SMGs or sub-machine guns can be used by the Sniper or the Engineer. They are very similar to Assault Rifles, but the are in most cases less powerful. The advantage to SMGs is the wide range of Attachments available, such as Corrosive or incendiary rounds, Grip, Silencer, or Red Dot. Some SMGs, most notably the Patriot and Bizon, have such great DPS that they only fall short of shotguns in close-range effectiveness. The ACR/G36 can be a great mid-range battle-rifle type weapon and others, such as the UMP and AKS-74, are effectively Assault rifles in terms of how they handle.
Weapons that fall under the SMG category are: