brass. The key to tuning the gas block for me is to find the heaviest bullet I will shoot from that rifle, and tune the gun based on that load as it will have the lowest port pressure. Generally that tuning point will work fine for loads that create higher port pressure as well. So in my case I tuned for a 175 SMK with Varget, but all my 168 and 150 loads shoot just fine with that setting. The only time Ive ever changed it slightly is when tuning it to run a suppressor, and even that was just closing the port down a single click.

And the issues you are seeing with CFE 223 are pretty normal. I find it OK for loads where I am not looking for accuracy, keeping the powder charge down, but its mostly blaster ammo. It tends to pressure spike bad in 308 when you start getting within a grain for max for me so I stopped using it.

 

[DMW1116]

I’m a little over a grain from max. It gives me great accuracy in 223 and this load in 308 was good too. However it’s temperamental enough I’ll try something else. I only have 1 magazine of these loaded, so now I can shoot them without tearing anything up. I just won’t be making any more.

 

[DMW1116]

I have 3 IMR 4064 loads that don’t have these issues. 2/3 shot less than 1 MOA during testing. I couldn’t get IMR 4064 to shoot well with these Amax bullets though. I have 2 more powders tk try before I go on the hunt for either different bullets or IMR 4895.