Revolvers

Piercer: The piercer pistol is used to do high damage in a burst to multiple enemies in a row. If you aren't using the alternate revolvers, aim for the head when using a charge shot. There isn't much to say about this one, use it against more tanky enemies (Malicious Face, Cerberus, etc.)
Marksman: One of the most powerful weapons in ULTRAKILL, I highly recommend using this weapon. The alt. fire for this gun throws a coin, which may not seem to do much, but, if shot/punched, it will do a more powerful, homing version, of the regular shot. This is best used against drones, like in 1-1, however, it is powerful in all situations. Simply toss the coin, and quickly snap your camera upward until your crosshair is on the coin.
Sharpshooter: I don't realy have much knowledge about this pistol, I would use it in enclosed spaces, or to deal quick damage to an enemy.

Shotguns

Core Eject: The Core Eject shotgun is a rather interesting weapon, not due to complexity, but just it's uses. I wouldn't advise using this as a main source of damage, however, it does do well in clearing out large amounts of Filth all clumped together. It's pretty niche, but if you shoot the core with the Malicious Railcannon, another weapon I will delve into later, it will increase the size of the explosion greatly. This is best used against hordes farther away from you, that are relatively large. A great use of this specific interaction is in P-1, where you can use it to easily kill every single one of the Flesh Prison's eyes. I recommend learning this technique.