Damn, that's nice. How did it fit in? The cartridge seems to be almost as long as tokarev.
>even if they're considered a bit outdated
SMGs aren't outdated, just misplaced, along with other weapons.
>Within reason, a lot of SMG layouts are almost as concealable as pistols
No, only very few SMGs are wearable and absolutely none are as concealable and compact as all but the largest of pistols. Concealable SMGs are a dead end. Wearable, on the other hand, have their place even if they'd not be as effective as full-sized ones or rifles. It really depends on whether you want to carry it on a sling or on a hip.
>Maybe a more modern, closed bolt design
It was done in a few years after spread of UZI and was called MP5. It became so popular it still is very common despite more interest in short barrel rifles.
>I don't know a ton about the various, uncommon loads but how about just some hot 9mm AP?
It can work but the effectiveness is questionable. Check out the thread, there's some comparisons and examples. I'd prefer lower caliber(.308-.312) high velocity monomaterial pointed spoon tip bullet to have at least some lethality instead of just hardly punching through a piece of armor.
>There's just something that turns me off about using uncommon rounds
Bad idea, almost all rounds that are currenlty in use are horribly outdated. Unfortunately, spread of these rounds makes development and production of new ones a lot less sustainable unless some government agency starts using them.