Frienda don't let friends shoot factory rifles (let alone factory black rifles).
It seems to me that most of the reason for a CAR is weight and something that's easy to carry and shoot. The reason for 20" is that extra velocity.
Why not go for a nice custom 18" fluted barrel by PacNor, CLE, WOA or ... Seems like that's the perfect combination to me. I'd also go with a 1:8 twist in a perfect world. BTW, 18" is what all of the hot new hoydy toydy boys are building over at arfcom. Hundreds of sheeple can't be wrong LOL.