Technically the painsaw isn't a gun and you forgot the flame thrower.
My skills with weapons aren't almost the same for each other as you think... These are my recommendations (not always how I fight):
Blaster: Aim ahead according to distance, and when i think i've got him on target, i fire.
Rifle: Spam, and ensure target is on crosshair at all times, else reload.
Painsaw: Hold the button in combat, try to get as close to the opponent as possible and aim at him. If he's running towards you, run sideways then run into him as he gets really close. If he doesn't run from you, circle him. If he runs, strafe jump to him. It's so funny when people are wondering "WTF! How is he that fast! Speedhack!"
Shotgun: "Triggerbot" style. Don't have to always aim on the target, but try to anyway. As the target flies accross the screen, click.
Lasgun: Just spam and aim. Great for chasing down aliens and as a survival weapon. Circling dragoons that are trying to chomp you are effective.
Mdriver: Similar to shotgun, except you have to be precise and is more effective long range. Don't use scope unless the target is hard to see or so.
Chaingun: Similar to lasgun, just don't spam long range. Use blaster in such situations...
Flamer: Honestly, I just use it like a painsaw, except i aim ahead and take notice of my inertia effecting the flamer, and I don't get too close.
Pulse: I use it like a flamethrower, without the inertia part. The closer you are to the opponent, the more effective. Pulse is the third most strongest damage dealing weapon in the game after flamer and painsaw.
Luci: Pretty useless unless there are some dragoons nearby. In such cases, charge up to full, and ensure you've aimed ahead of the target and fire. Prediction matters quite a lot. After firing primary, immediately hold down the secondary fire. If you accidentally didn't fire a full shot at a goon, this secondary will ensure his death in a quick combo.