Gunsmithing AR Bolt Lug Face

[ZenBiker990]

99.99% sure I know the answer, can’t figure out why I can’t find more about it.

Built an AR, new Vlotr upper, stainlaess match grade barrel 1:8 twist, 16”. Worst accuracy I’ve seen, 6” irregular group @100 yards. Started checking, headspace, lugs, barrel nut, all good,

Then I notice shiny marks on the rear of the barrel that look like contact with the front of the bolt lugs. Check w/machinist’s lipstick, shows postitive multiple contact.

In a bolt gun this is a killer. Have I stumbled on a similar issue here?

 

[mtgale00]

Normal... Dry charging may be making it stand out. The bolt hardness is likely 30+ points harder than barrel material. Somethings gotta give. I doubt this would ever be a concern when a chrome lined or nitrided barrel is used.

 

[Subwrx300]

My bolt face looks nearly identical to yours: mainly contacting on bottom. Three lugs (shiny), moderate contract/wear on middle half and not so much wear on the top. Two barrels (factory and aftermarket) bot shoot between .5 - .8 average.

I would tend to think it will be more related to ammo/barrel combination but just one opinion.

What ammo/load combos have you tried so far? Maybe barrel is not quite right (rough, out of spec chamber to bore alignment, bad crown?, etc,). Id reach out to Vltor to see what they have to say. 6" is preeeettttyy large even with bad ammo.

 

[TRP173]

From everything I have ever heard is there is no Standard for Match Grade so any company making Barrels can call them whatever they want.
What is the barrel make?

 

[Ledzep]

With no cartridge in the chamber, the bolt face will make contact with the barrel like what you're showing and that is what will cause the cam action of the bolt inside of the BCG.

With a cartridge in the chamber, the bolt face/cartridge head interface would be what caused the cam to actuate. If your rifle is doing that with a cartridge in the chamber, there may be headspace issues at play, but I think it's pretty normal to have those markings.

 

[ZenBiker990]

Barrel is 1:8; Black Hills 77 gr match, my usual combination, has always been a winner for me up to now. Headspace closes like this on Go Gauge. Put a piece of scotch tape on the head of the gauge .002” and bolt does not lock. I didn’t think bolt lugs were supposed to contact the barrel in battery, bolt/gas or otherwise. Always built bolt guns with .020” minimum between front of bolt lugs and breech end of barrel.. Barrel is considered a better quality tube, refraining from naming them for what may be a problem of my own making. My lathe is on a truck at the moment otherwise I’d take that barrel face back with a boring bar. Never had one shoot this badly before; wondering if it’s stacking tolerances or if I’ve made an error someplace else. Nothing to do but tear it down and double check, but I wanted to know if there was a spec for bolt lug clearance and the breech end of the barrel. I need to verify the twist rate as well to make certain it is what it says it is.

 

[Mordamer]

That's not your problem. What muzzle device do you have on the gun? I would suggest taking the muzzle device off and shooting it to see if that fixes your problem.

 

[natdscott]

Milspec on that dimension is about 0.010", and I spec my match barrels to have 0.003", not to exceed 0.005". Most AR barrels end up in the 0.015" area.

...so yeah, you might have a problem. Accuracy? I can't say...but I do know there's no BENEFIT to bolt gap being set that closely.

As for function, I see a possible problem. If a piece of tape will do that, gosh whatever would you do with a brass shaving or grain of sand?!

ARs don't blow up very well, so I'd check into ways to avoid an OOB firing.

 

[Ledzep]

Barrel is 1:8; Black Hills 77 gr match, my usual combination, has always been a winner for me up to now. Headspace closes like this on Go Gauge. Put a piece of scotch tape on the head of the gauge .002” and bolt does not lock. I didn’t think bolt lugs were supposed to contact the barrel in battery, bolt/gas or otherwise. Always built bolt guns with .020” minimum between front of bolt lugs and breech end of barrel.. Barrel is considered a better quality tube, refraining from naming them for what may be a problem of my own making. My lathe is on a truck at the moment otherwise I’d take that barrel face back with a boring bar. Never had one shoot this badly before; wondering if it’s stacking tolerances or if I’ve made an error someplace else. Nothing to do but tear it down and double check, but I wanted to know if there was a spec for bolt lug clearance and the breech end of the barrel. I need to verify the twist rate as well to make certain it is what it says it is.

The bolt ONLY contacts the breech face on an empty chamber. Think about it for a minute.

Pull your BCG out of the rifle and push/pull the bolt in and out of the carrier a few times. Something has to provide a force on the front of the bolt in order for the cam to work. On an empty chamber it's the breech face. On a live round it's the case head. Every AR ever made has those marks unless it's never been cycled without ammunition in it.

In order for it to have those marks AND have them be caused when a live round is being chambered, the headspace almost necessarily has to be long. Either that or configured EXACTLY at the same depth that the bolt nose hits the breech face.

 

[Ledzep]

Try your lipstick trick with a case, dummy, or live round in the chamber if you can do it safely.

 

[Ledzep]

Here's my bolt, a couple thousand rounds on it, but you'll notice the shiny spots where it's contacted the breech face and rubbed while camming on an empty chamber. Your accuracy issue lies elsewhere, probably muzzle device, crown, or something else.

 

[natdscott]

Ledzep, I agree with your logic, and my bolts look the same (I actually lube the front of the bolt jussst a touch), but I come back to his tape addition.

If what he says is true and a 0.002" piece of tape preents it from going into battery at ALL, then he truly MAY have a bolt gap issue.

0.002" clearance is only enough if you are a guy like me that cleans RELIGIOUSLY and OFTEN. My barrels and actions get NO more than 200 rounds between cleanings, and usually less than that. Paired with a lot of really polished stuff, and well-smithed barrels, carbon and gunk build up is less of an issue for me.

But the suggestion of gas tubes and muzzle devices as possible culprits should NOT be ignored.

-Nate

 

[Ledzep]

From what I'm reading it just sounds like his head space is on the tight side. Closes on a GO, does not close on a GO with .002" tape on the back of the gauge.

So he's GO or GO+.001. A little tight for a semi-auto, but still not indicative that bolt nose clearance is an issue. More of a reliability issue if anything than accuracy. And honestly, probably not a reliability issue until the thing gets nasty dirty.

 

[natdscott]

From what I'm reading it just sounds like his head space is on the tight side. Closes on a GO, does not close on a GO with .002" tape on the back of the gauge.

So he's GO or GO+.001. A little tight for a semi-auto, but still not indicative that bolt nose clearance is an issue. More of a reliability issue if anything than accuracy. And honestly, probably not a reliability issue until the thing gets nasty dirty.

Oh hell.

So it does:

Put a piece of scotch tape on the head of the gauge .002” and bolt does not lock.

I actually request that my barrels are chambered at -0.001"-0.000", not to exceed +0.002", but again, I have a very specific purpose, and I also hand load every piece of brass those uppers eat; the trouble is, for high pressure loads, almost ALL new brass comes out at -0.003" or more.

But yeah, SAAMI "GO" is indeed awfully short for general AR setup.

Signed,
Dumbass.

 

[ZenBiker990]

Appreciate the input; sounds like it’s headspace. It bugged me a bit when I checked it; no allowance for oversized cases or even the bolt crushing the case enough on the forward stroke to allow inconsistent lockup and internal ballistics and weapon harmonics. Back to the drawing board; Nothing to do but tear it down and check everything over I guess. I was just surprised when I started searching for specs on bolt/breech clearance or contact. Also, buddy with experience recommends checking the actual twist rate on the barrel; he says he’s seen 3 barrels from respected manufacturers where the measures twist was slower than stamped on the barrel. If that checks out, I’ll take it down and check for anything out of the ordinary. Wish me luck and thanks for the advice, guys. When I figure it out I’ll post it.

 

[natdscott]

WHOSE "match" barrel is this, anyway? Still haven't seen that defined.

Go for the easy things.

1) with a tight patch, check twist rate.
2) with more patches, fully clean the barrel with a good solvent.
3) check fired casings against a Go gauge or Whidden/RCBS/Dillon/Wilson gauge of your choosing...they will be ~0.001" shorter than the actual chamber. So if that figure is AT 1.4636, then it may be cut short.
4) May seem obvious, but use GOOD ammo, and try more than one. Something 62-77 grains from Hornady or Federal that says Match on the side, but not SuperformanceblastemflatterSupremes either. Just a standard 2750 7x grain load.
5) 6" is pretty heinous, but have you tried a known-good AR shooter. They aren't as common as they say they are, and ARs are not that easy to bughole.
6) Take off the brake, and shoot it that way.

 

[Mordamer]

Muzzle device...