"Ran in the house, and grabbed my clip; With The Mac-10 on the side of my hip"
Sub-machine guns or SMG's are one of the most situational weapon class since their effective range is strongly contested by Shotguns, Hand-to-hand weapons and flamers. SMG's feature a full-auto fire mode and a single shot mode.
What makes SMG's unique is that their spray and pray weapon trait changes their burst formula, highly increasing the rate at which the bullets disperse over range while detaching it from the spread parameter. This results in high effectiveness at very close ranges, but a noticeable drop in damage with each hex of distance between the shooter and the target. Generally, an SMG user is able to empty the whole magazine with about 100AP in one chain of short full-auto attacks. In order to achieve maximum damage, it is important to remain as close as possible. Similarly, someone who is being attacked by an SMG user can try to run away while under fire to reduce the amounts of bullets hitting him to survive the attack while leaving the SMG user deprived of both AP and ammo.
It is also worth noting that the MP5SD, silenced UZI and the Sterling L34A1 are sneak-related weapons and do not unlock their perks with small guns skill. They also have a different burst behavior to allow accurate ranged bursts rather than spraying the whole magazine from close distance.