It’s been a super busy year, and I’ve not been able to spend any decent amount of time trying to sight in my irons until today. I’m having an issue that I can’t sort out for the life of me.
My windage adjustment is cranked almost all the way to the left to hit dead center at 25 yds. I feel like I’ve triple checked everything: the sights appear to be mounted to the rails correctly. The rails on the upper and handguard seem to be aligned properly. The barrel appears to be straight and installed correctly. The sights don’t appear to be bent, and from what I can tell, there doesn’t seem to be any glaring issue in their manufacturing.
If you look at the rear sight from the top, it’s really obvious the aperture of the sight is way off of the center line of the upper.
Take a straight edge and make sure your rail is lined up straight with the upper, it could be canted. How many clicks left is your windage exactly from mechanical zero?
I’ve tried the straight edge. It’s REALLY close. I suppose it could be canted by a couple hundredths at the very most.
On the sight, there are 7 lines from mechanical zero to absolute left (0 being center, 7 being left) It’s on line 6. I believe each line represents 4 clicks. It’s literally almost all the way to the left.
it doesn’t take many degrees to throw windage way off. There is a possibility that the tube twists along its length depending on how long it is. Are you sighting in with the small aperture?
I had a this problem with a YHM rail. The first, and last, I ever purchased. The rail was aligned perfectly with the upper, but wasn’t machined correctly, i.e., the top of the rail was very slightly torqued or twisted at the muzzle end. I was able to determine this by removing the sights and placing the upper on a very flat surface- upside down, of course. Now mine was minor and didn’t quite require the amount of adjustment in the rear sight you’ve mentioned.
Edit- Probably shouldn’t mention it, but you’re sure the barrel nut is torqued correctly and fully seated in the upper? Jam nut too?
I don’t know which YHM rail you have (I’ll assume it is the one with the thick aluminum barrel nut and the the threaded rail that slips over it and is secured by the steel jam nut). We had those models on the BM carbines in Iraq. They were absolute shit and though we had a non-rail mounted front sight, I did configure a few just like that.
The rails were a pain in the ass to keep aligned and when guys mounted the slings on the their sometimes they would twist the rails. This was after I aligned and tightened them as best I could.
By just looking at it, it doesn’t look twisted - I haven’t had a sling mounted to it yet. But, I guess the machining could be off on the top rail where the sight is mounted.
I triple checked that the barrel was seated and the barrel nut is torqued correctly, and the jam nut is solid. What I can say is that the issue isn’t that the forearm isn’t twisting out of alignment from the rail on the upper. I am going to have a smith take a look at it before I disassemble it, but I’ll most likely try the flat surface trick to see if the machining is off
I had a smith look at it last night. If you hold the rifle vertically and look sort of down the top rail of the handguard towards the upper, it’s a little bit off. Maybe 1/64th" or so. I am a little leery that 1/64" off would make THAT big of a difference, but I guess we’ll see.
My next challenge will be to try to line it up. This may be tough with the YHM parts, as the handguard has set screws that are indexed into the barrel nut at a given interval - so I can’t just loosen the set screws and rotate it. What I can do is rotate the barrel nut a little bit (since it needs to be adjusted clockwise) but I need to verify that it won’t punch the gas tube. I think there is enough room to try it. That, or I need to sort out a different barrel nut/handguard combo…
Edit…I believe it’s not the barrel nut that it indexes in to, but rather the jam nut - if that’s the case, I may be able to tighten the jam nut the 1/64" without too much issue and the set screws can stay where they are…
I’m not sure what you mean by that, exactly, but the barrel nut is what actually sets the indexing for the whole unit: The closer to ‘perfect’ you have an opening in the bar.nut indexed/timed with the gas tube opening on the upper receiver, the closer to ‘perfect’ the alignment of the rail will be. I think the instructions tell you to tighten the jam nut all of the way and then install the anti-rotation screws, but it worked better for me to do it in reverse. I did ‘snug’ the jam nut up a bit first, but it was too hard to keep it lined up ‘perfectly’ if I tightened it up all of the way before putting in the anti-rotation screws. After installing the screws, I tightened up the jam nut the rest of the way, and it was solid as well as aligned.
If by chance it is that the rail itself is machined wrong, try loosening the jam nut, removing the anti-rot. screws, and rotate the forearm 180 degrees to see if the other side is in spec.
Sorry - I had that wrong. I just looked up the instructions.
I know for about 99% certain that the barrel nut is indexed perfectly with the gas tube. I will double check, but I believe it’s right dead center through the hole.
So I have a couple options, I guess:
What I’m probably going to have to try first is loosening up the jam nut slightly, and then rotating the handguard the 1/64th or what ever - and then tightening the jam nut back down - and seeing if the set screws still fit in the holes. I don’t know how much, if any, play they have. I thought about picking up one of those rail alignment tools, but for the time or two I’d use it, it’s not worth the $60. I think what I might be able to do is flip it upside down with the jam nut loosened and use the soft jaws on a vise to gently bring it in to alignment.
Secondly, I can try rotating the handguard 180 (or even 90, as it has rails on all four sides) to see if it’s more in spec.
Lastly, I can try to rotate the barrel nut a little bit, even thought I’m pretty sure it’s in alignment. Tweaking it a little bit may be the solution to it all lining up right.
There isn’t much play when just using the anti-rotation screws, if there is any at all. If the barrel nut is off by just a little bit, that could translate to the end of the rail being off/canted to where you have to compensate the windage further from the mechanical center/zero to make up for it.
It’s not ideal, but I used my detachable carry handle to hold the rail in place to check the fit and alignment.
ETA: Unless it is different than the YHM rifle-length ‘lightweight’ I have, the rail tube only has two holes for the anti-rot. screws, so 180 degrees is your only option.
Makes sense. I would say that the jam nut position could affect it slightly as well because it determines how close the guard can get to the barrel nut/front of the upper. But, with the set screws being the limiters, the jam nut won’t matter.
I think you’re right about the set screw holes - I hadn’t thought about that.
Will fiddle with the handguard/set screws/jam nut first, then will pull it all off to check the barrel nut alignment. But I swear it’s pretty dead on.
I know this isn’t a perfect world and you don’t want to hear this. Sell the YHM rail and get a good one. I have no idea what configuration you have but there are options like the Troy/ VTAC that are reasonably priced. Even Midwest Industries would be better than YHM.
I know some people don’t like the YHM rail, I had some problems with the install of my diamond rail, of course I didn’t follow directions. Your barrel nut is not lined up with the gas tube hole in the upper receiver, since the anti rotation screws go into the channels in the barrel nut it needs to be lined up perfectly or it will be canted, were the anti-rotation screws difficult to screw in at any point but not throughout the whole process?
When I finally got mine right it was a relief, it is solid and I like the size of it, they are cheaper than some of the more quality manufacture handgaurds but I don’t believe total crap if you know how to work 'em or learn the hard way to! though I will be keeping this rail I’m not confident in the aluminum barrel nut and I will be going with a DD lite rail on my next build.
though I will be keeping this rail I’m not confident in the aluminum barrel nut and I will be going with a DD lite rail on my next build.