AR-308 Accuracy problems. Gas tube alignment could be the culprit. Suggestions?

[aklaunch]

I have put together so many AR's complacency got me.

After going through a Criterion barrel, then upgrading to a Krieger barrel i still can't get the rifle to shoot.

Today i pulled the bolt out of the carrier and ran the carrier back and forth inside the receiver. I found out it was a bit sticky/binding. I ended up cleaning out the carrier key and that helped a bit.

The cause of the binding was not the gas block being misaligned, but the receiver inlet and/or the barrel nut not lifting the tube high enough.

Primer boxes have a thicker paper material. I used that as a shim and stuck it under the gas tube at the part where the hand-guard slides on.

With that shim..... The bolt goes back and forth like there is no gas tube there. Like it does on all my other rifles.

I would like to put devcon epoxy on the upper receiver gas tube inlet. Seems most epoxies i have on hand are rated for around 250-300° F.

How hot do you think the upper receiver gets right there where the gas tube goes in?
Other suggestions?

The only other thing i can think of is adding a weld on the barrel nut, then filing it back down.

I suppose i could try bending it. I am worried i will tweak it though.

 

[Molon]

The cause of the binding was not the gas block being misaligned, but the receiver inlet and/or the barrel nut not lifting the tube high enough.

It is not the purpose of the "receiver inlet and/or the barrel nut" to "lift" the gas tube. The gas tube should not be touching those parts.

The proper way to correct the situation is to determine which part or combination of parts is causing the misalignment and replace them. If you don't want to do that, then buy a tube-bender to tweak the gas tube.

....

 

[S197]

It is not the purpose of the "receiver inlet and/or the barrel nut" to "lift" the gas tube. The gas tube should not be touching those parts.

The proper way to correct the situation is to determine which part or combination of parts is causing the misalignment and replace them. If you don't want to do that, then buy a tube-bender to tweak the gas tube.

....

Molan correct. Pull everything apart and check for imperfections where the tube inserts into the gas block. Check if the pin holding the gas tube in place is causing tension on the tube. Check if the gas tube alignes with the receiver without the barrel nut. If everything lines up, then you probably over or under tightened the barrel nut.

if the tube is not bent enough to meet the receiver without binding then buy a cheap tube bender.

1/4 in. - 3/8 in. Tubing Bender

Amazing deals on this Tubing Bender at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

1/8”

1/8 in. - 1/4 in. Tube Bender

Amazing deals on this 1/8In-1/4In Tube Bender at Harbor Freight. Quality tools & low prices.

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[ron_c]

.

 

[msgriff]

The AR-15 gas tube offset is around 0.158". For the AR-10/308 using the same gas block dimensions, the offset needs to be around 0.220". Even though both platforms usually (not always) use the same gas tube, I've typically had to adjust the offset for large frame builds.

 

[aklaunch]

Gas tube bending tool ordered.

 

[bfoosh006]

FWIW... the SLR Adj. GB will present the gas tube at a slightly higher position then most standard GB's.

 

[aklaunch]

Thank you for that info. I plan on buying a suppressor in 2023

 

[HNation671]

Check your barrel nut, make sure it's not the type to be timed with shims, if it is then the timing must be aligned so the gas tube goes in without obstructions. Also since you changed barrels, make sure the gas block is the right size .such as 750 or whichever the barrel calls for, especially if it's a two piece clamp on. If your gas block is adjustable, open it up a click at a time until it cycles. I recommend adjustable gas block as there are some factory cartridges that don't necessarily cycle.

 

[stevieb92]

If your gas tube needs to be bent slightly higher toward the top of the inside of the receiver to slide into the carrier key perfectly, you can try this crude trick. Remove the bolt carrier group from the upper receiver assembly and lay the upper receiver down (with barrel and gas tube installed) onto a solid workbench with the bottom facing up. Look down into the charging handle channel where the gas tube is coming through the receiver. Find a small brass punch that fits into the width of the channel so you can very lightly tap the end of gas tube toward the top (which is facing down) of the receiver. Place the brass punch on the main part of the tube, not the enlarged stub at the end that interfaces with the carrier key. Give it a couple of small taps and then try sliding the carrier in to check fit. Of course, turn the upper assembly right side up and also check fit of the carrier into the gas tube. I've done this a couple times when the tube needed a slight adjustment to get the fit perfect. It's kinda crude, but it's worked for me.

 

[tucaz]

just curious but what are you using for a trigger and ammo? and what kind of groups are you getting?

 

[aklaunch]

Much has happened since.

For starters i did order the SLR adjustable gas block as Bfoosh006 mentioned. In fact it does present the gas tube higher.

Unfortunately i measured twice and ordered once. And i measured wrong. I bought the clamp on variety, and had to grind the block and my hand-guard a bunch to get clearance. Since then i have spent many trips to the rifle range wondering if i have enough clearance, and if the gas block was banging the hand-guard causing fliers. Back and forth i went swapping gas blocks. With no control load at the time, it is difficult to evaluate accuracy!!!!

My issue is that the SLR gas tube presents higher, and the OEM gas block is of course lower. I opted to grind on the barrel nut to get the OEM tube to line up instead of bending. I am unsure if grinding on the barrel nut causes uneven pressures when the unit heats up?

I have gone back and forth for the last time... Yesterday, for about the 3rd or 4th time i went out with the normal gas block that has lots of clearance and am unable to see a difference in groups. My main problem is that until a couple weeks ago, i have not been able to come up with a control load. Nothing repeats....

At this time Nos 165 BT's and 42 grains of Reloader 15 is working. A 10 shot group in worn out LC Nato brass yesterday came in at 1.207. And a 7 shot group shot in a big hurry with new Hornady brass came in right at 1 inch.. 6 into 1/2" and 1 flier....

I don't like the load... The POI is still moving a lot at charge weights above and below it. But it is shooting ok each trip. Unlike others.


I have chased my tail round and round with a new receiver, new hand-guard, new BCG, new reloading dies, different brass, different bullet lots and couple bent gas tubes and endless load development.

The latest improvement, and what i believe has brought me to the 1st repeatable load is going back to the original receiver. I have noticed better accuracy in general as the the barrel fit is a bit more snug. I suppose i combined this with leaving the JP high pressure bolt out also.

With a standard bolt from Brownells, and the OEM gas block, the rifle throws brass out the 12:15. With the JP bolt it hurls them into the next county out the 4 oclock.

At least as of yesterday, i have what i believe is something that shoots consistently due to swapping uppers and leaving the JP bolt at home. (not sure which was causing problems just yet) And now i can start swapping other parts to evaluate accuracy after the parts change.

Tucaz, i run a Giessele SSA trigger and handloads.

My head is harder than the barrel steel or i would have given up a long time ago.

I still believe i have a rifle problem. I just can't figure out what it is. Perhaps its the receiver to barrel extension fit on both the uppers that i have?

 

[aklaunch]

I just read a post from LRRPF52 about bedding the barrel. Seems i have a Zediker book around someplace that talks about gluing the barrel in.

 

[GhostFace]

I had accuracy problems with a .308 barrel. Once i burnt out the original barrel I went with the same brand of barrel that I replaced the original. I sent it back in to the manufacturer and they said the chamber had slight run out, they touched it up. When I received the barrel back this time I bedded it in with loctite. The barrel now shoots great. I doubt the change has much to do with them "touching" up the chamber but who knows, it could. I'm placing my money on the bedding.

 

[scott5456]

I had accuracy problems with a .308 barrel. I went with the same brand of barrel that I replaced the original with. I sent it back in to the manufacturer and they said the chamber had slight run out, they touched it up. When I received the barrel back this time I bedded it in with loctite. The barrel now shoots great. I doubt the change has much to do with them "touching" up the chamber but who knows, it could. I'm placing my money on the bedding.

Any more, I bed them all to start with. I've seen it make such a big improvement in groupings, I'd rather just do it upfront than to chase issues.

 

[GhostFace]

I'll be doing it for all of them here on out.

 

[aklaunch]

I ended up getting some shim stock, and going that route. The barrel is in the receiver, straight, looking good and not coming out. The receiver and barrel extension where a good match to begin with. Now they are bonded for life... It seems.

Will report next week when i can get to the range.

 

[bfoosh006]

Did you happen to lap the upper receiver face, prior to shimming / Loctiting the barrel in ?

About the 49 second mark...

 

[aklaunch]

I looked online for the tool and could not find one for AR308. Brownells has the small frame version on sale right now....

I had shim stock on hand from things that are un firearm related.

This morning drinking coffee, i was reading through some threads and found the tool at pacific tool and gauge. Too late now....

Oh well... I think today my wife will let me go to the rifle range.

 

[aklaunch]

I made it out to the range yesterday and was able to shoot a few 10 shot groups. No doubt things needed to settle in. I brought 12 cases with 41.5 grains of IMR 4895 and Sierra 168GR MK'S out to start the party. The first 2 went into the same hole, the next 5 shot group was erratic and had me worried, then another 5 shot group into less than an inch.

My Nosler 165 grain/RL 15 load shot pretty much exactly the same. 10 shots into 1.25.

The Hornady brass put 9 of them into .8, and a low flier made the group 1.25...... Argh!

Whatever.


For the 1st time in almost 2 years i have the rifle tuned to shoot as good as it did the day, season, or month before.

I can fine tune from there with confidence.