AR-10 / LR308 bolt gas ring fit?

[UncleDano]

I was having trouble with my 6.5 Creedmoor piercing primers on hot days.
I purchased a new JP Enterprise High Pressure bolt for this AR.
As I was swaping out the bolts, I got curious about the differences in the standard bolt and the high pressure bolt (besides the slightly smaller diameter firing pin).

I noticed first off, that the fit of the gas rings to the bore of the Bolt Carrier was different. The fit of the JP Enterprise was less than a "piston" fit compared to the Wilson Combat N/B bolt (It was tighter fit, but smooth "piston" like feel.

Gas Ring diameter on the JP Enterprise: .647"
Gas Ring diameter on the Aero Precision:.650"
Gas Ring diameter on the Wilson Combat: .660"

That's a difference of .013" total diameter. To me that's a big difference!

I have no tools that allow me to measure the bore diameters of each of the carriers. But I see that the JP bolt slides in with no effort smoothy. With the Wilson and Aero N/B carriers it's a smooth tight "piston" fit.

Now, I know that when you stand vertical a 5.56 BCG on the bolt end, it is supposed to support the carrier and not collapse down. But I also know that this is not necessary for this to be true with the AR10 BCG. The Aero Precision Nitride BCG WILL do this standing on end, but the Wilson and Aero Nickel Boron BCG will not, and with the JP Enterprise HP bolts they are even looser.

So, what have you learned, experienced with this? Is it something to be concerned about, or is it no big deal.
What is the proper clearance between gas ring and carrier bore, or is there no proper/recommended fit? Give me your thoughts.

By the way, the JP Enterprise HP bolt did stop the pierced primer problem.

 

[Darryle]

What size firing pin diameter? Measure the hole?

Are you using a JP firing pin?

If the firing pin hole is .065" and you are using the firing pin that came with the Aero BCG, firing pin is probably out of spec.

 

[UncleDano]

What size firing pin diameter? Measure the hole?

Are you using a JP firing pin?

If the firing pin hole is .065" and you are using the firing pin that came with the Aero BCG, firing pin is probably out of spec.

Yes, I am using the correct firing pin that came with the JP Enterprise bolt. It is only a few thousandths smaller at the tip than the standard firing pin.

 

[Darryle]

Measure the face depth versus the original Aero bolt, any difference?

Gas rings provide the seal, the key will provides the relationship of location depending on where the bolt is during the cycle, think of the upper as a track and the key as the train following the track.

Measure the 2 firing pins overall lengths, is the JP longer?

What primers? Are these reloads, 7.62x51 or 308 factory?

Mil-Spec primers are harder to eliminate an accidental discharge due to FP bounce, the firing pin is not captured, hence the reason you see the impression of the FP on rounds that were chambered from the magazine during firing but not fired themselves.

I have dealt with this in the past, tolerance stacking happens with ARs, you have so many variables, that you wind up on either side of ideal.

Have you tried the Aero bolt and FP? If not, that is my first suggestion. If you get cratering but no pierced primers, then I would try the Aero pin in the JP bolt, verify that it will fit thru the FP hole, if it runs with no cratering or pierced primers, the answer is tolerance stacking.

The solution is call JP and explain the situation, order a new pin, use the Aero pin with the JP bolt or lightly polish the JP pin until the piercing stops.

For polishing, I use a cordless drill set to the fastest rpm, chuck the pin up and use the finest paper you have, 600 grit or finer, followed by a run across some type of coarse cloth with some Flitz on it, just a touch to the face of the pin while moving in a fast arc so you keep the original radius of the pin, followed by a good polish. This may take 1 time or 20, you will get the hang of it, but don't rush or heat the pin excessively.

I have a drawer with 2 or 3 dozen out of spec pins, I dig thru them when this comes up measuring until I find one that is slightly shorter or longer as the case may require.

There is no true "Mil-Spec" with large frame ARS, issues arise more than they do with the AR-15 in the large frame.

Curious, What buffer are you running? Adjustable gas block? Setting?


****Edit to correct spelling errors*****

 

[Dankno]

I didn't pay much attention to diameters when I was installing my JP bolt for my 260, nor was I paying much attention at all during re-installation after cleaning it, which brings me to; BE CAREFUL DURING REASSEMBLY. The end of the single-piece gas ring from the JP hung up on my carrier and was destroyed in the efforts to un-stuck it.

As an aside, I had no function issues with the single piece gas ring and currently have no function issues with the standard 3-piece rings that came with my original bolt/carrier combo (Fail Zero if memory serves.) As the rings are considered a consumable, and with the lack of a "standard" as established as the small-bore AR15, I'd not worry too terribly about exact fit as long as function isn't compromised.

 

[ntsqd]

Why would the "support the bolt carrier" test not apply to the AR10's as well?

JP appears to build their parts to a standard such that I'd be hesitant to use just their bolt in someone else's bolt carrier. It may work just fine, but I see this sort of mixing and matching of different vendor's parts cause problems in many different disciplines, firearms being just one of those. I'm not a "Buy all of it from one vendor" guy, but I do believe in being careful about selecting all of a system's close fitting parts from one manufacturer.

 

[UncleDano]

Measure the face depth versus the original Aero bolt, any difference?

Gas rings provide the seal, the key will provides the relationship of location depending on where the bolt is during the cycle, think of the upper as a track and the key as the train following the track.

Measure the 2 firing pins overall lengths, is the JP longer?

What primers? Are these reloads, 7.62x51 or 308 factory?

Mil-Spec primers are harder to eliminate an accidental discharge due to FP bounce, the firing pin is not captured, hence the reason you see the impression of the FP on rounds that were chambered from the magazine during firing but not fired themselves.

I have dealt with this in the past, tolerance stacking happens with ARs, you have so many variables, that you wind up on either side of ideal.

Have you tried the Aero bolt and FP? If not, that is my first suggestion. If you get cratering but no pierced primers, then I would try the Aero pin in the JP bolt, verify that it will fit thru the FP hole, if it runs with no cratering or pierced primers, the answer is tolerance stacking.

The solution is call JP and explain the situation, order a new pin, use the Aero pin with the JP bolt or lightly polish the JP pin until the piercing stops.

For polishing, I use a cordless drill set to the fastest rpm, chuck the pin up and use the finest paper you have, 600 grit or finer, followed by a run across some type of coarse cloth with some Flitz on it, just a touch to the face of the pin while moving in a fast arc so you keep the original radius of the pin, followed by a good polish. This may take 1 time or 20, you will get the hang of it, but don't rush or heat the pin excessively.

I have a drawer with 2 or 3 dozen out of spec pins, I dig thru them when this comes up measuring until I find one that is slightly shorter or longer as the case may require.

There is no true "Mil-Spec" with large frame ARS, issues arise more than they do with the AR-15 in the large frame.

Curious, What buffer are you running? Adjustable gas block? Setting?


****Edit to correct spelling errors*****

First of all, the J P Enterprise Bolt fixed the primer piercing problem.
I am using Hornady factory ammo 6.5 Creed 140gr.

I have not tried the Aero bolt in this rifle, but will give it a try just to see result.

Firing pins are the exact same length.

The firing pins are not interchangeable; the JP point diam. Is .057" , the Wilson is .067" ( std).

I am using 3.8oz. buffer, tried 4.3oz, but case barely rejects with it.
Using adjustable gas block, adjusted into the bleed-off range.

 

[Harry Knutz]

First of all, the J P Enterprise Bolt fixed the primer piercing problem.
I am using Hornady factory ammo 6.5 Creed 140gr.

I have not tried the Aero bolt in this rifle, but will give it a try just to see result.

Firing pins are the exact same length.

The firing pins are not interchangeable; the JP point diam. Is .057" , the Wilson is .067" ( std).

I am using 3.8oz. buffer, tried 4.3oz, but case barely rejects with it.
Using adjustable gas block, adjusted into the bleed-off range.

That was the way I read your first post, Was that you bought the JP to end the pierced primer problem. Standard AR-10 Bolts have way to big of a firing pin hole from the factory, It usually shows itself to become a problem on 6.5 cartridges for some reason. Buying a bolt head with smaller firing pin and firing pin hole solves the problem. I don't see the gas ring size as being a problem with 0.003 difference shouldn't be an issue. JP stuff is high regarded.

 

[UncleDano]

Why would the "support the bolt carrier" test not apply to the AR10's as well?

JP appears to build their parts to a standard such that I'd be hesitant to use just their bolt in someone else's bolt carrier. It may work just fine, but I see this sort of mixing and matching of different vendor's parts cause problems in many different disciplines, firearms being just one of those. I'm not a "Buy all of it from one vendor" guy, but I do believe in being careful about selecting all of a system's close fitting parts from one manufacturer.

In testing, the AR10's don't seem to be affected as much as the AR15's with looser gas ring fit. And secondly, some DO stand up, and some do NOT as I mentioned.

The J P bolt is not built to a lesser standard or tolerence, but they purposely make the firing pin hole & firing pin tip a few thou smaller. Aero and Wilson do not make a high pressure bolt, so I must buy what is available.
The JP bolt actually tightened up headspace on this rifle.

 

[ntsqd]

I wasn't clear. The reason that I wouldn't use a JP bolt in another mfg's bolt carrier is because I expect that JP has carefully set their pairs of dimensions such that they will work the best together. All of the dims on their bolt may be within whatever spec there might be for inter-changeability, and all of the dims for their bolt carriers may also be within that set of specs, but they have likely set their own specs to be tighter than the stds. and to have carefully chosen their own dim tolerances to work best with each other. Putting their bolt in someone else's carrier may not get you the most optimized stack of dimensions.
Said differently, a JP bolt in a JP bolt carrier will offer carefully optimized performance where a JP bolt in someone else's carrier may or may not offer that performance. It's not a slight on either mfg., it's just the way that the tolerances stack up.

 

[tim_w]

The jp gas ring is made to have less resistance it will feel like a workout traditio ring set. Call JP or look at the tech articles and video on their site. Even their AR15 bcg will not usually pass the stand up bcg gas ring test. But it's not indication of a ring blow by situation.

If the seal is actually bad there is no damage that's happening. It simply will bleed more gas from the expansion chamber and cycle slower or short stroke. But my guess is it will seal fine and cycle as it seems to be. On large AR formats for hunting and or LR use for bore sizes 30 cal and under I almost always use extended gas ports location and adj gas block.

 

[Darryle]

I never use an adjustable gas block, I tune using a heavy buffer, I have rifles with buffers as heavy as 8.5oz. I don't want to restrict the one thing that makes the rifle function.

I know people do it all the time, I just don't.

 

[tim_w]

I never use an adjustable gas block, I tune using a heavy buffer, I have rifles with buffers as heavy as 8.5oz. I don't want to restrict the one thing that makes the rifle function.

I know people do it all the time, I just don't.

Understandable and reduces point of failures. Downside is extra gas carbon unburnt powder into the bcg and upper. No big deal on a hunting rifle as rds count between cleaning is never going to be high.