still trouble with c4 fsp rail

[castle]

i mentioned in my "updates to 6920" thread, that my centurion c4 rail was canted quite a bit at first. i removed it and reinstalled using the lug nut style alternating method for tightening the screws that clamp the bbl nut and it's much better, but the bbl is still not dead center in the rail at the end beyond the fsb. i've redone it about 6 times now, even holding pressure on the rail to keep it centered while i tighten the screws and still can't get it right? any thoughts? might the rail be machined slightly off? the bbl nut hasn't ever been off of my 6920. is it possible there's an issue with the bbl nut itself?

i'm not sure it matters much, as i likely won't mount any optics that far forward on the rail, but it bugs me.

any help is greatly appreciated.

[Failure2Stop]

Is the rail touching the gas block?
Got a pic?
Did you thoroughly clean the barrel nut and mounting area of the rail?
Is the HG top rail in alignment with the receiver rail?

[RichDC2]

I have used a carry handle half on the receiver and half on the rail before its tightened it down. Is the rail in contact with the bayonet lug or anything beside the barrel nut?

[castle]

i'll get a pic up shortly. it's not touching the fsb at any point that i can tell, although it's pretty damn close to contacting it at the receiver end. the rail is in alignment with the rail on the receiver perfectly. everything was cleaned well prior to the install. but i can take it off and check again.

[castle]

i could try the carry handle idea. i just figured that it would naturally end up where it should be after tightening the screws properly. even with the amount of force i was overcorrecting it with by hand it still ended up canted.

[Robb Jensen]

Your barrel nut might be canted or you might need to try another barrel nut. When I install rails like the Centurion, Troy, DD Lite/RIS II I use a Knights Armament alignment block designed for URX2-3 rails it holds the flat tops rail and the 12 o'clock rail in alignment while I tighten the rail down. It works very well.

[castle]

there's a 2 mm difference from one side to the other...

http://farm8.staticflickr.com/7061/6953879577_8b84652669_b.jpg
http://farm8.staticflickr.com/7192/6953874487_7345174132_b.jpg
http://farm8.staticflickr.com/7070/6953874477_84de67a1a1_b.jpg
http://farm8.staticflickr.com/7069/6953874463_c51b584a40_b.jpg
http://farm8.staticflickr.com/7065/6953874471_a292c6a3f3_b.jpg
http://farm8.staticflickr.com/7185/6953874451_18930da458_b.jpg

[castle]

robb jensen, how could i determine if the bbl nut is canted? would that mean it wasn't threaded on properly?

[Robb Jensen]

It could be a barrel nut that is slightly out of spec or a receiver that isn't square, the first thing I would try would be another barrel nut. And a means for holding the receiver and rail in alignment while tightening the fastening nuts.

[RD62]

there's a 2 mm difference from one side to the other...

http://farm8.staticflickr.com/7061/6953879577_8b84652669_b.jpg
http://farm8.staticflickr.com/7192/6953874487_7345174132_b.jpg
http://farm8.staticflickr.com/7070/6953874477_84de67a1a1_b.jpg
http://farm8.staticflickr.com/7069/6953874463_c51b584a40_b.jpg
http://farm8.staticflickr.com/7065/6953874471_a292c6a3f3_b.jpg
http://farm8.staticflickr.com/7185/6953874451_18930da458_b.jpg

Not trying to be an ass or anything, but I don't see the issue here?

From the pictures it looks like any cant is negligible to say the least, but maybe I am missing something here.

Is there contact between the rail and barrel/fsb or a performance issue somewhere?

[thopkins22]

It could be a barrel nut that is slightly out of spec or a receiver that isn't square, the first thing I would try would be another barrel nut. And a means for holding the receiver and rail in alignment while tightening the fastening nuts.

I agree...but I'd probably try spanning the upper and rail with a QD mount long enough to get purchase on both the upper and rail before buying a barrel nut.

Honestly I think it's a good thing to do when installing any rail to assure good alignment. Looking at the pictures, I'm not sure I'd bother with any other fixes unless you plan on running a visible or IR laser.

[castle]

thanks for the replies. rd62, no offense taken. i wasn't quite sure it was an issue myself. i just figured that it would be equidistant on both sides, so got a little concerned when it wasn't.

no plans to run a laser of any sort. took everything apart, and inspected the bbl nut which seems to be fine (at least to my eye), reinstalled with my carry handle spanning where the rail meets the receiver. it's a touch better after doing that.

after taking apart and reinstalling this thing about ten times now, i think i need to re-loctite the threads. as long as i can get it to where it is now, i'll be fine with it. it's just that spending $335 on it, i had expected everything to be in alignment. i'm fully aware that it could be my weapon and not the rail.

at any rate, everything else about it is spot on.

[castle]

just wanted to offer a quick update for anyone who happens to stumble across this thread in the future.

i dismounted everything, added loctite, and re-installed using my carry handle with approx 1/3 on the receiver and 2/3 on the rail to align the two. using some hand pressure out at the bbl while i tightened the screws in the alternating fashion. the rail is again rock solid, and now perfectly centered.

thanks for the help guys.

[Robb Jensen]

Good to hear. A means to alignment makes all the difference.