I have successfully assembled many, many AR15 uppers from all kinds of parts, but I am scratching my head about this one.

BACKGROUND: About 18 months ago I removed a pretty well used BCM 18" medium contour barrel from a rifle to switch to a lighter profile barrel (18" Faxon Gunner); this new rifle went together fine. The old BCM barrel, which appeared perfectly OK except for being too heavy, was put away for some future use. Fast forward a year, and I went to install that old barrel on a new upper just to have a spare and to consume a bunch of parts (new and used) that I had lying around. I mated a new (billet) upper to the old barrel and an old Midwest industries SS15 handguard, installing with my usual technique (tighten/loosen 3 times then crank to the next gas tube hole). However, I noticed that the barrel did not sit centered in the muzzle end of the handguard - there was a clearly visible difference in the gap between the outside of the barrel and the inside of the FF handguard tube on one side versus the other (several mm). I tried removing the Midwest handguard and installing a new Troy handguard instead, but the barrel still leaned to one side. After some thought, I figured the barrel must have gotten bent somehow and so - having no pressing use for the upper - I shrugged and put all the parts to one side.

THIS IS WHAT IS DRIVING ME NUTS: Today I tried assembling another upper. I planned to use a new 16" Faxon Gunner barrel and new forged upper receiver (Brownells IIRC) with the same old Midwest SS15 handguard. When I got it together, I was astounded to see that this new barrel is ALSO visibly leaning to one side.  I even confirmed with a different upper receiver and a brand new Odin Works handguard and the barrel is still bent in the same direction.

It seems extremely implausible that two barrels from two different manufacturers could be bent by chance. The ONLY common component to all these issues is the old Midwest SS15 handguard (and, of course, me). Is it at all possible that the barrel nut of this handguard is somehow capable of deforming the chamber end of these barrels or changing the barrel-barrel extension relationship? This seems crazy because the Midwest barrel nut is aluminum so I would expect it to deform before the barrel does.

Or is it just that some several mm of imperfect centering of the barrel in the handguard is normal, and I have been lucky not seeing such a phenomenon before?

How far off is it?  I have seen built uppers and some factory uppers with rails installed off center by a 1 mm or so. Never much more than that though.

You don't have bent barrels, the handguard is not sitting correctly against the mating surface on the receiver, it probably needs to be trued up a small amount.

I believe the barrels are fine, it is the squareness of the front of the upper.
Brownells sells a kit to lap it...

Or send it back for a new one.

If it is only off a mm or so I would leave it. If it is off nore than that you could trueup the receiver.

It is off enough that I can see it clearly at the end of a 15" handguard. What concerns me is that the same barrel leans the same way with two different upper receivers and two different barrel nut/handguards.

Actual measurement? Are you off by an 1/8" or what?

Two different receivers and two different barrels...???

I don't know what it could be but the cause is looking like you since you are the only constant in the equation.

it's significantly possible that both of your upper receiver noses are tilted the same way, and need to be lapped or trued.  on all the ones that I've lapped, it's usually a coin toss whether it would've tilted left, or right.  the up/down, not too worried about.

this

quick and easy to fix