For my second AR build I wanted to assemble an upper from scratch. For the handguard I had picked up a 13" Troy Bravo Rail to install on a Rainier Arms forged upper with a Bravo Company 20" SS410 barrel. My issue is that after putting everything together I notice that the barrel is not aligned in the handguard.

Hopefully these pictures show what I'm talking about:

http://i.imgur.com/h65iDeFl.jpg

http://i.imgur.com/7RsEl86l.jpg

My question is, is the offset shown in these pictures within the tolerances I should expect? I have searched the forums and followed the recommendations of equally torquing each screw slowly as well as using a picatinny scope mount to hold the handguard in place. No matter what I do the rail seems to end up in exactly the same spot nearly touching the gas block. Additionally, the interface between the upper and the handguard seems to have a noticeable gap after tightening down the fasteners.

Any suggestions on how to install the Troy handguard such that it correctly aligns with the barrel, or how I might determine where the error is coming from? I realize it could be an out of spec barrel nut, out of spec upper or out of spec handguard, but I'm not sure how to determine where the issue lies. Sorry for the noobish questions, I tried searching for as much information as I could before creating a post.

It could be the rail, barrel nut or upper. That being said this is the third thread in the past few weeks I've seen regarding misaligned troy rails.

Looks like the rail twists

To be fair the one was obviously an issue with the BCM upper.

Verify that your gas block's gas port is centered to your barrel's gas port? Then, verify that your barrel extension's alignment pin, the upper receiver notch, and the barrel's gas port (with gas block removed); when the upper receiver's picatinny rail is at top dead center, are also all at top dead center or (TDC). Check these because the picatinny rail on the handguard is aligned with the upper's picatinny rail in your picture.

Mine was like that upon initial install of a low pro gas block realized it was off center w the gas tube in the receiver, straightened and has worked flawlessly I was able to center it best trying to even out the space to vertically align the gas block. (Daniel defense low pro light block and bcm mid length upper

I adjusted my gas block a bit and it radially looks better, but it still seems to be off center (opposite the ejection port).

http://i.imgur.com/raD6jyGl.jpg
http://i.imgur.com/9tmuboll.jpg

full resolution:
http://i.imgur.com/raD6jyG.jpg
http://i.imgur.com/9tmubol.jpg

Does the gas block/gas port have that much tolerance that I could be that far off and still have the rifle function? I had done a function check and it extracted, fed and locked open fine. I can't function check the gas system until this weekend to know if it is still functioning after I adjusted it.

I'm lookin at your first pic with an iPhone but I me it looks like your gas block is clocked away from the ejection port. The rail looks like its towards the ejection port.

Before installing your gas block, did you look inside of the block to see how much of a difference the gas port's diameter of the block is compared to the barrel's gas port? An easy way to center the gas block; use an analog or digital caliper to measure from the barrel's gas port to the end of the barrel's gas port journal that's toward the receiver. Then measure from the gas block body's edge to the edge of the block's gas port to make sure the barrel's gas port is not covered up by the block when installed. Then with a pencil draw a line on the barrel's shelf that is inline with the gas port. Also draw two lines on the gas block that marks the width of the gas block's gas port. Install the gas block, and then center the line drawn on the barrel between the lines on the gas block. Make sure to take into account for the measurements taken with a caliper; whether or not the gas block can be seated against the barrel's lip or needs a gap (without the measurements you will not know).

or just eyeball it....yeah that looks about right.  Works fine.

The BCM barrel has a dimple which I used for coarse adjustment the first time, but the dimple is a bit larger than the set screw. I have taken it off and measured the fore and aft offset of the gas port and created a witness mark for the left and right which I used to line it up again and it looks pretty much exactly the same. I've also used a digital caliper to measure the distance of the outside of the barrel to the inside of the handguard and the barrel is offset to the right (looking toward the receiver) by about 1.5mm.