I took my newly assembled upper out to zero over the weekend and quickly realized something was not right. I needed upwards of 15 + clicks of right windage to zero which while not extremely excessive was too much for my taste.
My first thought was that the upper and rail were out of alignment but this was not the case. I used a rail vice block while assembling with loctite and upon inspection the rail is still inline with the upper. I mounted a carry handle across to verify and it fits perfectly so it’s not the rail alignment to the upper that’s the issue.
I then started looking at the gas block and it’s relationship to the rail. It appears the gas block looks to be closer to the left side of the rail and possibly ever so slightly rotated to the left as well. I’ve included a
pic of the gas block looking down the barrel.
My question is…what part is out of spec? The upper? The URX 3.1? It’s driving me a little crazy trying to see just what is out of alignment!
Rob
Parts:
BCM Middie 16" Govt with BCM low pro gas block
Aero Precision upper receiver
Shot of barrel in rail- Note the left to barrel clearance looks smaller than the right to barrel near the gas block
I’m not an expert, but just a thought; since you aligned the rails with an alignment block, the receiver and rail should be aligned. That can only mean the barrel isn’t seated in the receiver quite right or something is bent. I guess this is why the folks who depend on their iron sights go for the pinned FSP. They don’t have to worry about barrel-to-front-sight alignment.
In the case of a FF hand guard and rail-mounted front iron sights, all three parts, receiver (rear sight), barrel and rail (front sight) should be in alignment with each other or you’ll have to adjust for it in the sights.
Do you have a vise and receiver block? I’m not sure how to check barrel to receiver alignment, but it could be that your barrel isn’t fully seated into the receiver. Maybe hang your receiver vertically and use a plumb line to check whether the barrel or rail is crooked?
I do have a vise and receiver block. I can’t see how the barrel isn’t fully seated as looking at the chamber all is good and there is the recommended 50 ft lbs of torque applied to the KAC Nut which was loctited and progressively achieved during the install.
If you want to see what’s out of alignment, use a carpenter’s square against the upper’s rail. Just eyeballing it I can see the gas block looks to the right of the barrel nut. But can’t tell if it’s the pic not being square or the upper itself.
This. The issue arises due to tolerances in the upper receiver threads and the urx threads. You may have to try multiple upper receivers to get better alignment, but as IG noted, 15 clicks is within acceptable limits.
This is some interesting information. I did not know this.
I’ve been going through the same issue with an upper I had built with a URX 3.0 rail. My barrel was way more off than the OPs and my cut FSB was making contact with the rail while shooting it. At 100 yards I stopped adjusting after 18 clicks left.
Is there an upper receiver that isn’t affected by this? Meaning, are these tolerance issue more prevalent with a colt upper vs. a noveske upper (Just throwing those out there not being specific)?
I’ve had very good luck with MUR uppers. Two upper builds with URX rails and minimal windage adjustments needed for the irons. I had them installed by professional armorers though. I’d have to be installing alot of urx rails to justify the cost of setting up to do it properly, as it involves alot more than just buying the wrench, IMHO. Ayan at boltcarrier.com does a very good job.
OP, your situation is a little different than what I’ve seen in the past. Usually, the gas block sits more to the left (when viewed from the muzzle). Odd.
IMHO the PRI rail block is not an adequate solution to mount a URX rail correctly… Everything - the rail, the upper, the block - flexes if you apply more than 40-45ft/bl of torque to the barrel nut.
I would tend to agree. I know alot of guys have made it work, but the KAC vise block or a set of custom vise jaws like Ayan uses are really needed to keep everything straight during installation.
You need real old school american made vice for this with atleast a 6" jaw. An old school one where the jaws are straight and no play, using the KAC fixture is good.
I use an old Columbian vice that belonged to my great grandfather that never really got used for anything and it is still like new and straight.
A mill vice with custom jaws like what Ayan uses is also good.
Harbor Freight / Lowes / Home Depot china shit isn’t going to cut it for this.
For a URX install, everything needs to be perfect. You need a high end barrel, a name brand flattop receiver, a real vice, and the KAC tools.
