300 WSM does well in shorter barrels, my 18" WSM is a bolt gun, but I've been impressed, definitely wouldn't go much longer on an AR, and I'd be most concerned about getting a long enough gas system. KAK offers this 19" upper that looks pretty much like what I think I'd go with.
https://kakindustry.com/300-wsm-upper-receiver-19-inch-barrel-15-inch-mlok
