I’m a big fan of the M1 carbine and if I was going to be stuck with just the one gun and an ammo can of ammo for it, the 30 carbine would definitely win out in my opinion; no question. It’s very light, fast to use, and as powerful as a 44 magnum handgun. If I couldn’t have an AR carbine, the M1 carbine would be my next choice, followed by the AK.
The reasons I like 9mm carbines are largely just due to the specifics of my situation. I already have defensive carbines in 5.56 and 300 blackout, so that’s covered. But the 9mm carbine has some pretty neat advantages for other things. In an AR style platform, it lets me get in shooting time with the same controls and ergonomics as my main gun, but with substantially cheaper ammo. Also, it lets me shoot close-in targets and drills substantially closer than you can really safely do with the 223, when shooting at steel like I mostly do.
Since I cast my own bullets for 9mm, it’s extremely cheap to load for. The 30 carbine can use home-cast bullets, but I have no first hand experience with it and I’d assume you’d have to be more fastidious about bullet hardness because of potential issues with gas system fouling.
And honestly, in a pinch I’d actually be pretty comfortable with a 9mm carbine for defensive purposes if pushed into a corner on it; as long as I got to choose the load. I constantly am reminded about two saddle guns that were used by family members to take a whole bunch of deer over the years; one 25–20 and one 32–20. One was my dad and the other was my wife’s uncle. For years, they successfully took a lot of medium-size deer completely adequately and humanely; and the 9mm carbine from a carbine exceeds both of those calibers power-wise. There’s been at least five, and maybe more, factory loads that exceed 700 ft/lbs of energy from my 9mm carbine, so as long as I did my part adequately I believe that’s adequate power; again, at least in my particular situation. I’m not law enforcement anymore, so I can’t see me needing to shoot through car doors or anything, or needing to engage anybody at any substantial distance either for that matter.
Basically, to me a 9mm carbine compared to other low- to mid-power carbines is much like a 9mm pistol compared to other-caliber pistols. It’s basically “OK“ in my opinion; not fantastic but not bad. I wouldn’t ‘want’ it to be my only defensive-carbine option, but wouldn’t be genuinely terrified if I was forced to use it in a pinch.
And less importantly, there’s also the convenience benefits of being able to use and handload just one load for rifle and pistol use; at least for the kind of stuff I mostly do. Same with magazine commonality. Absolutely not a “top-tier“ consideration, but it’s awfully handy.
Fwiw, somewhat off topic but the magazine-commonality thing is something I like enough that this weekend I started having thoughts about selling off my HK carry pistol and going back to glock. I really like the HK, but objectively the Glock does pretty much everything it can do (partly due to the Tau SCD on it; I won’t iwb carry a glock anymore without one), but the glock also shares magazines with the 9mm carbine and 9mm braced AR pistol. Haven’t decided for sure yet, just started thinking about it while doing a Rob Pincus pistol class yesterday. The class suffices for the ‘enhanced’ chl renewal qualification and only cost $50 more (plus ammo cost) than the mandated renewal course by itself, so I made a day of it. I used the HK for the first half of the day, and Glock the second half of the day, and both were more than adequate. So I may or may not sell the hk; not decided yet.