I was wondering if any of you guys have a flat top and a FF rail that is level but when you look down the top center the rails are about 1 mm canted ? I'm not complaining but just wondering if this is common . I was told before and after my custom upper was built the machining specs could differ.
I have another upper with a m73 forend and it lines up PERFECTLY. I'm inclinded to order a DD rail on my next project. Thanks for your input.

Pics?  

 If it is just twisted slightly, you might be able to loosen the handgaurd nut and twist the rail to where it needs to be.  There is a little slop between the pins and the holes in the barrel nut, you might have enough movement to get the rail lined up. Without pics it will be tough to see how far it needs adjusted.

Weazer

I have a couple of pics but don't have a hosting sight. I can E-mail them to you IM me. Also the gunsmith said he used a level when setting up my upper and insists not to adjust it. I trust him, I just want to know if it is common.

I'm not sure how you mean but the only adjustment would be in the barrel nut. It has to be right on for the rails to line up.

Quoted: Pics?    If it is just twisted slightly, you might be able to loosen the handgaurd nut and twist the rail to where it needs to be.  There is a little slop between the pins and the holes in the barrel nut, you might have enough movement to get the rail lined up. Without pics it will be tough to see how far it needs adjusted. Weazer

This is true. There is a certain amount of trial and error to align the rail properly. Unfortunately if you can't get it lined up by adjusting the ring then the front sight and rail have to come off. Also, this is not a problem specific to LaRue, all FF rails that have their own barrel nuts must be adjusted this way.

Just throw a carry handle across your flattop and the LT FF handguard, then tighten the LT retaining nut. Should result in perfect alignment.

Quoted: Just throw a carry handle across your flattop and the LT FF handguard, then tighten the LT retaining nut. Should result in perfect alignment.

That's a great trick for aligning Yankee Hill tubes that haven't been tightened yet because they spin freely and are held in place only by the locking ring...  But with LaRue and DD that index on the barrel nut, you're talking about trying to move the already tightened barrel nut with a carry handle clamp?  Possibly, maybe, but not definitely!  

Quoted: Just throw a carry handle across your flattop and the LT FF handguard, then tighten the LT retaining nut. Should result in perfect alignment.

This trick doesn't always work. I used this method on a Samson MRFS  and the Front Flip Up BUIS (FUBUIS) was still off (off beyond what I had for windage adjustment). I even visually checked the rail and it looked perfectly straight comapred to the rails on the upper. Of course, a fraction of a degree on the rail level will throw the tip of the BUIS a few degress off.

I think the only way to make this work is to use a full length (or almost full length) rail like the PRI. For me, I just changed rails (to a DD M4 12FSP-M...the Samson was just too heavy) and did a YHM Front FUBUIS/Gas Block combo. Now, with the same Rear BUIS, it's dead on. Now, unless it's something like a LMT MRP (monorail type system), I don't fully trust mounting a Front FUBUIS on 'em.

its the slight misalignment of the barrel nut.


there is a bit of room inside the holes for the gas tube which leave room for misalignment but imo also leaves room for out of spec gear to fit. trial and error before finally tightning the locking ring all the way usually gets it.

tighten barrel nut
install tube and snug locking ring
use flat edge to see if its level
if off its usually canted to the right when looking from the chamber area to the muzzle
remove tube and "tap" the barrel nut and try again.

i have installed four larue handguards and noticed the canting on the second one (first one came out perfect) after looking at it all and thinking about it i came up with the above.

Ask Denny how he does it.  He nailed mine, perfect mechanical zero.  CMT/Larue 9in w/ Troy sights front and rear, dead on.  

"Ask Denny how he does it. He nailed mine, perfect mechanical zero. CMT/Larue 9in w/ Troy sights front and rear, dead on."

WORD!  Same for mine. It's an ancient Chinese secret passed down from father to son for over a thousand years.
I was there when he did it....I could tell you, but then three Chinese guys in pointy hats would fly over the roofs to your house and cut you into peices.  Those stains are a bitch to clean out of carpet dude.
http://i51.photobucket.com/albums/f360/reddyrob/GUNS/shorty.jpg