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Posted: 3/11/2006 9:46:36 AM EDT
After having built dozens of AR's and being an engineer, I sat down and really studied the barrel to receiver design/construction of the AR and I'm curious about this torque issue on the barrel nut.

As I understand it, there is no sealing effect at issue here as the gas seal is created by the bolt to barrel extension fit. Mechanically, whatever torque value you put on the barrel nut, as the barrel heats and cools in cycles of firing, (if I remember all this correctly) the torque value will never decrease, only increase as the metal is cycled. (Someone with a bit more metalurgical background chime in here.) I think this is why we hear so often of crazy experiences removing "frozen" barrel nuts.

Now it would seem that IF this is true, and bearing in mind that the barrel nut is locked mechanically by the gas tube passing through it's slots, why do we need a large inital assembly value of torque on the barrel nut? Given that we are torquing a steel nut/barrel index pin onto an aluminum upper receiver, I can only see bad things happening with large torque values - like cracking the 7075 T6 of the receiver.

As an experiment, I tried torquing the barrel nut to a rather low value. In fact quite a bit less than 30 ft lbs. I completed the assembly and test fired several hundred rounds then disassembled. The rifle shot fine and came apart quite easily afterward. I examined the upper with magnification to see if there were any detrimental effects such as evidence of barrel/receiver movement or gas blow by. There were none.

I would really like to hear from some of you who have really considered or tested this issue. Can anyone tell me why we need 30 ft lbs (or as some suggest MORE) from a safety/reliability/ function standpoint? I know the manufacturer suggests this - but what real world effect is changed by torquing to greater values?

This all may be too much. Guess I should have just posted a pic of my latest build, but inquiring minds want to know.
Link Posted: 3/11/2006 4:02:41 PM EDT
I've read on this board of some people going less than 30# and having the barrel nut loosen enough to slop around against the gas tube. Granted, it cannot loosen completely, but it can move if not torqued enough. It has also been a common complaint from those using vertical grips on a rail. A little side pressure on the grip can cause loosening of the barrel nut. You might as well go the extra amount (30# plus) just to be on the safe side.
Link Posted: 3/11/2006 4:05:22 PM EDT
Tag.
Link Posted: 3/11/2006 5:47:51 PM EDT
My personal take on this is that 30 ft pounds is not really a large torque value for that diameter nut, and thread pitch. I do agree though that a final torque value around that range is a good number to end up with, even after lining up the gas tube. That should snug things up adequately. How these parts perform together cold is one thing, but they can get pretty hot under a f/a load or rapid fire, and that could compromise a lesser torque.

Some armorers may have a tendency to overtighten these parts at the factory, but the "frozen" nut reports could very easily be caused just by not using a thread lubricant. The resultant corrosive bonding through oxidation and electrolysis generated from the dissimilar metals could make normal disassembly very difficult.

TC
Link Posted: 3/11/2006 5:54:34 PM EDT

Originally Posted By TopCatUS:
My personal take on this is that 30 ft pounds is not really a large torque value for that diameter nut, and thread pitch. I do agree though that a final torque value around that range is a good number to end up with, even after lining up the gas tube. That should snug things up adequately. How these parts perform together cold is one thing, but they can get pretty hot under a f/a load or rapid fire, and that could compromise a lesser torque.

Some armorers may have a tendency to overtighten these parts at the factory, but the "frozen" nut reports could very easily be caused just by not using a thread lubricant. The resultant corrosive bonding through oxidation and electrolysis generated from the dissimilar metals could make normal disassembly very difficult.

TC



When I took my barrel off my carbine at my armorer's course, it was a major PITA, because I failed to lube it when I assembled it previously.

Torque... I would say it's important to hit that 30 ft-lbs because that is the accepted industry standard, and if you ever have any problems down the road... the weapon is "IN-SPEC".

Just my $0.02
Link Posted: 3/11/2006 6:28:24 PM EDT
Marine Corps says 31 to 80 pound feet, so I guess around 55 is the sweet spot.
Link Posted: 3/11/2006 6:38:19 PM EDT
There is no real "sweet spot"

There is too much mythology on this issue.

The torque specs are there for a reason... but people pay way too much attention to them.

A minimum torque is given to allow the barrel nut to be at the necessary torque to general not back off.

Then - you simply tughten the barrel nut until the next gas tube hole can be aligned. Sometimes this is 31#. Sometimes, if the nut is just barely past one gas tube alignment, and has a long way to get to the next one... it takes a TON of torque to get there.

I have seen barrels that loosended off in the field. I did two installs on AR pistols a while back - and both shot loose (aluminum barrel nuts)

I have seen a 20" rifle shoot loose, which results in the barrel nut just rocking against the gas tube (no big deal) and the barrel is now allowed to rotate in the upper receiver slightly, allowing for a tiny amount of windage change (not good)

Neither affect reliability.... but the torque values are there for a reason.


That being said, I never use a torque wrench. I snug them up good, three times, then tighten to the next gas tune alignment position. It doesnt hurt to use the torque wrench, if you have trouble "feeling" 30lbs. My elbow "clicks" at 30# now.

(and I ignore the 80# max stuff) You tighten to the next gas tube. If that goes over 80#, so be it.)
Link Posted: 3/11/2006 6:40:55 PM EDT
On assembly if a blowback AR, like a 9mm, it is important to pay special attention to the torque - since there is no gas tube. Also, since there is no gas tube, if it DID back off, there is nothing to keep it from backing off all the way, creating a headspace issue and the barrel is allowed to bull away from the upper receiver.

On pistol caliber AR's, I CRANK those barrel nuts down tight.
Link Posted: 3/11/2006 7:05:40 PM EDT
Thanks for the input guys. I especially wanted to know if anyone had ever had a nut back off - question answered. I see the problem with sight alignment and movement. I've never had one back off, but perhaps the most telling argument for adequate torque might be the free floating rail system moving the whole thing around.

Interesting thought in any case. The heritage of the AR configuration came from the little survival rifle that was designed for the Air Force and carried on to today. The barrel / nut assembly is just like an AR without the gas tube and, of course, it's a .22 LR. That little nut just gets a hand tighten.

Again, thanks for the time and input.
Pete
Link Posted: 3/11/2006 7:16:32 PM EDT

Originally Posted By persimmonpete:
but perhaps the most telling argument for adequate torque might be the free floating rail system moving the whole thing around.



The LaRue Tactical railed handguards have an anti-rotation device which further attacks the issue of railed handguards twisting.
Link Posted: 3/11/2006 7:36:52 PM EDT
I just put a LaRue rail on mine today. Torqued to 30 ft. lbs. and backed off several times, then torqued to 30 ft. lbs. for the final time. f'n barrel nut gas tube web right in the middle of the receiver gas tube hole. Had to finally go to 85 ft. lbs. to get the thing to line up right. Didn't hear or feel anything strange while coming down on it. I guess we'll see. It was either go way less than 30 or go higher. Came out nice though; boresighted just before dark and the front sight was almost perfect inline.
Link Posted: 3/11/2006 7:39:04 PM EDT

Originally Posted By blueheeler66:
I just put a LaRue rail on mine today. Torqued to 30 ft. lbs. and backed off several times, then torqued to 30 ft. lbs. for the final time. f'n barrel nut gas tube web right in the middle of the receiver gas tube hole. Had to finally go to 85 ft. lbs. to get the thing to line up right. Didn't hear or feel anything strange while coming down on it. I guess we'll see. It was either go way less than 30 or go higher. Came out nice though; boresighted just before dark and the front sight was almost perfect inline.



That sounds exactly like it *should* be.
Link Posted: 3/11/2006 7:44:21 PM EDT
Glad to hear that.
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