Chamber Scratching Brass

[Hawk in WY]

Rifle: Armalite 10T .243 Stainless 24 inch barrel.
Brass: Norma mostly but fewer neck scratches and splits with Winchester.

Sized cases are fine. Cleaned dies anyway.

Fired cases showing neck splits with fairly hot load and Norma twice fired brass.

Shoulders bumped back 3 to 4 thou.

Hot loads 100% thin neck splits.

Milder loads mostly Winchester brass show some neck scratches and rare neck splits.

Seems to me the chamber is scratching the brass leading to neck splits with hot loads.

One to two thousand rounds through this rifle with problems only recently.

How can I polish the neck area of the chamber or is there some other cause?

Thank you for any advice or comments.

 

[MtnCreek]

I'd be hesitant to try to polish the neck area for fear that I might affect the leade. It would be best if you could scope it to see what, if anything, is in the chamber. It could be a strip of brass or carbon in there. I'd clean it well. Maybe take a loop type jag with a tight patch and turn it to see if that fixes it. If not, I'd be looking for someone with a scope.

 

[Hawk in WY]

Good advice.

Off to find a bore scope.

 

[MarinePMI]

This sounds more like a generous necked chamber, causing you to over work the neck when sizing (causing the split necks). If that is the case, it is what it is. Annealing the case necks may resolve the split necks, but it's no guarantee.

 

[L.R.Novice]

Check the barrel extension particularly on extraction. The ejector can jam the brass into the sharp sides on the barrel extension lugs gouging the necks leading to splits on the next firing.

Edited! Barrel extension not muzzle

 

[Hawk in WY]

Check the barrel extension particularly on extraction. The ejector can jam the brass into the sharp sides on the barrel extension lugs gouging the necks leading to splits on the next firing.

Edited! Barrel extension not muzzle

This is getting complicated.

The problem started recently after maybe 1,000 flawless rounds.

I see no marks on the brass before the firing where it splits.

The only way I know to check the barrel extension is to remove the barrel. Ugh.

Would only want to do this if I was fairly certain the barrel extension is the problem.

Anything else to look at?

Thank you for the lead.

 

[FatBoy]

at face value, I agree it sounds like this is a combination of overworked brass and hot loads. If you shot factory loads for most of the first 1000, then sized and loaded them hotter than factory the necks are likely splitting during firing and as the case is more violently pulled from the chamber it is being dragged across a sharp portion of the barrel extension.

When you chamber a live round, or just a sized case, and extract it by hand does it scratch?

Have you recently changed mags?

 

[Hawk in WY]

When you chamber a live round, or just a sized case, and extract it by hand does it scratch?

Nope. Important Edit: A sized case ejects with one distinct scratch.

Your diagnosis sounds correct. I am not over pressure but close to SAAMI Max according to QuickLOAD and my chronograph.

Anything I can do other than going to a milder load?

 

[Velocity Chad]

Is the chamber clean with no carbon ring? Time to invest in a borescope.

 

[FatBoy]

can you post some good pics of the scratched but not split, as well a couple split cases?

 

[MarinePMI]

I'm not sure a bore scope is warranted, or needed. I think Fatboy nailed it.

 

[Hawk in WY]

I agree.

Posting pictures is beyond my skill level.

Scratches and splits look just like you would imagine.

Scratches are above the shoulder to below the case mouth.

Splits are from above shoulder through to case mouth. Usually two; 30 to 45 degrees or so apart.

I do not see scratches on cases before I load them.

I do not believe this is a matter of a scratch turning into a split on the next loading.

Either scratches or splits not scratches before the split. Some brass and some loads split. Some scratch.

I agree Fatboy's diagnosis sounds correct.

So the solution is?

 

[MarinePMI]

Anneal your brass and move forward. If scratches become a problem, then work on a milder load. It's just the nature of semi-autos; cycle timing can (and usually do) affect what loads you can run.

 

[Hawk in WY]

Important Correction

An unloaded sized case ejects with a scratch.

Problem is a combination of the barrel extension as Fatboy identified and possibly cycle timing as MarinePMI identified.

Great job guys.

Now what to do about this?

Biggest problems are with 75 gr V-Max loaded below SAAMI max but hot.

AR-10 T's are known to only cycle properly with 80 to 100 gr bullets. Armalite has issued a Notice to this effect.

I will try 87 grain V-Max unless anyone has a better idea.

 

[FatBoy]

If you can anneal it will help but If the neck area of the chamber is that big, you're going to be lucky to get firing #3.

I would work up a milder load and start looking for a replacement barrel that has a chamber more suitable to reloading. I'm sure that chamber is fine for fire-and-forget brass, and it will probably digest any case you put through it, but unless you plan on buying loaded ammo or new brass every firing, most likely the barrel needs to go.

 

[L.R.Novice]

I’m sticking with the barrel extension as the cause. Scratches and other imperfections can cause issues. If you are scratching rounds by hand ejecting then they are likely getting really gouged when live firing. Check a couple after you fire them. Also get some cerrosafe and make a cast of the chamber. You can see imperfections and measure the size to verify that is not the problem.

 

[Hawk in WY]

You guys are the best.

I don't like the answer but I believe you have found the source of the problem.

The answer seems to be don't expect to reload brass fired in this barrel.

My one niggling concern is the rifle handled well over 1,000 rounds which I reloaded successfully until just recently.

I will try the cerrosafe and see what I find.

Thank you all for your help.