DIY Upper- How Hard

[pinzgauer]

I just want to get into making sure my upper receiver extension face is square to the barrel extension flange, and that those are square to the bore axis. Aside from accuracy, I think these are important for reliability and bolt life, as imbalanced loads on the lugs will shear them quickly when the barrel is not square with the upper.

What are your suggestions for purchasing or making a tool that checks for upper receiver extension face squareness? I've been thinking about a tool that fits inside the upper raceway from the front, and can turn on the face of the extension. If you wanted to get really picky, you would true the inner flange of the barrel nut, so it will apply evenly-distributed compressive force against the barrel extension flange and the receiver extension face.

I'll defer to others with more experience on this, as I've not done this on my builds. From talking with folks and reading, the jury is out how much it really helps if you are using in spec components. If you somehow end up with out of spec components it can make a difference.

In general, the theme is that it cannot hurt, and if you are grossly out of spec it will help.

Others indicate that the face of an in-spec upper receiver (what you would hone) deforms as needed when torqued, so it's an unneccesary step which just removes your hardened surface, etc. Which somewhat makes sense. There are also debates around whether the barrel extension can even cant enough if in-spec to to allow unequal bolt lug loading. So who knows.

The Brownells tool (or similar) appears to be what nearly everyone uses, and it's not expensive. But if you are going to go to that trouble, it does seem like you'd want to true up the flange as well just to be sure.

As for a gauge, the trick would be a block that goes in the receiver, with a rotating arm that holds a dial indicator on the face. you'd have to make sure the tool itself did not have play in it. Or the poor man's equiv would be to mount your barrel into the receiver under slight pressure, but not via barrel nut. Then look for gap with feeler gauge, etc.

If I ever do a build with a premium barrel I'll probably make the attempt. But for now I can safely say that unsquare receivers have not been a limiting factor in my builds! (Watch me break a bolt lug tomorrow from jinxing myself)

I've seen some discussion around trying to check trueness of an existing mounted barrel, usually involving longbed mills.