Gunsmithing AR-10 Problem with headspace

[crumpmd]

OK. I did not chamber or set headspace on this rifle. It was done at Krieger by them. I got it all put together and test fired it. It shoots sub-moa but the brass looks bad.
The brass is heavy with soot all the way from the case mouth to the rim. And the neck is squashed down so the case is shorter overall.

a new case

Once fired

And the neck is different now for sure , a new and once fired side by

So what does it look like? Excessive headspace? Would that have led to incomplete seal on the case neck + shoulder?

 

[wnroscoe]

Re: AR-10 Problem with headspace

Being .002" shorter isnt alarming to me as a new unfired case expands to a larger diameter after being fired. This alone would shrink a case slightly.

What load were you shooting?

 

[427Cobra]

Re: AR-10 Problem with headspace

I think a Tubb flat wire buffer spring and a JP adjustable gas block will cure your problems.

 

[UNCIVILIZED]

Re: AR-10 Problem with headspace

Likely it's just how the chamber was cut, & that it varies from your chosen brand of ammo's specs by a few thousandths. Given what you state, the shoulder of the chamber is where most of the variance is.
+1 on the post firing brass size thing. And it's not uncommon to have case mouths get dinged by ejection. Some gas guns are notorious for it.

Your cases are just stretching to fit the chamber when they're fired, & then spring/shrink back a miniscule amount post firing. Such is common with most metals.

 

[DRAM40A1]

Re: AR-10 Problem with headspace

I would guess not enough pressure to seal the chamber. These look like they were fired through a suppressor?????

 

[gibar]

Re: AR-10 Problem with headspace

Another possible cause maybe your loads are too light not letting the case seal to the chamber.

 

[crumpmd]

Re: AR-10 Problem with headspace

All excellent insights.
I am not worried about the case month ding.
These were 139 grain Scenars loaded with 35 grains RL 15. That is 2 grains less than the full power load I run in my bolt gun. And they were shot will a Silencer.

So shooting a hotter load may seal the neck and shoulder?
I will give it a try tomorrow.

 

[LoneWolfUSMC]

Re: AR-10 Problem with headspace

If they were shot with the can on, that accounts for the soot on the cases.

 

[crumpmd]

Re: AR-10 Problem with headspace

Never knew that. Only shot my can on a bolt gun Til today.
I think I have to load Some hotter ammo and shoot no silencer.

 

[palma]

Re: AR-10 Problem with headspace

Isn't AR brass filthy anyway? Case length will probably be longer than when you started after a resizing. Gun shoots well. What's the problem?

 

[Hateca]

Re: AR-10 Problem with headspace

Be very careful developing a load for the .308 AR type gas gun if you also shoot with a suppressor. You can have a good load with the can off and get a high pressure issues or high bolt speed once you put that can on.

It is possible that the load will work without the can and not with the can installed.

 

[exhogflyer]

Re: AR-10 Problem with headspace

I have noted in all my AR style guns sooty brass after shooting, I don't think that in itself is a problem.Remember that gas tube?Brings all kinds of nasty shit back to the bolt/chamber area..

 

[crumpmd]

Re: AR-10 Problem with headspace

The soot is not the problem. It is a symptom. the case is 3 to 6 thou shorter. The angles on the shoulders are different. Maybe that is how it is supposed to work but I have never seen a case shorten when fired and sized. I'll talk to Krieger tomorrow about their chamber reamer. I am thinking I ought to recut the chamber.

By the way the cases look about the same with or without the silencer.

 

[JRose]

Re: AR-10 Problem with headspace

I've given up on running a can on my AR10, just to aggravating.

 

[737shark]

Re: AR-10 Problem with headspace

what do the necks measure after fired?

How wide is the case after fired?

What kind of brass are you using?

 

[crumpmd]

Re: AR-10 Problem with headspace

Lapua brass, about a buck each.
I'll get back to you on the neck and case body measurement. I don't recall measuring them. The length was just obviously shorter so I measured it. I would not have believed I could see 5/1000 inch difference but when I lined it up....