How does an AR-10 headspace?

[problemchild]

The reason I ask is because I got a new bolt (not bcg) and the rifle is jamming the rounds into the chamber. LC brass works OK but FGMM sticks a tiny bit in the chamber when behind the bolt. SBS brass jams in so tight you have to beat it out when inserted by the bolt. Yet the same brass, or live rounds, will drop in and out of the chamber with ease and not stick when dropped in by hand. Its only when the bolt feeds the round into the chamber does it stick in the chamber. Why would changing a bolt only cause this? This brass worked with the old bolt (old bolt is gone).

[Jpach]

What gun is this for?

[problemchild]

What gun is this for?

An ar-10 type DI rifle.

[Jpach]

Oh ok, I was just asking because I know you have the LMT. Im looking for headspace info right now but I cant seem to find any, all I know is that its different than DPMS .308 headspacing

[rsrocket1]

The AR10 is a 7.62x51 so it headspaces from the face of the bolt to the shoulder of the case. Your cases could be just fine in body thickness, but if the shoulders are not sized properly, the bolt will not close on it. Check the headspace on the cases to make sure that the headspace is 1.635" or less.


A Hornady headspace gauge is a handy tool for this.


You could see if the shoulders are too high by painting a shell with a magic marker and chambering it. If the ink is rubbed off on the shoulder, you need to bump it back more.

[problemchild]

Brass is 1.612 and my chamber is 1.613. Shoulders are the correct size as the round will freely drop in a JP case gage and out again.

Any other ideas why the brass would stick ONLY when fed in by the NEW bolt and not the old?

The AR10 is a 7.62x51 so it headspaces from the face of the bolt to the shoulder of the case. Your cases could be just fine in body thickness, but if the shoulders are not sized properly, the bolt will not close on it. Check the headspace on the cases to make sure that the headspace is 1.635" or less.


A Hornady headspace gauge is a handy tool for this.


You could see if the shoulders are too high by painting a shell with a magic marker and chambering it. If the ink is rubbed off on the shoulder, you need to bump it back more.

[tommyid1]

Is the bolt somehow shaving brass on the way in or something? Look for a burr or marks on the case after you extract it. Is the bolt free in the carrier. Does it lock and unlock smoothly by hand?

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[motorwerks]

Oh ok, I was just asking because I know you have the LMT. Im looking for headspace info right now but I cant seem to find any, all I know is that its different than DPMS .308 headspacing

I'm not trying to be a jerk, but how can the headspacing be different? Its the same round. I know the go, no-go, field gauges are the same and can be used for everything chambered in the caliber they were made for.

[problemchild]

Is the bolt somehow shaving brass on the way in or something? Look for a burr or marks on the case after you extract it. Is the bolt free in the carrier. Does it lock and unlock smoothly by hand?

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This may be hard to believe but I think the new bolt is driving the round further into the chamber and it has changed my measured headspace from 1.613 to an unknown smaller dimension.

I just bumped the shoulder back on one piece of brass to the max my die would allow which is 1.5895 and it feeds in and out smoothly. I am going to bring the shoulder forward until it sticks. 10 fired pieces of brass measured to the shoulder fired with the new bolt average 1.6145. My handloads are bumped back to 1.612. My die is a small base dillon carbide die so Im good there and it was working OK before the bolt. The bolt is not shaving brass.

On edit and further jacking with the brass and gun I think the taper of the brass and diameter is larger from about the mid point to the base. I just measured some factory new hornady match and factory new black hills. The base right before the extractor cut-in grab area measure .464 for black hills and .466 for FGMM and hornady match. My brass measures .470/.471 at the base. Even with my die all the way down and destroying the shoulder I cannot make my base smaller than .470. I have a SB resize die and it sizes to .470 as well. I have no way to make the base smaller. I can see scratch marks on the stuck/removed round near the base.

Why the hell would a new bolt cause the brass to start sticking like this? Im lost on this one.......

[problemchild]

Found this. I will check it tomorrow.

[Jpach]

I'm not trying to be a jerk, but how can the headspacing be different? Its the same round. I know the go, no-go, field gauges are the same and can be used for everything chambered in the caliber they were made for.

No worries, I am no expert at this stuff, but I recall that the reason why it is not smart to use a DPMS bolt with an Armalite barrel/barrel extension is because of the way it is headspaced.....

Yeah, again, I am no expert but that is what I recall. And there is some diagram on the net showing the differences between DPMS and Armalite headspacing.

[SixPointEight]

The headspace on an AR can change with a new bolt, even with the same barrel!

Think, the cartridge is held in a recessed cutout in the bolt. When fired the case will expand to fill the chamber, then, usually shrinks to about .001 smaller than your headspace measurement. If you're resizing so your brass fits with bolt A, bolt B may hold the case in a different position, changing the distance between the bolt face and the datum line(headspace). In this case, switching bolts tightened up your headspace.

ETA, not only the recess in the bolt face, but tolerances in the locking lug area could affect this measurement.

[problemchild]

The headspace on an AR can change with a new bolt, even with the same barrel!

Think, the cartridge is held in a recessed cutout in the bolt. When fired the case will expand to fill the chamber, then, usually shrinks to about .001 smaller than your headspace measurement. If you're resizing so your brass fits with bolt A, bolt B may hold the case in a different position, changing the distance between the bolt face and the datum line(headspace). In this case, switching bolts tightened up your headspace.

ETA, not only the recess in the bolt face, but tolerances in the locking lug area could affect this measurement.

I left a message for the engineer at the company and the lead gun tester. I'll find out whats going on and post it later. All I kn ow is there is no way to make the brass fit now. Even brand new black hills and FGMM factory rounds are sticking. Something aint right. Probably have to ship the upper back to have it adjusted.

[bwiese]

Your gun must not be an Armalite AR10.

Armalites are known to like Fed GMM (esp 168gr variant) - in fact, the
AR10-T is specifically tuned for it.

Do you have a chrome-lined barrel? [I'd bet not.]

Brownell's sells 308 Go/No-Go headspace gages. Get 'em.

[goodlookin1]

Ima put my money on either a DPMS bolt in an AR-10 platform or vise versa, a AR-10 bolt in a DPMS platform.

But i've been wrong before.....