barrel nut installation

[kimberacp]

When tightening the mil-spec barrel nut, if it does spoke does not clear for pathway for the gas tube, do I loose the nut, file off the spoke that is the way or use barrel shims???

What would be the correct method???

thanks

[Flatland Shooter]

First I always put a very small amount of anti-seize compound on the upper's threads. Initially I torque the barrel nut to 35 ft-lbs. I then increase the torque just enough that the gas tube will pass freely into the upper.

I've been told you can torque up to 85 ft-lbs but so far I've never needed to go that high to get alignment.

Bill

[APL-G35]

I'd torque it correct and cut the nub off. Mil spec nuts are relatively cheap.

[ac4wordplay]

I'd torque it correct and cut the nub off. Mil spec nuts are relatively cheap.

I think "correct" torque is a wide enough range that correct alignment is not a problem.

[usmc1094]

As said its 35-85 so you shouldn't have a problem getting it were you need it. I wouldn't cut anything off.

[MickB]

You put anti-seize compound on the upper's threads. Tighten to 35 ft/lb. Back off and tighten again. Repeat one more time. By doing this you are seating the nut on the upper's threads.

Then tighten one more time, as soon as the nut gets tight, keep going till you get the correct alignment of the gas tube.

Correct alignment means the gas tube passes through with no binding.

Final check is dropping the carrier into the upper (minus the bolt) and checking gas tube/carrier key alignment. You want the gas tube to slide into the carrier key with no binding. Adjust gas tube in the upper as necessary using a large flat blade screwdriver till no binding of the gas key occurs...

Mick

[a.roberts]

I had one upper/barrel/nut combo that refused to time between 30-80 ft lbs. I chucked up the barrel in a lathe and took a little off the front face of the barrel extension. Problem solved.

From the three options listed I'd use shims. Or try a different barrel nut.

eta:

Also, grease is what you are supposed to put on the threads.

Moly grease if you want to get picky.

Edited December 21, 2013 by a.roberts

[Alaskapopo]

I thought the specs were 35 to 70 pounds did not know 85 was the top end.

Pat

[usmc1094]

I thought the specs were 35 to 70 pounds did not know 85 was the top end.Pat

I've only done 2, but neither made it past 45 pounds

[mpeltier]

I thought the specs were 35 to 70 pounds did not know 85 was the top end.

Pat

It's not Pat. Per Colts Armorers manual CM118 dated 01/2010 revision 3, initial torque is 30 ft pounds, (backed off then re-tightened as mentioned above) maximum allowable torque 80 ft pounds.

Interestingly the Colt monolithic bbl nut is tightened/loosened/(adjust torque wrench)tighten, in the sequence of 30, 45, 60 ft pounds.

[GunCat]

I'd torque it correct and cut the nub off. Mil spec nuts are relatively cheap.

I think "correct" torque is a wide enough range that correct alignment is not a problem.

That has been my experience (using 30-80 ft/lb min/max)

[bikerburgess]

if the nut doesn't time inside the torque window, you can use a tool to lap the face of the upper where the barrel seats and try again till you get it where you want it.

http://www.brownells.com/gunsmith-tools-supplies/rifle-tools/receiver-tools/ar-15-m16-upper-receiver-lapping-tool-ar-15-lapping-tool-sku080-000-182-20220-44377.aspx

Mike

[MarkCO]

I have found that barrel torques less than about 45 ft-lbs can cause problems:

1. Loosening and then moving the gas tube to one side resulting in gas key or gas tube wear.

2. Higher level of heat related walking of POI.

Just a few items to keep in mind.

[Alaskapopo]

I have found that barrel torques less than about 45 ft-lbs can cause problems:

1. Loosening and then moving the gas tube to one side resulting in gas key or gas tube wear.

2. Higher level of heat related walking of POI.

Just a few items to keep in mind.

Thanks for the tip. I am about to put an upper together with a Nordic Barrel and will torgue it to at least 45.

pat

[AKJD]

Colt manual CM102 for the M16A1 lists 30ft-lbs as the initial torque spec and states that additional torque may be applied to provide clearance but does not give an upper limit.

Army TM 9-1005-319-23&P Unit and Direct Support Maintenance manual torque specs are 30-80 ft-lbs and uses the 3 times torquing method for better thread fit. This is how I was taught in the Army.

I've installed a few barrels and have encountered a few that wouldn't align, while in the Army when alignment wasn't achieved we tried different barrel nuts until we found one that worked. Now I try another nut or two, if that doesn't work I lap the front of the receiver until it works. Doesn't take much usually.

I use grease on the threads but have use anti-seize in a pinch. Some claim the graphite in the anti-seize can cause corrosion problems, I haven't seen it but that doesn't mean it doesn't exist.

[63expert]

You put anti-seize compound on the upper's threads. Tighten to 35 ft/lb. Back off and tighten again. Repeat one more time. By doing this you are seating the nut on the upper's threads.

Then tighten one more time, as soon as the nut gets tight, keep going till you get the correct alignment of the gas tube.

Correct alignment means the gas tube passes through with no binding.

Final check is dropping the carrier into the upper (minus the bolt) and checking gas tube/carrier key alignment. You want the gas tube to slide into the carrier key with no binding. Adjust gas tube in the upper as necessary using a large flat blade screwdriver till no binding of the gas key occurs...

Mick

This^^^^^

[cas]

I back off.

After shooting an accurate, reliable rifle for years, with a loose barrel nut (FF tube/nut rocked back and forth) I don't worry about torque specs.

Edited December 22, 2013 by cas

[MickB]

Initially setting the torque wrench to 35 ft/lb and backing off a couple times (NOTE - do not back off with the torque wrench), you set the torque wrench to the upper spec limit (80 ft/lb) and simply tighten till the gas tube hole aligns. This will guarantee being over the 45 ft/lb that Mark describes.

You still need to check gas tube to carrier key alignment regardless.

Mick

I have found that barrel torques less than about 45 ft-lbs can cause problems:

1. Loosening and then moving the gas tube to one side resulting in gas key or gas tube wear.
2. Higher level of heat related walking of POI.

Just a few items to keep in mind.

[openclassterror]

At our shop we make a free float rail system that attaches with a special nut that has infinite timing. After much experimentation we determined that MOST of the time, any torque beyond 45 ft/lbs is detrimental to accuracy, with everything else in the system identical (assuming minimal clearance on barrel extension OD. If barrel is sloppy in receiver tube, you have another set of accuracy problems). If you use the multi step torqueing process and anti-seize, it should never come loose at 35 ft/lbs. Grinding the teeth off a standard type barrel nut usually causes trouble with clamp-on plastic handguards because most types have small bumps in the barrel nut groove to keep them from moving around, which correspond to the teeth on the nut when timed correctly. If not timed properly and the offending tooth is just ground away by the gas tube, the hand guards will not fit in their correct location. The Midwest free float hand guards (among others) also use these teeth to orient the top rail upright, so again, just grinding off the top tooth is badness. Best thing to do is try another nut, shim, or lap end of receiver. I try to never go more than 50 ft/lbs if at all possible. Also, if you are not using a properly fitting fixture block, you risk twisting or bending the receiver if you push it to the high limit.

Tom

[Alaskapopo]

Made a stupid mistake on my last build forgot to use anti sieze or greese on the barrel nut. Question is my upper going to be ruined next time I need to do a barrel change or is this a fixable mistake?

Pat

[mpeltier]

Made a stupid mistake on my last build forgot to use anti sieze or greese on the barrel nut. Question is my upper going to be ruined next time I need to do a barrel change or is this a fixable mistake?

Pat

Steel or aluminum bbl nut?

[Alaskapopo]

Made a stupid mistake on my last build forgot to use anti sieze or greese on the barrel nut. Question is my upper going to be ruined next time I need to do a barrel change or is this a fixable mistake?

Pat

Steel or aluminum bbl nut?

Steel but its ok I remember that I a friend of mine had installed the barrel before I removed it to send it off for some work and then later re installed it. He put anti sieze on it so unless it all rubbed off when I re installed it I should be ok. Thanks guys. Was worried for a bit.

Pat

[MarkCO]

Pat, you should be fine. A lot of these "additional" procedures are meant to enhance the poor surface finish and poor machining of mil-spec parts. The parts you use are better, so you should not have an issue.

Just realizing how the barrel mates up to the receiver, where the optics are mounted, it seems intuitive that a more secure connection will result in less thermal drift and better repeatability, however, there seem to be a few folks saying the opposite.

Just think it though. If one were to weld the barrel to the receiver, you would essentially have a more rigid connection. When I started to see all these high speed videos of how much the rifle barrel whips, I started to contemplate the connections and play with the torque levels and even did some modeling of the connection to see how the materials interacted. What I found, is that as the torque level was increased, the first shot POI shift, accuracy and thermal drift were all improved. I ran a rifle with the barrel nut torqued over the "maximum" and it shoots just at 1/3 MOA in the indoor tube. Take that for what it is worth.

Anodized aluminum to stainless steel has no corrosion issues. However, bare aluminum to stainless steel has a corrosion potential that is well known among materials engineers. Salts, which can be present in some environments and which are produced in the combustion of gunpowder, accelerate this process. I would never advice anyone to remove the anodizing on a receiver where it will be in direct contact with stainless steel. Some barrels and extensions are in fact stainless steel.

[GuildSF4]

Anodized aluminum to stainless steel has no corrosion issues. However, bare aluminum to stainless steel has a corrosion potential that is well known among materials engineers. Salts, which can be present in some environments and which are produced in the combustion of gunpowder, accelerate this process. I would never advice anyone to remove the anodizing on a receiver where it will be in direct contact with stainless steel. Some barrels and extensions are in fact stainless steel.

Galvanic corrosion is a horrible thing. Too many engineers and techs know nothing about it.

[openclassterror]

The anti-seize is not to hide a poor finish, but to prevent galling at high torque conditions. Many times I have removed really tight barrel nuts and found the anodizing badly worn on the threads, esp on factory DPMS installs. This creates the steel-on bare aluminum galvanic interface that the last several posters mentioned. Anti-seize allows the threads to torque smoothly without friction (heat) welding and tearing the surface of the material.