Anybody have one or know if this will be troublesome in operation? I'm having a bbl made but my bbl guy only has the specs on carbine length and rifle length gas systems. I'm thinking carbine length gas system because I have a 12" VTAC handguard I will be using on this build and I want to keep the gas block under the handguard.

Thanks,

Bravo

It will work

I will work, but with that barrel length I would opt for the rifle length system.

If you have an 18" barrel I would assume that you are looking to get more accuracy out of your rifle. A rifle length system would give you a softer recoil impulse and possibly make your rifle more accurate.

Putting a carbine length gas system on a barrel longer than 16" seems to be a waste of a barrel to me.

YMMV

For 18" and your description I would recommend the Intermediate gas system length.  Carbine is WAY too short.

Intermediate is 1.500" shorter than rifle length which should be easy enough for your barrelsmith.  If you don't want to believe what someone on the internet tells you, call Vltor and bother Eric for the spec (as I already have).

A low profile gas block on an Intermediate will fit inside a 12" free float handguard which answers your other concern.

ETA: Intermediate gas barrel with PRI low profile block, DD M4 rifle length handguard.

Nice rig

This^.

You can also add Noveske to the list of people to call on this..

It HAS existed, I remember seeing in a gun magazine/catalog from 2005 that Wilson combat made an AR that had an 18" barrel with CAR length gas system.

Due to the extreme amount of dwell time that would have, I'm sure it would need a smaller than typical gas port in order to not be over-gassed, and have problems with excessive recoil and parts wear. M-4 barrels (14.5" CAR) have a 0.63" gas port, so if you have an 18" CAR barrel made, just as a guess, I would ask for a gas port somewhere between ~0.50" and 0.60".

Use a rifle gas on an 18". You wont regret it.

^^This

No way I would spend all that money building an oddball rifle that will almost certainly have "issues" (sooner or later) just because you don't want the gas block visible.  There's a reason mid-length and intermediate length gas were developed.

I would either find someone who can build it in intermediate length gas, or just go with rifle length gas.  I'm all about aesthetics, but not at the expensive of performance or reliability.

And really, there are a lot of cool things you can do with an exposed gas block.  I think a flip-up front sight base gas block (like the Mk 12 Mod 0) looks awesome on an 18" rifle personally.