Centurion C4 & URG top rails wont line up

[Aiden Fontana]

I cant seem to get the top rail on my C4 handguard to interface with the top rail on my upper. I've taken it off 2 or 3 times and it will not line up properly. Could the fact that I'm using a BUIS to connect the rails be an issue? (it's a LaRue A1 fixed QD sight) When I throw it on with the rail loose it interfaces, but as soon as I tighten the handguard down, the QD lever on the sight won't budge, and I have to loosen the handguard screws and try again. I was wondering If anyone's had a similar problem that can give me some advice. Thanks.

[GH41]

Did you follow the C4 instructions? Not tightening the screws in a criss cross pattern is probably your problem. Using something longer than a sight like a carrying handle will help to.

[Iraqgunz]

Proper barrel nut alignment is important.

At this time it is advisable to secure the upper in a vise or an otherwise hands free arrangement. Insert the screws from the top and tighten the 2 screws closest to the receiver first. (Do not over-tighten screws. Screws should be just about hand tight). Then tighten the 2 screws closest to the rail. (Again do not over tighten). Once all screws are in place begin to tighten them in alternating criss-cross pattern. Make sure that the rail is aligned with the upper receiver assembly. All screws should now be complete as tight as possible by hand and rail should not move.

[Dionysusigma]

I had this issue too, and can confirm Iraqgunz's excerpt (directly from the installation instructions) were what I didn't do, the first time. :moil:

After taking the whole thing off and starting over, I even followed suit with the screws further along in the side rails as well, and now everything is right where it should be. I used a Magpul XTM cover to help out, which is much less rigid than your BUIS. I went [forward left] 1/4 turn, [rear right] 1/4 turn, [rear left] 1/4 turn, [forward right] 1/4 turn (and over and over) until I felt one of them tighten down fully. Skipping that one, I kept up with the others (omitting the tight one) and when another was tight, skipped it in its cycle too. At the end of it all, I went through again and checked each one (keeping up the crossing-pattern) to double check, and it lined up perfectly.

[Iraqgunz]

I actually compiled and wrote that for Centurion. I use a TA51 mount for most rail alignments and it works fine.


I had this issue too, and can confirm Iraqgunz's excerpt (directly from the installation instructions) were what I didn't do, the first time. :moil:

After taking the whole thing off and starting over, I even followed suit with the screws further along in the side rails as well, and now everything is right where it should be. I used a Magpul XTM cover to help out, which is much less rigid than your BUIS. I went [forward left] 1/4 turn, [rear right] 1/4 turn, [rear left] 1/4 turn, [forward right] 1/4 turn (and over and over) until I felt one of them tighten down fully. Skipping that one, I kept up with the others (omitting the tight one) and when another was tight, skipped it in its cycle too. At the end of it all, I went through again and checked each one (keeping up the crossing-pattern) to double check, and it lined up perfectly.

[Amicus]

I actually compiled and wrote that for Centurion. I use a TA51 mount for most rail alignments and it works fine.

While my experience can not compare with IG's, I followed the instructions and (as is my habit) used a TA51 to assist alignment. My C4 went on just fine. A removable carry handle should also work well -- the longer the clamp, the better for alignment purposes.

[Dionysusigma]

I actually compiled and wrote that for Centurion.

Huh, never knew that... thanks, then!