I tried to post this before, but my posts have somehow disappeared.  Don't know what's up with that.

Anyways, my next build will be intended to fill the gap between my 20” AR10 and 16” carbine, with EoTech.  It will have a magpull stock, and be equipped with either a TA11 or TA31 variant.  I’m thinking I want an 18” barrel, but I’m not really familiar with the options.

This rifle should be quite light—I don’t really want a heavy match barrel.  Further, reliability trumps match accuracy for this build.

I’m thinking my best option may be to have a 20” chrome lined government profile barrel cut down—what do you guys think?

I’m also tossing around the idea of a 16” mid-length system.  The only real problem I have with this idea is the similarity to my carbine.  For what I want to do with this rifle though, the extra barrel length may just be a liability.  Any thoughts?

I would just go with the midlength, but thats just me. I have an 18", and it hasn't got much on my middy. I dont shoot it much. Wanna buy it?

I'm different-- got a couple 14.5-16" middys, but my favorite upper to shoot is my lighweight 17" with a rifle length gas system.  The difference between it and a 18" is slight.

With 18-inch barrel, you have a couple of options.

You could cut down a 20" gov't profile barrel like you said and ADCO would be the popular choice to do it.

Your other option is to buy a 18-inch Armalite rifle with barrel work already done by ADCO.

Armalite 18-inch M15A2 dealer link

There are no major issues with the 20" barrel with a rifle gas system cut down to 18", as long as the gas port is not less approx. 5" from the muzzle according to gunsmiths in this site.

It's not fair to compare a 16-inch mid-length gas system with the 18". Compared to the 16" carbine gas system, the mid-length have an advantage in smoothness, more handguard area for your hand, more area for accessories and a standard bayonet will fit properly.

If you don't mind the extra weight then the 18" should be fine, but if you like the lighter feature then the 16" would be a good choice.

Supposedly Sabre Defence may have some 18" SS barrels, mid-length gas system, medium contour available soon. Talking to their production manager, he says they are part of a contract batch for DEA. I'm hoping to get 2 or 3 of them if they become available.

Sabre has no accuracy guarantees (AFAIK), and I bet they'll be a pretty heavy profile.

I went with the 18" from MSTN because every little bit counts and as ZAK say's the differ is slight.