300 blackout market needs a NR barrel. I would buy a light to medium weight for sure myself.
300 blackout was designed to burn all of it's powder in a 9-10" barrel. I think at 18.5" you'd start to lose performance. Personally I think the 300 blackout really only makes sense in a short barrel, suppressed carbine shooting subsonic ammo. Highly effective for CQB/CQC applications.
My nr - 30 carbine (18.6 barrel)runs on the same powder as my 300 blk which has 16 inch barrel. Both rifles run fine on the H110 or 296 as well as A-1680. I also have a 300 blk sbr which will run fine on D4198 and or the pistol powders. Can't see why the 300 wouldn't be ok in 18.6 inch. Use a flat base bullet for max loading