Yes I mean Armalite
chamber 308
barrel length 20 " maybe 24 " with same contour as the AR-10 T barrel
Twist; 1-12
plain end
Thank you
Ok...that helps.
First off...the AR-10T contour isn't what you'd typically call a "heavy" barrel as they are ~1" under the handguards, 0.875" at the journal and ~0.750" to the muzzle. If you want a heavier profile than that, they are certainly available, but that's your call. You don't likely need a 24" barrel in a .308 gasser to get the most out of the rifle/cartridge within the practical limits of its usefulness.
Second, a slower, 1:12 twist will somewhat limit your options as most ready-made gas gun tubes for the Armalite AR-10 pattern are primarily a little faster (like 1:11.25 to 1:10). Frankly, I wouldn't sweat the twist too much unless you are just really planning on mostly running lighter weight bullets from the rifle that the faster twists may not play well with when all is said and done.
Now...with the above features/options that you want in mind, you are most likely looking at a custom barrel using any number of different mfg's blanks as the basis for your barrel whether it be a Krieger, Rock Creek, Bartlein, Shilen, LW, Douglas, etc. Check with Paul Craddock at Craddock Precision and see what he's got in Armalite AR-10 pattern that may meet/exceed your requirements. He's great to work with and generally is only 3-6wks out on complete barrels provided he's got the blank available you want to use.
If you decide you want to give/take a little on your requirements, there are a host of different options for an off-the-shelf, Armalite AR-10 pattern, 20"+ barrel, including various options from Rainier Arms (Select, Match, UltraMatch), Noveske (21" Pac-Nor blank to Noveske's specs), Armalite (SASS, 10T, etc. barrels), SAOD's DMR and Target barrels, etc.