how to use the weapons in a usefull way! FINALLY!

pistol: let's face it. you'll only going to use it on possessed, or for sniping weak enemies.

shotgun: sadly, you cant snipe with the primary fire, but you can with the grenade launcher. if you upgrade it enough, you will eventually use the grenade launcher more than the primary fire! i know i do. the charged triple shot is just for dealing high amounts of damage. it's even effective against bosses and useful for staggering enemies.

super shotgun: it's only going to be used for high health enemies you just so happen to be close to.

heavy assault rifle: it's litterally useful for only 2 enemies. the hell knight, because of the micro missiles. and the possessed/ imps because of the scope.

mainly used in circle strafing.

chaingun: it's mainly used at close range, so good luck sniping with it. but with the mobile turret mod, you can sweep entire rooms of enemies.

plasmagun: it got nerfed. hard. it's no longer a secondary powee weapon. instead, it's a slightly more powerful assault rifle. which means it will only be effective against certain enemies. unless you get quad damage. then it becomes unstoppable. the heated vent mod is usefull if you want to take down multiple enemies, while the stun mod is used if you feel like stunning them.

gauss cannon: with it's two mods, you can either use it as a sniper cannon, or a mini-BFG. the primary fire is just a super shotgun, but concentrated into a single beam. at least damage-wise.

rocket launcher: only use against big slow enemies or multiple small ones. otherwise it's just a waste of ammo. the homing mod is useful for REALLY strong demons. while the detonation mod will only be used for multiple demons.

chainsaw: only used when low on ammo. or for fun. whichever you choose. you get ammo for using it.

the BFG-9000: it kills everything, everywhere. useful for when you have the "saving throw" rune equipped.

you will probably only use it for boss fights and getting rid of entire armies for health.

melee: only usefull for glory-kills, and staggering imps and possessed.

Here are the two main ones I used in the game: 

Heavy assault rifle with Rocket:  used it most of the time ... made me feel overpowered 

Gauss cannon with sniper cannon:  for far away targets 

Played on nightmare but couldn't help the feeling that weapons are too powerfull 

yay. a reply. so tell me. what in this list conflicts with your personal opinions on the weapons uses? tell me how wrong i am so i can die.

Well, I suppose I could write up my own opinion on those weapons then.

Pistol: adequate early on for when you need to snipe enemies off in the distance, surprisingly useful against distant Imps with the charge-up shot. Quickly becomes irrelevant as a weapon due to the chainsaw and prodigious supplies of ammo (otherwise would've been an excellent ammo-conserver).

Combat shotgun: Primary fire is only really good early on; later on you'd only really use that on the possessed or perhaps to finish off staggered enemies. The secondary fire modes retain the weapons usefulness throughout the game, however.

The explosive shot mod gives you another tool against the shield guys, it's also great for taking out pesky imps in general and has decent range. Burst-fire is mostly just for fun, however, it is useful when you're fighting cacodemons or mancubi and have tons of shells to burn up, because of the significantly added range, and those two enemies are particularly nasty at close range, so the smart thing is to take them out at range.

Plasma Rifle: Primary fire is very useful early on against tanky enemies, like the Possessed Soldiers and Hell Knights, because these enemies are slow enough and wide enough that they're easier to hit with continuous fire and you can quickly whittle down their health with it; it's even decent at slowing down Hell Knights significantly while pouring it on them.

Later in the game, you're really only going to be using the alt-firemodes, but these are both extremely useful. Stun-bomb is generally great for temporarily taking enemies (multiple or single) out of a fight in a pinch, and it is a useful weapon for dealing with shield guys. The heat-blast mod is invaluable in some combat situations where you're getting swarmed by imps; in those situations, it can be a regular mini-BFG to just clear out your private space and get rid of several very pesky enemies in one fell swoop. Great mod.

Heavy Assault Rifle: Primary fire is serviceable, and useful against imps, but mostly becomes irrelevant when you get mods for it. Micro missiles are generally good, but I mostly use it to deal with imps, because it usually kills them rather well (without staggering them) and the micro-missiles have enough homing capability that you usually just need to fire them in the general direction of the imps. Works well enough against slightly tanky enemies too. Fully upgraded, it's very versatile and also effective against tanky enemies, but I tend to prefer using bigger guns against those.

The Tactical Scope I tend to use sparingly, but is very effective against distant Imps and works well enough against possessed Soldiers and Lost Souls.

Rocket Launcher: Early game, the primary fire is pretty paltry and I only really use it against shield guys, possessed soldiers and to clear out groups of smaller enemies in some situations; later, however, the fully upgraded remote detonation mod upgrades the primary fire so that it does twice the amount of damage and has twice the stopping power, which makes it an ideal weapon against tanky enemies (even the Hell Knight, which is funny, because you do not want to use the RL against those guys early on).

Lock-on burst is perfect against Summoners; a single burst staggers the dumb bastard and then you can finish it off easily enough (does feel a little cheap at times though). You can even use it effectively against Hell Knights, but it's still rather risky, due to the time it takes to get a lock.

Remote Detonation is pretty meh, until fully upgraded, then it makes the RL a beast.

Super Shotgun: Still the most over-powered weapon in the game. There's barely an enemy in the game that you can't use it effectively against, however, it's a bit risky to use it against Cacodemons and Mancubi, due to the increased difficulty of dodging their projectiles at close range (and the Mancubus has a couple close-range attacks that are very difficult to avoid when engaged). I use it frequently against Imps due to how reliably it removes them from the battle (and its ability to kill multiple of the bastards with a single shot). Due to the player's high mobility, you can easily get close enough to put some serious hurt on the enemies and it has superb stopping power.

Gauss Cannon: Primary fire is ok against tanks, but I find it mostly useful for killing shield guys, since it utterly ignores their shields. Precision Bolt is nice enough, but very utilitation; mostly just to get a more powerful shot off at a distant tanky-enemy. Siege Mode is quite unwieldy until fully upgraded, due to the fact it stops you in your tracks in a game that strongly encourages you to keep moving; once upgraded, it's great for dealing with large enemies in general or clear out a crowd, but until then, you have to time your shots extremely carefully and you also have to rely a little bit on luck as well.

Chaingun: Good against most enemies, just not against the pinky. It's more effective at killing Imps (without staggering them) than the Heavy Assault Rifle and it also has decent stopping power against larger enemies. Primary fire is functional. Gatling Rotator is just a minor upgrade to the primary fire mode, but it is desirable in certain arenas where you really don't to slow down even slightly, because it allows you to pour out a lot of firepower, while remaining very mobile.

The Mobile turret mod is desirable when you want greater firepower over mobility.

I almost never use it against Mancubi, due to there being far better alternatives against those, but I do like to use it against Imps, Hell Knights, Cacodemons, Revenants and Barons of Hell. Generally, it's a good weapon against fast enemies, both large and small, because it's a hitscan weapon, and it works against tough enemies, because it has just enough stopping power to slow them down significantly enough.

Chainsaw: I rarely use it, because I usually find I have plenty of ammo, but I do like this weapon and its function a Hell of a lot. Excellent panic weapon for when you need to kill a big enemy quick.

BFG 9000: You feel overwhelmed? This baby clears out some much needed space. I use it when there are far too many Imps and they just keep coming. I use it when there's suddenly a lot of large enemies and they're getting harder to avoid. The battle is going like shit and the enemies keep coming, I fire it up!

usually, the hell knights are pretty fast. so i use the HAR (Heavy Assault Rifle) to circle-strafe them in close quarters, MicroMissile them also in close quarters, and snipe them as well as weaker enemies at range. the description even says it's for really fast enemies, so that limits it's effective use to hell knights. (but feel free to use it however you want) here is a free strat for you: if you see a hell knight, whip out the machine gun, use the micro missiles to slow them down, and then circle-strafe them with it. once they are orange, you can either glory-kill them, or keep circle-strafing.

Heavy Assault Rifle.  It is my favorite.

Once you get upgraded to no-reload and then infinite ammo, its micro missiles rule.  Even taking the cyberdemon down with it is a breeze.  Next to the BFG (which IMO is little more than a hail mary weapon), the Assault rifle can be the most powerful weapon in the game.