BAR and STG44
This is used in a very similar way to the SMGs but because it is heavier its recoil is stronger. By crouching though you'll get pretty good results even at long ranges and whilst prone you'll be able to use it as an MG with smaller clips.
Just remember to aim then fire and try to fire from crouched or prone and you'll do pretty well generally. Because the ammunition is pretty heavy you can also fire through thinner walls and barricades so if you see an enemy hiding behind a thin piece of cover try firing through it and you'll be able to pick up a number o kills like that. These heavier automatic weapons make great scouts in enclosed areas because of their power any enemy force encountered can be thoroughly hosed with these large projectiles and if not killed it'll at least make them easier to dispatch by the rest of the squad. Also because of the more distinctive sound of this gun it’s easier to pinpoint the location of the engagement.
A while back I briefly used a semi-automatic script which allowed me to set automatic guns to fire single shots. To fire bursts required me to either switch this off with the appropriate key or to click the mouse fast. I found the accuracy when doing this was extremely high when crouched and it got me in the habit of doing it even without the script. Because of the power of the projectiles plus the accuracy of firing single shots these guns make excellent rifles, and have the added advantage of a huge clip. It's often a good idea to use single shots on distant targets especially when you are ambushing them, making sure that you hit them with 4-5 single bullets is more likely to kill them than spraying 30 at them and the recoil throwing most of them off target.
Both guns are brutally effective at point blank range and you're best defence is either to get a very lucky snap shot that kills them before they can draw a bead on you or to run and pop back and surpass them. So remember to keep your finger on the trigger at all times in enclosed spaces ready to spray anybody foolish enough to mess with you.
One of the recent additions to DOD, the FG42 Bipod version in deployed mode