Who decides what is OP? The one getting killed by a particular weapon or the one doing the killing? Of course the guy getting killed, I hate being killed as much as the next guy but this is a FPS and it is based in the future where I am sure the guns are more powerful. When a grenade explodes at your feet you should die! When some one shoots you in the head with a sniper rifle you should die! When a heavy opens up on you with a HMG you should die! etc, etc, etc...

The easiest way to tell if something is OP is when it slowly takes over the kill feed until you are forced to adopt it in order to compete. (In previous builds it's been Swarm Launchers, Creodon Breach ARs, HAVs, Minddrive SMGs, and remote explosives.)

There are people saying that when grenades explode at their feet they shouldn't die? I haven't been seeing that at all.. I've been seeing people complain about a short fuse on grenades that make them too effective and easy to use. Why else would people rely on their grenades for kills more often than their primary weapon? Luckily when things are OP, it's easy to spot because once people realize that it's broken, they start to exploit it and you see it often.

I also don't see people saying that headshots with snipers shouldn't kill, most of the people I've heard complain about snipers say that they need to be more difficult to use; perhaps by adding sway.

The HMG comment, IDK. Simply because of the way you stated it, just because a heavy starts shooting at me doesn't mean I should die. I could just use cover and run around, avoid him and wait for teammates and overwhelm him.. or toss a OP nade at him lol.