Case heads bent on extraction?

[hounddawg]

I have noticed that I have problems with concentricity on ammo that has been fired in my AR's. I use a Hornady tool for checking which rotates the case on the head and the meplat. I do not have any problems with cases that are fired in bolt guns so I am guessing the heads are getting bent on extraction in my gas guns.

Opinions ? and does anyone know of a link to the Army study on accuracy with bent case heads Bart sometimes mentions?

 

[lugerstew]

You sure that the rims aren't slightly bent? I found 3 GHL 308 brass yesterday with bent rims, so I couldn't get a steady reading on the headspacing.
I set them down on a flat surface and thats when I noticed that they rocked slightly back and forth. They were fired from an AR-10

 

[44 AMP]

There is no semi auto I am aware of that is designed to keep the fired cases in the best possible condition. The only consideration used is that the fired cases leave the gun reliably and no consideration is given to the condition of the brass, ONLY that it is gone.

Ever see what comes out of an H&K 91? Compared to that the M1A is reloader friendly!

Also remember that unless its a match grade purpose built rifle (and you are paying for that) there's no need for the mechanism to be "perfect" only "good enough to work well".

SO, yes, its possible your bolt face isn't perfectly square to your barrel,

Now, here's the question, for what you want to do, does it matter??

 

[RC20]

I see it as a curiosity thing like flash hole size.

I have seen the H&K 91 brass (or something similar, forget the cartridge, FNL?

There is some need for easier extraction as you don't want to rip your extractor off.

 

[Shadow9mm]

Do you have a normal or enhanced power extractor spring setup? What gas length system are your running? What buffer weight are you running. How far is your brass getting thrown from the gun? how many rounds through the gun, or rather, have you ever checked or replaced your buffer spring?

I'm guessing its an over gassed system just being rough on the brass. If that is the case you could tune things to shoot a bit softer and hopefully not tear your brass up.

 

[hounddawg]

@lugerstew - yep bent rims is what I meant
@shadow - AR -15 20 inch barrels on both the 6.5 and the .223, less than 1K rounds. AR10 18 inch barrel less than 750 rounds. No mods. No idea how far the brass gets thrown with either since I use a brass catcher on both

Both rifles are mostly just used for plinking but I am going to give the NRA mid range prone competition a try. I get 10 round groups about 1 - 2 MOA out to 800 (.308 and 6.5 Grendel) consistently which should make me competitive but would just like to get the ammo as accurate as possible.

 

[Unclenick]

Harold Vaughn proved by measurement that any uneven contact the case head has when pressure forces it back against the bolt face introduces off-axis recoil moments that do affect group size. Board member Bart B. has pointed out military match team armorers determined experimentally that new brass outshot reloaded brass that had been ejected from Garands and M14's due to their bolt faces not being trued to the chamber axis or due to extractors bending the rims.

A stiffer recoil spring may help, but an adjustable gas block is the most positive approach to reducing how energetic gas-operated extraction is in the ARs. The drawback is you may not find one setting that is best for every bullet weight you load, but you can almost certainly find something better for all of them and that is less prone to bending a rim.

To salvage bent rims, try making sure the bent portion of the rim is supported by your shell holder during resizing to tend to bend it back. The flat surface the Forster Co-ax press provides is a good one for this. You can also mount a case in the Wilson case trimmer's shell holder and turn it backward and use the trimming cutter to shave off anything uneven. The rims can be rolled back to profile in a lathe using a smooth wheel in a knurling tool and moving it against the case head with the case held in a Wilson trimmer holder chucked in the lathe.

 

[FrankenMauser]

Could be extraction.
Could be an out of square bolt face.
Could be a bad shell holder.
Could probably be a few more things.

I had to fix a bunch of .458 SOCOM rims a few years back, after pulling bullets under extreme neck tension.
I ground a mandrel out of a Lee .45-70 trimmer pilot, which let me put pressure only on the web of the case head, outside of the primer pocket, but aligned the tool with the case mouth. Although I intended to do the reforming in my reloading press with some kludged die parts, I quickly found that it was far easier and far faster to just whack the mandrel with a hammer, with the case sitting on the flat of the anvil on my vise.
Since fitting in the chamber and shell holder again were the only requirements, I didn't care if they were slightly out of square.

 

[Shadow9mm]

@lugerstew - yep bent rims is what I meant
@shadow - AR -15 20 inch barrels on both the 6.5 and the .223, less than 1K rounds. AR10 18 inch barrel less than 750 rounds. No mods. No idea how far the brass gets thrown with either since I use a brass catcher on both

Both rifles are mostly just used for plinking but I am going to give the NRA mid range prone competition a try. I get 10 round groups about 1 - 2 MOA out to 800 (.308 and 6.5 Grendel) consistently which should make me competitive but would just like to get the ammo as accurate as possible.

There are a number of things it could be as. Most common though, in my experience, is an over gassed system opening too early and yanking the brass out.

You could swap in an adjustable gas block, but that can get expensive and requires you to take your gun apart.

Personally I would try a H or H2 buffer to slow down the action opening a smidge, its an easy swap. The advice I received on buffers was, run the heaviest one the gun will lock open on.

 

[jetinteriorguy]

A few years ago I picked up some 7.62x39 brass so I could shoot my AK more without depleting my factory stash. Now talk about bent and messed up rims. So I put together an AR so I could shoot more without destroying the brass. It took an H2 buffer to tame it down to a reasonable level, it still bends rims once in a while but at least it’s a tolerable level, I’m getting a nice predictable 1.25-1.5 MOA group from this rifle and I’m more than happy with that.

 

[hounddawg]

I’m getting a nice predictable 1.25-1.5 MOA group from this rifle and I’m more than happy with that.

yeah I just need to keep reminding myself that this is not F class and the ten ring is 2 MOA. Only a few guys shooting it at my club and most are using .223's so a mid 190 score would take first everytime

 

[kilotanker22]

I agree with Unclenick. I had a similar problem with an AR rifle in 5.56. An adjustable gas block helped tremendously. I think it has to do with too much gas coming through the gas tube too quickly. Essentially, my theory is that extraction is beginning before the bullet exits the bore and pressure drops. That could explain the rims getting bent as they are extracted, since the case is still expanded against the wall of the chamber for a few milliseconds when extraction begins.

I might be way off base here, but it makes sense to me and an adjustable gas block solved my problem. I experienced this problem with a carbine length gas system. I have never experienced this problem with a mid-length or rifle length gas system. I presume, since the gas travels further to get to the bolt carrier with a longer gas system, the effect is less than with shorter gas systems.

 

[hounddawg]

will be reloading the ones I shot yesterday in the Grendel tomorrow. I still had 10 loaded in the virgin cases and runout was .0005 to .0025. If the ones I load tomorrow are out of kilter I will order a new adjustable gas block