I want to use a shaved FSB for the gasblock under this older 12' light rail. The problem is, I just don't see how it will fit through this rear flange which measures just about 1". Any input is appreciated.

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so it's a 1 piece tube? how bout a side pic?

Looks like a DD. You wll have to remove the gas block before you slide the flange on. Then put the block back on and istall the rail.

Yes, it's a DD and the flange and rail are welded together. The gas tube actually goes through the small hole at 12'oclock in the first pic.

I'm thinking the only way to do a carbine gas system with this rail would be open up the flange so that a shaved block & gas tube would fit through, or maybe DD would swap me for a new style.

Is it new or used?  If new, I would return it.  If used, take it to a machine shop and have them hog it out.

Remove the FSB, Install the rail and reinstall  the FSB. I don't know if you'll have access to the taper pins though.

Or buy a low profile gas block.

First off, he knows how to install it.  Second, it is sooo easy to install, so he should do it himself and learn something.  I am sure ADCO can do it, but it isn't necessary.  This mod will also make the rifle serviceable at home in the future.  Third, his gas block WILL fit.  The problem is that it is most likely pinned on and the old DD Light rails required that the GB be installed after the rail, making a GB-under-the-rail install a serious pain.  
If his front sight base is pinned on it is going to be somewhere between a serious pain in the ass and impossible to re-pin the front sight base after the rail is slid onto the barrel.  I just installed the newer version of this rail and the difference is that the area that the gas tube goes through is milled out to the main hole in the handguard, allowing it to be installed over a lo-pro gas block.  Sure, he could tear off his cut down fsb and install a set screw lo pro gb, also through the handguard holes, but I would rather have a cut down fsb for security sake.  It would be inexpensive to have that area machined out at a good machine shop.  YMMV.

With that said, if you can get the newer version, do it.

ETA: What about putting the barrel/gb assembly into the handguard before installing the barrel on the upper receiver?  Push it far enough through to allow tightening the barrel nut, then install the six allens last.    I think you could tighten it with the gas tube in there.  That way you could pin it together on the bench?

ETA again: Nevermind. The barrel nut wouldn't fit through the hole.  It's been a long day...

Sell it and get a newer 12.0 LITE rail.

I have seen numerous people post that they are looking for the "old" style LITE rails. It might not help with you gas block issue, but you will make enough to buy the new rail and probably a little extra.

I think I have it figured out, and will post some more pics when further along. Thanks for all the replies.

I think it came out pretty good. What do youguys think?

Here's a finished pic

I like it.