In both Squad and PS MG role is the one that I find to be the most suting for me, and I spent some time thinking about historical and gameplay-related tips and tricks to help you become the machine gunner your section needs.
Also, most of what's below also applies to MG operators on vehicles.
What is a machine gun?
In a war of bolt action rifles, the machine gunner is the only section member capable of providing sustained, long range, automatic fire. It's safe to say that a machine gun contains anywhere between 40-80 percent of sections total firepower. Where 8 guys can fire 20 rounds, a machine gunner can fire 200. A main role of MG operator is making sure, that the enemy section is incapable of returning accurate fire, and that they will die if they try. A good machine gunner knows, that the key to being useful is knowing when to aim and when to fire. Finding a good ratio of quantity and quality of fire is what makes a mediocre MG a great one.
Suppresion
Suppresion is a in-game mechanic supposed to represent a soldier being terrified for his life and unable to return fire. It's represented by your screen growing darker and darker, your muzzle shaking and lower stamina regeneration. Soldiers are much more likely to be supressed by fully automatic fire, than by ocassional round whizzing past them. Accurate automatic fire can render an infantry man completely useless, even when behind cover - that's what makes machine guns so important. As a machine gunner your task is to kill enemies when in direct line of sight, and supress them when they're in cover - making sure that they stay there.
Example - enemy is fortified inside a house. They have a sniper and a few riflemen sitting in the windows, killing incoming friendlies as they try to overrun the house. In such case, your task as a MG operator is to place yourself outside effective range of the enemy (200-400 meters), place down your bipod and lay down sustained fire into the house, lighting up the windows, balconies and wherever else the enemy might peek for. Supressing the enemy will allow your allies to get close and into the house (preferably close enough to throw grenades in).
This example (replace a house with trenches, halfwalls, foxholes) is a primary type of action a MG operator should undertake when on the offensive. Finding an enemy strongpoint and making sure that they're incapable of using it to fire from is what a MG is for.
Territory control
What a machine gunner should do when defending a position. The answer might seem easy - find somewhere to place a bipod on and wait for the enemy to cross your sights.
Even though that might work some of the time, remaning in the same position too long is a one way ticket to getting overrun or naded, blown up or you name it.
During the defensive manouvers, when protecting a town, compound or any other stronghold, the first task of a machine gunner is to find a good spot to provide fire from. The larger ground it covers, the more ground it denies from the enemy. Look for multiple story buildings, bell towers, hill tops. Many buildings allow you to climb the rooftops (e.g using a vehicle to get on top). The more you can see, the more people you can kill. It's the best to identify a few of useful spots, because you will want to relocate once enemy the enemy starts supressing you back or if they'll start coming from different directions. Getting yourself stuck in one spot is never good.
Once in a decent spot, wait for a firefight to begin and start firing. Now, the important bit - you need not to see the enemy in order to fire at him. I cannot stress that enough. Have you seen one guy crawl trough a hedgerow, and now he's dead? There's a good chance there's five more around him. Light that spot up. Whenever you think that there's a wall, foxhole or anything like that the enemy might be using as cover, shoot it, and shoot it alot. Make sure that if they're there that they stay there. That obviously stands both on defensive and the offensive. Pretty much, after identifying enemy position, it is your task to make sure that there's a bullet hitting every square inch of that post code.
Sustained fire
Suppresion and kills are best delivered, using sustained fire covering fairly large amount of ground. To be able to do that, you need to learn how to fire your gun to fire enough bullets, with good enough accuracy. With the Bren, practice firing three round bursts. With the MG42, don't be afraid to let loose with 10-20 round burts. Opening up with a full auto belt dump sounds cool, but even with a bipod it's hard to remain on target, and it also chews up the ammo real fast. In order to be able to fire longer, and more accurately developing proper trigger discipline is key.
Ammo and a lack thereof
During prolonged firefights, you might find yourself in a situation where you're about to go dry. A machine gunner is one of those roles, where you might actually live long enough to see yourself run out of ammo. If there's no ammo crate nearby, and the logi is nowhere near to place one, consider just dying, respawning and taking your spot back again. To prevent a situation like that from happening it's good to contact a logi squad to make sure you have something to rearm from. It might not be worth to drop an ammo crate on the forward line of defense that you will soon most likely abandon anyway, but if you happen to find a spot well worth fortifying, make sure to let logi guys a heads up to drop some supplies nearby.
Landing your shots
So, suppresion does not require your shots to be all that accurate, but if you want to kill a guy then you need to make your shots land. Since as a machine gunner, your biggest advantage is that lots of firepower gives you lots of range, you should always try to place yourself in a position that puts you close enough to a fight but far enough so you don't get easily sniped or flanked. That means, that you need to learn how to kill people from a distance.
Firstly, zero your weapon for proper range. Be default, "X+scrollwheel" allows you to range your weapons sights. Setting that for the range you're engaging that means that you don't need to hover your sights above the target and lob shots into them, allowing for more precision. Secondly, learn how to lead your shots. Running targets can be quite challenging, but an old rule that at 100 meters you don't lead, and at 200 and more you lead one body width per 100 meters should make it easier. Also, practice makes perfect. The more dudes you drop the easier it will to start dropping them more consistently.
Few notes worth mentioning
1)Once contact with the enemy is established, and you have a general idea where they're coming from, fire in that direction. If the enemy is in coming from the treeline, but you can't see them, spray that treeline. If they're coming from the trenches, keep firing at the trenches. If you think that you don't have anything to fire at, you most likely haven't paid enough attention
2)Suppresion is hard to measure. It doesn't gain you points on the scoreboard, sometimes it might even be hard to see if you're doing any good if a job. Patience and experience is key.
3)Since most of the time a MG gunner is fair bit away from the rest of the squad, while at the same time being a primary target for snipers, rifle grenades or you name it, having a designated medic is a fine idea. If your squad lead is willing to designate one to follow you it's worth to have one nearby.
And here, also. When I told No.501_Spetre that I'll be writing something along those lines, he kindly asked me to add this priceless piece of information aimed at riflemen down on the ground. I quote "Don't fucking line up alltogether like a bunch of retards on a schooltrip. Spread around, but don't walk off on your own. Don't peak angles that your friend just took one to the nugget from. Don't sit with 6 other guys in one foxhole wondering why did you all die to one grenade."
Anyway, hope that gives you some idea on what makes a machine gunner different than any other role in Post Scriptum, have fun and I'll see you 18th on the release day.
