IMHO, a semi-auto version of a compact SMG with a shoulder stock seems about perfect as a sidearm or PDW for second-line troops, especially if we are talking about young men with little training doing their mandatory military service.
Pistols and full-auto SMGs are hard to master, and the latter use up ammo fast, but the M1 Carbine shows that a handy shoulder-fired weapon is easy to master. A Micro Uzi, B&T MP9, Skorpion in 9mm or this Peruvian gun, all of them fitted with a shoulder stock of some kind, can be a holster-carried carbine.
With hot 9mm +P ammo it would approach M1 carbine numbers but in a compact packaget that can still be fired as a pistol in a pinch. If reliable, I'd rather carry that than any ordinary 9mm pistol.