Cast a vote to designing an SMG if there aren't enough votes for an LMG.
A mag-fed submachine gun that fires 9mm rounds. It sacrifices some rate of fire for increased reliability.
-1 to complicating design action with second design. Hollow points bullets should be designed later
In general, actually adding features seems not to detoriate a design. If the rolls are bad, they simply not happen and we switch to the existing rounds.
This usually the case for non-integral stuff. Worst case scenario, I'll tell you that it just can't be done in one turn, or require a lucky roll that doesn't really penalize.
So, here comes your first attempt at a submachine gun. You've made big belt machine guns, and you've made a semi-automatic pistol before, and unlike the AS-F14, this uses the small 9mm cartridge. A lot of the difficulty is making weapons like the AS-F14 comes with containing the powerful 7.62 cartridge. Speaking of which, I should probably specify that to be the 7.62x54mmR, so you can invent the 7.62x39 later. Or the 7.62x99 or something.
AS-MC16: This is a sub-machine gun, firing the 9mm pistol round. It uses a new, closed-bolt blowback system perfected from the AS-F14. This system is reliable and keeps dirt out of the gun. The bolt system takes up about fifteen centimeters, all of which is past the trigger, then a magazine, a 30 round drum, inserts into the receiver from the left. The barrel after the receiver is short, about twenty centimeters, and has a slotted metal hand guard. The stock is wooden and stops at the receiver, where brass is ejected from the bottom of the gun. The short barrel gives poor accuracy, especially when hot, and the 9mm bullets have much less range than a rifle or machine gun. The system fires about 400 rounds per minute. Its complexity makes it Expensive.
Well, you've made a sub-machine gun. The dice were with you. Time to revise.