Basic LMG is cheap. I am okay with it as is.
Vulcan is fine where it is at. Even though it is good for small PD, it may still need IPDAI to be reliable enough against some missiles.
The only one I am dissatified with is DLMG. It seems too expensive for a paltry increase over the basic LMG, and I almost never use it for PD. It is only useful for max DPS in an assault role. At least basic LMG is cheap. Probably needs more DPS or cost 4 OP or... something.
Heavy Machine Gun is powerful with buffs. With IPDAI, it tracks well enough to be reliable PD. With ePD (and other range buffs), it has almost enough range to match Arbalest and function like an assault weapon, but with much better efficiency. HMG is more of an assault/PD hybrid, kind of like Devastator but kinetic instead of HE.
Single Flak fires too slowly to be worth more than it is. If flak misses, the missile will probably hit before flak gets another shot. If missiles come in waves, some will get through between flak shots.
Double flak is fine where it is. It is good for PD and not much else. Nice when IPDAI is not an option.