Ayan is a hell of a guy!
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3 factors are at play:
Relationship of the barrel to the rail
Relationship of the rail to the upper receiver
Relationship of the barrel to the upper receiver
There are factors that effect each of those.
Tolerance stacking in a few minor areas, or a significant deviation in one single area can result in the barrel not being perfectly centered in a floating handguard; without having any effect on performance.
These issues are pretty much never a factor in a non-ff handguard because deviation is masked by connecting the handguard to the barrel. A system that never before cared if the barrel extension perfectly flushed out on the front of the upper receiver is now expected to demonstrate virtually no deviation there. The system never functionally cared if the top of the upper receiver was perfectly parallel to the barrel. All of these legacy aspects can combine with application of torque to cause the barrel to not sit perfectly centered in an otherwise perfectly functional gun with quality parts all around.
From Ayan's description, it sounds like the receiver rail and URX rail don't quite agree on what "parallel to the barrel" is.
Well mine came back a couple days ago, Rainier had it turned around in like 2 days. I wont be home 'till next week, but had some pics sent to me and it looks perfectly aligned. Colt upper, Compass lake Krieger barrel, and BCM BCG so it was quality parts.
I'm going to say it again.. machine the end of the URX rail.
My recent SBR build had the same issue, just not as extreme.
Everything sits perfectly centered now after the back of the rail was cleaned up. I can't post pics because the parts are getting cerakoted right now.
I got an AAC MPW today with the URX 3.1 factory installed and it's noticeably crooked as well, but to the opposite side of the OP's rifle. I haven't shot it yet so I don't know how many clicks it would take to zero the irons. What's the maximum number of clicks from center that would be considered acceptable?
My SR15 IWS Mod. 1 upper has the same rail, but I cannot detect any crookedness by eyeballing it. In addition, the rail sits flush to the upper receiver, unlike the MPW which leaves a significant gap.
An update on my URX upper. Ayan @ boltcarrier.com took the time to hand fit 3 different upper receivers I sent him and found one that mated up perfectly with a new rail as the existing one had been beaten up from wrenching it apart several times. It was an AO precision upper with a rectangular forge marking for anyone whose curious. I received the perfectly installed railed upper a few days back and will hopefully get a chance to run some rounds through it soon. Many thanks to Ayan who will get all of my AR related business going forward.
Rob