Hmmm....I have a Beretta CX-4 Storm carbine in 9mm, it is nice just because everything just fits me right and it's dang accurate (and it looks cool).
9mm and .223 seem to be the most popular calibers, which is important when considering ammo cost and selection. I'm going to limit my ammo to 9mm, .223, and 12 gauge.
So, I dunno, I could see my CX-4 being pretty practical now that I have a couple of 25 round mags. It is just 30' long, easy to handle and aim. Enough for good penetration up to 100ft and extremely accurate, with lots of selection for ammo, which I can switch out with my pistols. Barrel doesn't overheat, even if I unload the whole mag at once.
However, for sheer ft/lbs of energy to the target, a 12 gauge or .223 rifle cartridge is def much more oomph for the same cost of one of those carbines you mention.
Are you using 9mm to practice at the range or something? That's also a nice benefit. Plus you can get cheap, reliable 9mm hollow points, with less potential for over penetration into the next three houses in a suburban environment. Seriously, check out http://theboxotruth.com/docs/bot1.htm
. Even a 9mm hollowpoint can plug up and go through the equivalent of four walls with deadly force. A .223 practice round will go through a couple of houses easy. Every bullet is a potential lawsuit.
Shotguns are nice because you can load them up to not overpenetrate, but they hit a lot harder than the 9mm. A single 00 buck shell is like delivering 9 to 12 .380 bullets all at once all in the same general area. -boom- $0.75 per shell or so, but it doesn't seem like I need to shoot so many shotgun rounds for good practice...not sure why.
.223 is sorta a must have, I spose, especially if you're going to hit anything beyond 50 yards with any stopping power. At least $0.75 for a good hollow point and $0.50 per target round, they're expensive to practice with. About four times the power of a 9mm though and much more accuracy at longer range.
The major benefit to 9mm carbines to me is the low cost. For that I'd just go for a Hi Point. Here's a good selection page. http://www.best9mm.com/carbines/index.html
Oh, besides, the Just Right carbine doesn't handle +P 9mm rounds, which is actually hardly anything in terms of chamber pressure, so I would avoid them like the plague (no pun intended). Just means the strength of the chamber isn't very high, which is a bad thing.
If you really want a 9mm and want some good accuracy and good powder burn in a longer barrel, I'd go for a Glock 34 handgun. They are cheaper and awesome. It is actually my favorite gun. Good 6" long sight radius. Low perceived recoil. Easy to maintain and clean. Otherwise, I might go for the Kel Tec carbine. I just have a personal, irrational weakness for the Beretta CX4, I guess. It just feels good.
My two cents,