Try to imagine an Owen SMG with the trigger moved to the front pistol grip. Discard the rear pistol grip and butt stock. Make up the length with a 19" barrel. Overall length would still about the same as an extended Sterling (28"). The longer barrel would increase muzzle velocity by 12 percent and muzzle energy by up to 44 percent. Increased muzzle energy would keep traditional pistol ammo (7.62mm Tokarev, 9mm Luger, .45 ACP) viable at longer ranges.
Fixed sights are offset the left of the magazine and are calibrated for 100 to 300 yards/metres. For fighting around corners, it might need a second set of sights along the right side.
The key to installing a longer barrel without increasing overall length is a bullpup configuration with the magazine located behind the trigger. If you use a wrap-around breechblock (ala. Uzi) the trigger mechanism could impinge on the (overlapping) front end of the bolt, shortening and simplifying the trigger mechanism. Simple construction is a driving goal. Keep it simple with a fixed firing pin. Most components are pressed sheet steel (automotive industry). Limit precise machining to: barrel, bolt face, extractor and a few trigger components. The precisely-machined bolt face is squeezed (or spot-welded) into the (thick sheet steel) bolt body. The trigger bar/strap works in tension to pull down the primary sear.
Install just enough wood, Bakelite, leather or canvas to prevent soldiers' hands from freezing to the gun during chilly ambushes.
Not sure if it needs to fire single shots from a closed bolt ?????? In that case, the firing mechanism would resemble the internals of a semi-auto Uzi with the main spring(s) driving the firing pin.
Not sure if it needs a selector switch. ???? Single shot is mainly used during initial training or police work. If a selector switch is installed, it should be installed within thumb reach of the trigger hand. Similarly, if a primary safety is installed, it should be incorporated with the selector switch and rotate forward to fire. Rotating allows the soldier to confirm selector/safety position in the dark.
Definitely install a grip safety on the pistol grip with its own, separate sear. The secondary sear can engage several notches in the bottom of the bolt, reducing the risk of accidental discharge.
A top-mounted magazine (aka. Owen) could eject straight down, eliminating grumbling from left-handlers. For reliability, limit magazines to 10 or 30 round boxes with double row and double feed. When not firing, the mag and mag-well pivot forward to lay along the barrel, easing movement (for support troops: artillery, sappers, signallers, etc.). Add a dummy mag-well to carry a second mag. A dust cover protects the mag-well when folded. Similarly, the ejection port has a spring-loaded dust cover similar to an AR-15.
Add a stud or ring to hang the gun from a belt, (vehicle) bracket or shoulder strap. Pressing with one finger disconnects the ring for "quick draw."
A stamped sheet steel receiver (aka. M3 "Grease Gun") would reduce parts-count. But the bolt and recoil springs slide on two rods (ala. Uzi). To reduce opportunities for dust, the cocking handle slides along the top of the forearm and pushes a (non-reciprocating) rod that impinges on the front face of the bolt. The rod slides through a hole adjacent to the chamber.
A tubular (or stamped sheet metal aka. PPSH) barrel shroud/breech could simplify construction. A bayonet lug - on the muzzle end - would be handy when fighting in someone's house (FISH) .... if you run out of ammo before running out of bad guys. Just use a standard bayonet already issued with service rifles (e.g. spike bayonet for Commonwealth troops).
The forearm includes a socket for a (already widely issued) flashlight such that the switch falls readily to the forward hand.
If you really want to get futuristic, hang a grenade launcher under the barrel. Limit grenades to baseball size (aka M79 Blooper).