You've ordered a few tools from Brownells and you're assembling an AR from parts. It can be a lot of fun to build your own rifle and there are plenty of online tutorials and books available to show you how to do it. When it comes time to torque the barrel nut, you find that you have to really struggle to line up the gas tube hole.
If you loosen the nut to the previous hole, it seems too loose and if you apply a cheater bar to your wrench it feels like you're over-torqueing the nut. The problem could be that the face of the upper receiver isn't square or there is a burr so the nut feels like it's tight but the barrel shoulder is really only bearing on a small percentage of its seat.
Not to worry. Armorers long ago figured out a simple way to square the receiver face. We turned a piece of round stock to a diameter just shy of an inch so it would slide into the receiver and left a square shoulder that is used to apply lapping compound to the receiver face. A half-inch diameter cylinder is machined opposite the shoulder so the tool can be turned with an electric drill.
If you don't have a lathe you can buy one of these tools from Brownells (part #080-000-182) for $35. I have a lapping tool I made in the early 1990s that is very similar to the current Brownells product.
Secure the upper receiver in a vise. I used a Precision Reflex upper receiver vise block (714-000-006) to hold the one in the pictures. Lubricate the main shaft of the tool and make sure it will rotate freely in the upper receiver.
Apply lapping compound to the shoulder of the tool. Brownell's recommends 180-grit but the coarsest grade I had on hand was 500-grit and it worked fine. Try not to get any compound down into the receiver. Turn the tool with a drill, checking your work periodically and replacing compound as necessary.
Starting out you will notice that only a portion of the receiver face is turning silver as the anodizing is worn away. That is the high spot we are going to remove. Keep working until the entire ring is silver and smooth. Clean up the receiver and try torqueing the barrel nut again. Easier, isn't it? The barrel shoulder is now bearing evenly on the receiver face the way it should be.
Norcross made his own squaring tool in the 1990s (top). The Brownells version below it gets the same job done, and you can buy one for about $35.
Insert the lubricated pilot of the tool into the receiver and apply lapping compound to the tool's working surface. Don't get it inside the receiver.
Secure the receiver
Secure the receiver in a vise and drive the tool with a drill motor to lap the receiver face. Check periodically and replace compound when needed.
After some lapping, it's easy to see the high spot on the receiver that is preventing the barrel nut from snugging down to its proper final position.
Finished receiver face
The finished receiver face is smooth and square. Now it will be easy to turn the barrel nut to proper orientation. The gas tube will fit right through.