Honestly I think pistol caliber carbines are quite impractical. I'd much rather have a can or two (most likely only one when you factor in the tax stamp cost). Especially a nice .308 can that can go on a 300 blackout SBR and a regular AR-15 in 5.56. But thats my opinion.
A pistol caliber carbine is not impractical. Also selling the berretta is not gonna get two cans maybe not even one 30 cal can.
A 300blk build is a great idea though