Bull Barrel Shims

[STS556]

I installed a DPMS stainless barrel on my new build. Actually just getting around to finishing up my FUAC lower. The barrel is just over an inch where the barrel nut is. The teeth on the barrel nut are just slightly off and I can't install the gas tube, and the hand guard is slightly canted. A regular shim won't fit and I cant seem to find any shims online for a bull barrel. If anyone here has a bull barrel and needed shims, what did you find that worked for shims?

 

[JamieZ]

You dont need shims for a bull barrel,,,,you need regular old barrel shims,,,,,,,,,,,,arent all barrel extensions the same size since they all fit into the same size milspec upper? You just cut the shim and put it on, then torque your nut.

Heres a video I found explaining it.

AR-15 Barrel Nut Shim Set

bisonarmory.com

 

[STS556]

That video shows putting shims on the barrel on the receiver side. Won't that change how things line up? Like the gas tube being too short at the BCG and the head space? I have always put the shims when needed between the barrel nut and flange.

The barrel I have is 1.050" at the receiver and tapers down to .936 from the gas block to the crown.

 

[Mosinitis]

You don't need any shims.

 

[JamieZ]

That video shows putting shims on the barrel on the receiver side. Won't that change how things line up? Like the gas tube being too short at the BCG and the head space? I have always put the shims when needed between the barrel nut and flange.

The barrel I have is 1.050" at the receiver and tapers down to .936 from the gas block to the crown.

No

 

[dsdmmat]

http://ar15barrels.com/data/barrel-nut-index.pdf

they go under the nut on top of the barrel extensions shoulder.

 

[JamieZ]

NOTE: Use of these shims will NOT affect bolt / barrel headspace

A blurb from the bison armory website I posted

 

[GUNSICK]

Did you lube the threads up? Some Aeroshell 33ms should help getting it torqued down a little more.

 

[dsdmmat]

A blurb from the bison armory website I posted

While that is true because the distance Between the bolt face and the barrel is not changing. Placing the shims on the receiver face does change where the gas port is in relation to where the bolt carrier is when the bolt is in full lock up and the length the bolt has to travel to be in full lock up. Now it may not be enough to affect the function of the rifle but It does move everything outward.

 

[JamieZ]

While that is true because the distance Between the bolt face and the barrel is not changing. Placing the shims on the receiver face does change where the gas port is in relation to where the bolt carrier is when the bolt is in full lock up and the length the bolt has to travel to be in full lock up. Now it may not be enough to affect the function of the rifle but It does move everything outward.

If it affects it, it would have to be the biggest piece of junk bought on the internet and there would have to be a gas block issue causing it more than the gas tube going into the gas key a certain amount of distance.
Your gas tube goes into the gas key on the BCG roughly an inch plus.

Hope I understood what you're saying correctly

 

[dsdmmat]

If it affects it, it would have to be the biggest piece of junk bought on the internet and there would have to be a gas block issue causing it more than the gas tube going into the gas key a certain amount of distance.
Your gas tube goes into the gas key on the BCG roughly an inch plus.

Hope I understood what your saying correctly

The AR has a lot of generous tolerances, changing things will usually only become an issue when every part you have is on one extreme or the other of those tolerances ( tolerance stacking). You are correct you would have to have a lot of things going against you for the shim on the receiver to affect your rifle in a negative way.

Shims on the receiver face change the measurements I mentioned, Shims beneath the barrel nut do not. As stated .001-.004 may not be enough to affect the function of the rifle but, it does change those measurements I mentioned, that is all I am saying.

I prefer not to mess with those tolerances and just go with the shim (when needed) under the nut.

 

[Robin]

Wouldn't placing the shim on the receiver face affect head spacing? I know its a negligible amount but that is a critical measurement. Outside is where it belongs, heck all you want to do is change the barrel nut stopping point at torque. I'm no expert and have only done a few or more but it seems like a no brainer to me.
Robin

 

[dsdmmat]

Wouldn't placing the shim on the receiver face affect head spacing? I know its a negligible amount but that is a critical measurement. Outside is where it belongs, heck all you want to do is change the barrel nut stopping point at torque. I'm no expert and have only done a few or more but it seems like a no brainer to me.
Robin

No the headspace is not changing because it is the relationship between the bolt face, bolt lugs locking on the barrel extension and barrel face. That measurement is not changing. The measurement that is changing is the travel of the bolt to get into the locked position.

 

[STS556]

Did you lube the threads up? Some Aeroshell 33ms should help getting it torqued down a little more.

Lubed the threads good.

 

[STS556]

I ended up torquing the barrel nut to around 60 in-lb. I didn't want to go any further because I was afraid of cracking or stripping the receiver. It still didn't line up very well. I lined the nut up as close as possible and trimmed the teeth with a small stone on the Dremel. I trimmed a tooth where the gas tube goes and a couple of teeth where the tabs for the hand guard go. All is lined up now. Thanks for the reply's all.

 

[Mosinitis]

If it is close to lining up but doesn't, unscrew it and re-tighten. Repeat if needed. Each time it will go further and further. 35-75 or even more torque is fine. Some believe the more the better.
Use clamshell vise block, don't use reaction rod or all bets are off.