Just make friends with a GOOD machinist
He'll make you whatever you want--legally of course.
Not sure of availability of the cheaper "kit house" barrels or how they shoot but if you want the best "possible" accuracy your barrel should start out as a match grade blank. My 2 current barrels are from Lilja blanks but my next ones will be 5C's from broughten. I do my own work making the barrels and gas blocks.
I'm sure somebody here knows smiths currently offering 300W AR barrels as an option.
Are you going to use a gas system or hand cycle since you say accuracy is the focus?
As for accuracy my first Lilja barreled upper 16"1x8 twist once put 5 rds into a big hole at 100yds with sub 220s and N110. That upper had a carbine length gas system that I never found the best powder to get it to cycle before I sold it to a buddy. My testing is very limited though. Past 100 I haven't done much at all. The week before thanksgiving my 11"(remainder of barrel from 16" build) with 5.1" welded-on upper I took deer hunting put 3 rds into 3" @200yds with 2(lower hits) out of the 3 showing slight tumbling. 5x scope. I thought it was pretty good for a slapped together for the heck of it barrel.
I've never shot past 200yds with subsonics.