Hi all, new here. I have a question about receiver / rail alignment and what is considered acceptable. I recently finished assembly of a new middy upper. After it was done I noticed that the forend rail was sitting slightly below the receiver rail. Since most of my other uppers are perfectly aligned, I spent some time trying to correct it. I tried different barrel nuts, a different receiver, and finally a new rail. The only thing that stayed constant was the barrel itself. As it sits now, the forend top rail sits about 11 thousandths below the receiver rail. Is this pretty normal? Should I be bothered by it? It seems like more of a cosmetic issue I guess, but I really wish this lined up a little better. Especially since these are all good quality parts. Appreciate any input. Here is a pic to show what I mean:
Something could be out of spec. What brand is the upper receive and the rail?
Some of the two piece rails have to be help “up” as the screws are tightened down in order to get them to line up. I have used an A3 carry handle in the past to do this.
Thanks. I tried using a carry handle to line everything up, but once the bolts were snugged up it seemed to pull everything down to the way you see it. I considered out of spec part(s), but at this point I’ve swapped everything but the barrel. The parts in the pic consist of a Rainier Gen 1 upper (Mega I believe) and a Centurion 12.0. I then swapped the upper for one of the new BCM uppers (saw your thread on them) and a Fortis Rev 14. The BCM / Fortis set-up might be slightly better than the above pic, but still off. I’ll try and get a pic of it. It’s about .011 off.
Hmm, those components sound good. Before you bolt up the rail, Try pushing upward on it as you tighten down the A3. Put the end of the A3 (where the nut is) on the end of the rail and the rest on the upper receiver.
Thanks, I’ll give that a shot. Just to clarify, you’re saying to put most of the carry handle on the receiver, and just the very front part of it on the rail?
Ok, I’ll try that later on today. The BCM receiver is on there currently and the alignment was pretty much the same as the Rainier, so I think I’ve ruled out the receiver as the issue.
I have a similar problem with my DD RIS II rail lining up with my Colt upper… Not only is the rail just a tad left of the upper, there is also a noticeable gap. Any advice?
Left to right alignment should be an easy fix. My problem is a bit more complex since the top rail alignment is more a function of the parts themselves and not an adjustment. You just need to loosen the bolts on your DD and rotate it slightly and re-tighten. Try using a carry handle to bridge the gap to aid in alignment. I don’t think I’d worry much about the gap, it kinda is what it is on that. I assume the barrel nut was properly torqued and is still tight?
I was able to straighten out the rail but there’s still a small gap. If this is normal I’ll be able to get over it, but it has been bugging me for awhile now.
I just went and checked the DD rail on my BCM and there is a slight gap, about the same as the one in your picture on mine as well. It would not appear to affect anything other than perhaps aesthetics.
Of note, I used an ACOG mount to straighten my rail up to my upper when I installed it.
Well I tried re-mounting the rail using the carry handle again and there was no change really. It’s still about .010 off. I guess I’ll have to live with it. I considered maybe trying another rail that doesn’t use the factory barrel nut, but I’m not sure I want to spend any more money on this. Here is where it’s at right now:
You’re really splitting hairs here. Its not really that bad and probably unnoticeable and insignificant in general. If it doesn’t cause any malfunctions and your irons can be zeroed, I’d move on.
