Look into the Kel-Tec Sub2000. Great little rifle, super light and compact, extremely quick target acquisition. And most take Glock mags, so you can run 17rd G17 mags or even the 30+ capacity G18 mags.
I chose the KelTec over the others (Marlin Camp, CX4, Hi-Point, etc) because its just so light and compact. And I liked the capacity potential. I've had zero problems with it and consider it a go-to HD option. The CX4 felt a bit bulkier to me and cost twice as much, though I'm sure it's a good gun. The Hi-Point is awkwardly weighted and extremely top-heavy, like everything they make. But it too is a decent gun.
The only downside about the Kel-Tec is that it's a blow-back operated gun that gets absolutely filthy after a few boxes of ammo. But you can't beat it for a quick, light, high capacity 9mm carbine at $400.
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