AR-15 barrel indexing

[Slater]

Some folks have had issues with the barrels on their AR-15's being improperly indexed, so that the rear sight has to be cranked all the way to one side in order to get a zero.

Would this be evident when looking through the sights or is it something that isn't readily detectable?

 

[Omaha-BeenGlockin]

I've always torqued my barrels to spec and set up A2 sights in the center and never had a problem.

I'd say re-torque the barrel.

 

[Shawn Dodson]

No, you won't detect it by looking through the sights.

Use a barrel block (to hold the barrel) instead of a receiver block (which holds the receiver) when tightening and torquing the barrel nut.

Apply molyb anti-seize compound (available from Bushmaster) to threads on upper receiver. Torque barrel nut three times to max value (80 ft-lbs). Loosen after each torque. Then set torque wrench to minimum torque value (30 ft-lbs) and torque barrel nut. If barrel nut slot does not line up with gas tube receptacle, set torque wrench to 80 ft-lbs and torque until slot lines up or max torque value is reached. If max torque is reached and slot doesn't line up, replace barrel nut and repeat process.

 

[Slater]

So even if the front sight picture seems aligned correctly (not canted) when looking through the rear sight, there could still be a potential problem?

 

[Shawn Dodson]

Yes. It's unlikely that you're going to detect it until you shoot it, unless it already has been sighted in and the rear sight has been cranked way off center.

 

[Toten Kopf]

Do not use a barrel block when torqueing the barrel nut. That is the "old" way of doing it. And if anything, you can damage the index slot using that method. When using this method the receiver will want to "torque" in the direction of tightening.

However, use the barrel block to install the flash hider/muzzle brake instead of the receiver block. This will keep the barrel from moving during installation and won't have any effect on the receiver. The barrel block is positioned on the rear portion of the barrel, not the front (exposed) portion.

Use a receiver block for installing the barrel. This way provides less "torque" on the receiver (which is softer).

Also, I use a sight indexing rod to ensure correct alignment with the receiver and the barrel. This is a precision machined rod that lays on top of the carrying handle. The pointed end aligns with the front sight. As long as the pointed end of the rod stays in alignment with the front sight during barrel instrallation you will have perfect receiver to front sight alignment.

 

[Slater]

I've heard folks saying that Bushmaster used to have issues with this. Have they improved in that regard?

 

[Shawn Dodson]

Do not use a barrel block when torqueing the barrel nut...using this method the receiver will want to "torque" in the direction of tightening.

Likewise when using a receiver block the barrel rotates in the direction of torquing.

If you're damaging the receiver then you're overtorquing the barrel nut.

Cheers!