Indexing AR-15 barrel to extension

[FelixEstrella]

Any hints on the correct method for indexing an AR-15 barrel? Visually lining up the gas hole with the extension index hole, after stretching a piece of masking tape along the top centerline of the barrel seems to be all I can think of without specialized tools.

[workinwifdakids]

The best way I've found is to apply torque until you can't tighten any more, and then back it off until it looks good to your eye. When you attach the gas tube to the block, if it won't index properly into the upper receiver, you can go clockwise or counterclockwise (depending on how much resistance it's showing) until the gas tube slips into the receiver properly.

In short, line-of-sight is good enough. The masking tape is a good idea, though.

[FelixEstrella]

The best way I've found is to apply torque until you can't tighten any more, and then back it off until it looks good to your eye. When you attach the gas tube to the block, if it won't index properly into the upper receiver, you can go clockwise or counterclockwise (depending on how much resistance it's showing) until the gas tube slips into the receiver properly.

I think you're referring to indexing the barrel nut to the upper receiver.

I'm referring to indexing the barrel extension to the barrel. If this isn't done right, the sight tower won't be vertical when the barrel is installed into the upper receiver.