I've got a couple of barrels that were made by Wilson Arms and sold by Nordic Components. The are 18" 416R SST with 1:8 twist and chambered in .223 Wylde. They taper down to .800 until you get to the .750 gas block section. Muzzle is threaded 1/2"-28.
I used a lot of these for 3-gun builds and have these two new ones left over.
$250 each plus shipping