Well for long range / stationary work you can't beat the bipod. Certainly if you are going to use the bipod the HB is a must for super laser gun, if not the compensator is the way to go as long as you don't use over 5 bullets per burst, then it kinda loses it's potential. But I couldn't recommend the HB enough
Flash hider is next to useless except if you have your effect level too high and would like to remove some muzzle flash. IMO it's not really an issue tho.
What LMG may I ask? MG4 is better that the M249 if people were wondering. PKP and M240b are the high dmg kings with the MG4 king of the low dmg high ROF LMG's, Symthic.com for all your nerdy needs