View Full Version : Interesting issue with my KAC URX 3.1 build
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
http://i138.photobucket.com/albums/q260/robe2103/photo8_zps3cddb36f.jpg (http://s138.photobucket.com/user/robe2103/media/photo8_zps3cddb36f.jpg.html)
Shot of barrel in rail- Note the left to barrel clearance looks smaller than the right to barrel near the gas block
http://i138.photobucket.com/albums/q260/robe2103/photo9_zpsc999e847.jpg (http://s138.photobucket.com/user/robe2103/media/photo9_zpsc999e847.jpg.html)
crosseyedshooter
06-12-13, 20:50
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.
crosseyedshooter
06-12-13, 21:54
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?
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.
15 clicks isn't excessive, regardless of your taste.
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 is a known issue with KAC URX rails. You must test fit the parts before assembling.
You might have to try several uppers before you get alignment.
This is a known issue with KAC URX rails. You must test fit the parts before assembling.
You might have to try several uppers before you get alignment.
That is probably true. I may just go with another rail setup.
This is a known issue with KAC URX rails. You must test fit the parts before assembling.
You might have to try several uppers before you get alignment.
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.
davidjinks
06-13-13, 10:24
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.
This is a known issue with KAC URX rails. You must test fit the parts before assembling.
You might have to try several uppers before you get alignment.
davidjinks
06-13-13, 10:29
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)?
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.
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)?
Iirc, vltor murs are known for their tolerances.
Iirc, vltor murs are known for their tolerances.
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.
David, who assembled your upper?
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
http://i138.photobucket.com/albums/q260/robe2103/photo8_zps3cddb36f.jpg (http://s138.photobucket.com/user/robe2103/media/photo8_zps3cddb36f.jpg.html)
Shot of barrel in rail- Note the left to barrel clearance looks smaller than the right to barrel near the gas block
http://i138.photobucket.com/albums/q260/robe2103/photo9_zpsc999e847.jpg (http://s138.photobucket.com/user/robe2103/media/photo9_zpsc999e847.jpg.html)
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.
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.
This has been my experience as well.
This has been my experience as well.
I'm wondering how the upper and rail were secured during installation?
I'm wondering how the upper and rail were secured during installation?
The PRI rail vice block was used.
The PRI rail vice block was used.
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.
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.
I used a 40 year old American made vice my dad bolted to his hand made solid as hell work bench so the vice isn't the problem either. The one tool i didnt purchase was the KAC vice grip tool because $350 was just a bit much for a rail install tool. I still am leaning toward an issue with either the rail itself or the upper being out of spec. I'll see upon disassembly i guess.
I used a 40 year old American made vice my dad bolted to his hand made solid as hell work bench so the vice isn't the problem either. The one tool i didnt purchase was the KAC vice grip tool because $350 was just a bit much for a rail install tool. I still am leaning toward an issue with either the rail itself or the upper being out of spec. I'll see upon disassembly i guess.
The problem is that the PRI clamp isn't really what you should be using. That combined with tolerance differences between the threads are probably going to mean you will continue to have an issue. You can try different uppers and dry / test fit everything before applying loctite until you find a good match OR send the upper to Ayan for install. His services are very reasonably priced. I know that you spent good money on the wrench and want to do it yourself, but you can easily recoup the funds in the secondary market.
IMHO, KAC never should have started selling the wrench. They caved into pressure and started selling it, I understand why. Unfortunately, without a good fixture to secure everything, it's a total crap shoot as to how the rail will align.
CharlieMike
06-15-13, 07:04
Frankly the PRI vise fixture is the wrong thing to use so it is difficult to know if it is a rail/upper tolerance stacking issue vs an incorrect installation.
I think the best DIY tool for URX installation is the YHM Upper Receiver Picatinny Rail Vise Jaws (http://www.midwayusa.com/product/425110/yankee-hill-machine-upper-receiver-picatinny-rail-vise-jaws-ar-15).
Still, if the face of the barrel extension seems flat and well aligned with respect to the inside of the upper (feed ramps align etc.) then you're probably good to go. It looks good -- I've seen much worse. :^)
Dang. I just sent off the parts for my upper to be assembled by rainier arms with a 3.1 rail, including a krieger SS from compass lake and and a colt upper from SAW
At least they have a good rep, and claimed to have all the parts to properly assemble it. I'll see for sure when it gets back.
Dang. I just sent off the parts for my upper to be assembled by rainier arms with a 3.1 rail, including a krieger SS from compass lake and and a colt upper from SAW
At least they have a good rep, and claimed to have all the parts to properly assemble it. I'll see for sure when it gets back.
I didn't know that Rainier did URX work. Cool.
Ayan's type of vice block works best.
The URX can "kick off" due to several reasons - play between the threads of the URX and the Upper, or being pushed by the URX rubbing on the Upper.
The other issue that can cause this is that the upper it self may not be true where the barrel extension mates with it - and the barrel is kicking over.
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.
+1... No way in hell the PRI vice block combo thing can hold the URX & upper receiver in alignment. you may need several upper receivers to test fit, and then you still may have to machine the end of the URX to get it timed flush. Then Cerakote the machined area... I think the finished product is worth it though.
CharlieMike
06-21-13, 14:42
Just FYI, I took the upper apart and reassembled it and I can reproduce the problem. I think the OP did a fine job installing the rail -- maybe slightly excessive loctite :^) -- but I don't think the issue was his fault.
The rail is perfectly aligned while it is in my vise but the moment I take it out, it skews about 6/100" measured near the end of the rail.
I think I have a fix for it. We'll see in the morning.
-ayan
Just FYI, I took the upper apart and reassembled it and I can reproduce the problem. I think the OP did a fine job installing the rail -- maybe slightly excessive loctite :^) -- but I don't think the issue was his fault.
The rail is perfectly aligned while it is in my vise but the moment I take it out, it skews about 6/100" measured near the end of the rail.
I think I have a fix for it. We'll see in the morning.
-ayan
Ayan, please update us on your fix. Very interested in seeing what you find out.
Just FYI, I took the upper apart and reassembled it and I can reproduce the problem. I think the OP did a fine job installing the rail -- maybe slightly excessive loctite :^) -- but I don't think the issue was his fault.
The rail is perfectly aligned while it is in my vise but the moment I take it out, it skews about 6/100" measured near the end of the rail.
I think I have a fix for it. We'll see in the morning.
-ayan
I'd just like to take a second to thank Ayan for contacting me and offering to help out with the upper. He is truly a class act and I'll surely be doing business with him going forward.
Rob
I'd just like to take a second to thank Ayan for contacting me and offering to help out with the upper. He is truly a class act and I'll surely be doing business with him going forward.
Rob
Ayan is a hell of a guy!
Failure2Stop
06-21-13, 15:47
Just FYI, I took the upper apart and reassembled it and I can reproduce the problem. I think the OP did a fine job installing the rail -- maybe slightly excessive loctite :^) -- but I don't think the issue was his fault.
The rail is perfectly aligned while it is in my vise but the moment I take it out, it skews about 6/100" measured near the end of the rail.
I think I have a fix for it. We'll see in the morning.
-ayan
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.
That is probably true. I may just go with another rail setup.
Let me know when you do, I'll take that "terrible out-of-spec" URX 3.1 off your hands.
davidjinks
06-21-13, 16:44
I'm pretty sure you meant this as sarcasm, however some people may get the wrong impression of your intent. I don't believe anything is out of spec in regards to the rail, upper or the whole enchilada.
Let me know when you do, I'll take that "terrible out-of-spec" URX 3.1 off your hands.
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'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.
Who machined the rail for you? Did you take enough off to flush it up with the upper receiver and "time" it?
Who machined the rail for you? Did you take enough off to flush it up with the upper receiver and "time" it?
I did, and yes of course it is timed with the upper receiver for a flush fit. :cool:
jumbopanda
06-28-13, 20:39
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
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