Is the gas port positioned differently (i.e. closer to the receiver) on an M4 barrel compared to a 16" CAR? Are there only two common gas tube lengths?

No and No. The CAR/M4 is the same gas tube.There is the rifle and CAR/M4, but I don't know if the new "mid-length" barrels will have a different length tube or just a hidden gas block like the Dissapator. Then there is the really short one for the 7"-8" barrels.

Deleted by Corwin1968 because now he's totally confused (again!!) and thinking the information he posted may not have been accurate.

Where do you get four?  I count three.

After experimenting for years with unreliable 10.5" to 11.5" shorties, the military figured out that the critical dimension of the barrel was the length from the gas port to the muzzle.  When you take a CAR barrel and extend the front end to standard length, you get 14.5".  So there are two standard lengths of gas tubes, one for the 20" and one for carbines.

The so-called "mid-length" is made by lengthening the section before the gas port (along with the handguards) so the barrel comes out to 16".  But it's really kind of a funky solution as far as I'm concerned.  My favorite configuration is a standard 14.5" M4 barrel with a KKF flash hider.

Why is it a funky solution?  14.5 inch barrel properly goes with carbine and M4 handguard gas system.  Rifle handguards witha 20 inch barrel/gas system.  We wouldn't even have 16 inch barrels if not for NFA.  Midlength puts the gas block at the proper distance from the muzzle, but longer sight radius, and the barrel looks less unprotected.  Pull it looks more like someone shrunk a 20 inch barrel, instead of streched an M4.  BALANCED LOOKS!  I'm going to get one, as soon as the ban goes away and they come out with one in chrome with 1 in 7 twist.

Actually, I didn't mean funky in a bad way - I probably should've said "unique" because I agree with you.  It's definitely an interesting  solution lengthening the center section rather than just tacking on some extra tube at the muzzle which always looked weird to me.

I've personally never built an AR with a mid-length barrel, but I think I'm gonna have to give it a try and see how it shoots.



Definitely wait for the chrome!!

[Edited to ficks speling misteaks.]