A local shop had an as-new .40 S&W Beretta carbine for $500, but I could not find a way to justify buying one. Dammit.
You really can't get much velocity with any pistol round, even in a carbine. Even a .223 kicks any pistol round's butt.
I thought about using 2400 or some other slow burning powder to take advantage of the long barrel, but I found out you really can't do that with a blow-back style weapon.
In the long run, my XDm .40 was far superior - it holds 17+1 rounds, and has all the other advantages of a pistol.
Still, I would not knock anyone who wants to get a pistol carbine! I bet they're a lot of fun.