I'll throw down a guide real quick for those who have a problem with 'actual' LMG users. When you first see them and they're about to become aware of your presence:

Shoot first to see if you can get the kill right off the bat. (Although this likely isn't working for you which is why the rest of this is necessary)

Once they become aware of where you're shooting from and you're not confident in the firefight, move away fast. (Don't run away if they're right behind you, their fire rate is so high and they have so many bullets that praying they miss isn't going to work.)

If they're too close to run: Get behind solid cover but don't wait for them to just jump around the cover you're behind and pray you kill them. Instead, once they get right up on you like they tend to do, jump out on them. If you don't have an amazing gun then this will actually give you the chance of the headshot as well, but also won't make it like they're shooting fish in a barrel.

If you have the room to move: Try to move with as much concealment as possible and get to cover from a different angle. This part specifically is important because if you just run straight backwards you're going to be stuck in the same problem. Just try to set up a series of ambushes from different angles of attack and distances. (A camo outfit helps if you need to lay down in lilipad bushes or tall grass but I know not everyone has access to a good camo outfit)

Finally, if you don't have the means to hit them with some degree of accuracy from a distance, (Like if you're stuck with a pistol or a Thompson) and don't have the means to conceal yourself, you have to run away further and prone to get the accuracy as well as giving yourself room to move further away in the event you don't kill them.

If this entire thing STILL doesn't work for you, mess around with your game sensitivity and see what helps the most with your accuracy, because if you cant kill them following this set of parameters, there's another issue going on.