Edward, Mid length is essentially too long for the fast burning powders used in 300BLK. That's why you don't see any. I would check with Right To Bear : righttobear.com/300-blackout-AR-pistol-SBR-barrels-s/2024.htm

Carbine length from everything I've read is the better option for shooting supers,assuming you are building a rifle, and not an AR pistol, if you are building an AR pistol (barrel shorter then16") you can run into cycling issues shooting supers, the work around for this is to use an adjustable gas block (pistol lenght) But for a rifle I'd go Carbine lenght, A carbine length gas system does not require an adjustable gas block and requires no adjustment if shooting supersonics. You can use a standard gas block which is a little cheaper than and adjustable one.
----------------
Odin works makes 300Blackout barrels with Carbine lenght gas, so if your wanting 16" or longer id go with that,if your going the pistol route, then pistol lenght and an adjustable gas block.
Odin works 16" carbine gas here
primaryarms.com/odin-works-300-blk-barrel-carbine-medium-16-gas-block