AR Barrel Shim Question

[lencac]

Hi Guys
Couple questions;
Do folks shim the barrels on AR type rifles for the purpose of keeping the barrel nut alignment within the optimum torque range?

And then, just in general, on any high powered bottleneck cartridge rifle what do you suppose is the exit pressure at the muzzle as the projectile leaves the barrel. My guess is 4K - 5K psi

 

[Frosty Dave]

Folks have been known to shim AR barrels to reach the nut tooth-to-torque reading happy place and somewhere out on the vast expanse of the internet is even a chart to convert shim thickness to expected nut rotation.

But shimming wouldn't be most folks preferred method of getting there.

Personally, I'd just change torque wrenches until I got the reading I wanted.

 

[briansmithwins]

Shins aren't usually required. I've never seen a .mil armorer's manual that ever called for them.

Flash hiders now...

BSW

 

[DougW]

I have changed or added barrels to uppers so many times I lost count. I have tried the torque wrench method and I do not like the prospect of jamming a steel nut on aluminum male threads. I tighten and loosen the nut about 6 or 8 times, then tighten up enough to align the gas tube. I do use a torque wrench on the wrench, but I never hit the 40 ft/lbs setting on the wrench. I have never had a headspace or barrel issue on any of my rifles. YMMV-

 

[TBH]

As I recall, the torque is between 20 or 25# to 75#. The idea is torque the greased nut to the min, then tighten until your gas tube clears. I read somewhere if one of the teeth on the nut is in the way, mark it, remove the nut and remove the offending tooth.

 

[gotigers]

Barrel nut torque for an AR is 30-80#. Rarely do I need more than 40# to align the nut.

Do not remove a tooth.

I've had a couple of stubborn barrel nuts. I just tried a different barrel nut. A milspec nut should align easily above 30#. I bought one at a gun show that would not align. I was hitting 80#. I tossed it in the trash and tried another. The milspec went on easily. Barrel nuts are cheap. Get a couple.

Tighten, loosen. repeat a couple of times. That will help.

Use a good moly grease on the threads. I wouldn't use Loctite or copper based anti seize.

I've never needed shims.

 

[MistWolf]

I know of one well respected AR armorer that will use shims to keep the barrel nut torque to around 40#. He says this gives him the most consistent accuracy

 

[Tirod]

Barrel nut shims were invented for freefloat designs that only had one channel for the gas tube to pass thru. It was a simple solution, but it takes installing it and taking it apart multiple times to get just right.

That's why the free floats that use the GI nut still sell well. No fuss no muss.

As for the optimum torque range, the minimum is 30 foot pounds, the maximum is 85 to prevent stripping the threads on the nose of the upper. I don't consider a range of 55 foot pounds to imply an "optimum" window. You just screw the thing down to keep it from coming apart. Too many other items affect accuracy far more than to worry about finessing the torque.

Milspec is 2MOA which is a ten inch circle at 500m. About half the center of mass on a human.Works on whitetail deer,tol. How much accuracy you need beyond that is an artificial requirement imposed to do something the AR15 wasn't invented to do, but it can do it well if that is your intent.

 

[MistWolf]

I don't consider a range of 55 foot pounds to imply an "optimum" window. You just screw the thing down to keep it from coming apart. Too many other items affect accuracy far more than to worry about finessing the torque

He's built, rebuilt and shot a lot of ARs. His experience is that he sees more consistent accuracy when the barrel nut is torqued to ~40 ft/lbs. It's not difficult to do, if you so desire