On my assaulters,

M4 with EoTech and EPR rounds for medium-long range maps, though it can do some close quarters work.

MK18 with EoTech and SOST Ball rounds for short-medium ranges.

MP5 with EoTech and the last ammo type (forgot what it’s called), great for CQC and fast assaults.

MP5SD with the same setup as the standard MP5, obviously great for CQC and stealth.

On my support class,

M249 with EoTech, best gun IMO for support if you’re going to be on the move constantly.

M240 and MK48 and XM338 with EoTech or the Elcan, depends on range. Best used when there is cover and you need to suppress an area.

On my marksmen,

M110A1 with LPVO 1-6 or 1-8 depending on range.

Might occasionally use the MK14 with the same setup if I feel like it.

On my grenadiers,

M4 with EoTech and EPR ammo.