“I couldn’t hit it sideways”

[zeleny]

So I built an AR-15 and an AR-10 (https://photos.app.goo.gl/VgAn4ZmVhVkV5PlI3) with JP Rifles’ components on Turnbull case hardened steel receivers. The AR-15 is dimensioned milspec and I got it to run like a top and shoot 55gr Armscor ammo into a single hole at 50 yards, so far. Not so with the AR-10 that wears an extra high Picatinny rail. I have tried two scope setups, a USO MST-100 in 30mm steel Badger Ordnance rings with an inch insert, and a Schmidt & Bender 3-12x50 PM II in 34mm steel Badger Ordnance rings. Each runs out of windage adjustments before I can align the point of aim with the point of impact at 50 yards. All optics are mounted on the upper receiver rail. The rifle runs fine otherwise.
23697
I am conjecturing two possibilities:

The axis of the barrel threads is misaligned with the axis of the bolt carrier;
The axis of the Picatinny rail runs in a plane different from that of the axis of the barrel.

How can I measure these alignments?

[schüler]

I've seen similar issues when an inclined rail is combined with an inclined uni-mount but that's not what you have. I assume the rings/inserts are level plane? Some inserts will allow inclination.

I would think a 50yd zero would still be doable... but have you tried 100yd?

A cleaning rod in barrel with a cleaning rod or orher straight edge placed flat on receiver rail might indicate gross angular differences.

Collimator or bore laser might be helpful.

[gtae07]

My first take would be to detach the upper, pull the BCG, and sight down the bore at a convenient reference point while the upper is held in or resting on something. Then eyeball the rail and see if it's anywhere close.

This is how I boresight new sights before taking the gun to the range. I also have a laser boresighter tool that I can use at home. Both should at least get you on paper at 25 yards, then you can back out as desired. Any gross misalignment should be visible this way.

[StraitR]

A bore laser would no doubt tell the tale at distance, but I would think that much runout should be noticeable when looking at how centered the barrel is in the handguard, if in fact the upper and handguard are square. Run a straight edge (steel yard stick) down each side of the receiver and rail to confirm they are square.

Easy test, given it won't zero at 50, is to separate the upper/lower, remove the BCG, set the upper on something sturdy and sight a target at distance through the bore, then compare to what you see in the scope.