FrankenMauser
New member
You shim the joint between the barrel extension and the receiver with round shims specifically made for the job.
There are many places that sell them, and even a few different versions. This is just one example: Bison Armory AR-15 shim set
Each of those shims effectively adds 0.001" to the length of the barrel boss on the upper receiver, which stops your barrel nut slightly more than 1/3 of a notch/hole short of where it torqued without the shim. It lets you achieve alignment for the gas tube, with the torque setting you want to use. ....Or it lets you get a proper torque value with barrel nut - barrel extension - upper receiver combinations that are just stubborn and won't align otherwise.
There are many places that sell them, and even a few different versions. This is just one example: Bison Armory AR-15 shim set
Each of those shims effectively adds 0.001" to the length of the barrel boss on the upper receiver, which stops your barrel nut slightly more than 1/3 of a notch/hole short of where it torqued without the shim. It lets you achieve alignment for the gas tube, with the torque setting you want to use. ....Or it lets you get a proper torque value with barrel nut - barrel extension - upper receiver combinations that are just stubborn and won't align otherwise.