Have you ever heard of bending an Upper receiver?

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That pin, by the way, is also used to help align the barrel nut. No?

Yep. They are using a bent gas tube shoved through a hole drilled through the charging handle channel's indexing spline. It'd make assembly as easy as can possibly be. I saw that and slapped my head. It's truly an awesome tool IMHO.

Just keep a tiny bit of forward tension against that tube, and as soon as the barrel nut lines up enough it'd pop through and tell you you were home.

That tool would make upper receiver building a "point, set, and match".