Rifleman, Assault, and Support are unlimited (no upper number of players) classes. Machine Gun is limited to two players per team. Rocket and Sniper are limited to one or two players per team I'm not sure which.
The support weapons, BAR and STG44 have a nasty kick which can be compensated for slightly by crouching and greatly by going prone. You are very vulnerable changing from upright to prone and back because you cannot fire at this time. I used to die very often when dropping to prone to decrease the recoil effect. Now I spend more time crouching. You are slower and quieter when you crouch.
When using support weapons pull back on the mouse as you fire to keep on target and fire in short bursts. You can also fire at their feet and let the recoil walk the shots up to their head. It takes practice.