Thinnest round handguard for ar15?

No. The Midwest Industries Gen II SS is one of the thinnest I have seen so far, but does have a dedicated 12 o'clock rail, and comes with 3 rail sections. It is also the lightest aluminum tube I have used-very nice unit.

You really have to make sure you have a low profile gas block if installing a carbine or MLGS under a longer tube, that is how narrow it is.
 
For a full slick rail, the Clark is probably going to be the lightest. I looked at them for a lightweight build I was doing for my girlfriend, but couldn't get past the fact you have to glue your forearm on.

Another option you might want to look at is the Seekins MCSR; they're crazy light as well for still being aluminum and having a top rail. That's what I went with and I was stunned how light a 12" rail could really be (8.6oz, 6.3oz for a 9"), and with none of the craziness with glue and strap wrenches. MCSR V2 Mod Quad Rail
 
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The AP Industries carbon fiber handguards are probably the lightest on the market right now. Their 7" free-float carbine handguard is 3.8 oz.

I have one of the rifle-length one, and it's 5.4 oz.