There is
a handgun for normal quick attacks
a magnum for blowing their brains out real quick.
a rifle for long ranged attacks
a machine gun to take them out fast.
a shotgun so they can't get close to you
a Rocket Launcher just so everyone can get out of your face
a hand cannon... that.. just... kills.... EVERYTHING.
o yea.. and the knife if you want to count that.
There are 3 different types of grenades Flash Incendiary and a regular.
the hand cannon can only be gotten after beating the game and completing mercenary mode for all characters with 5 stars in each stage, with ALL the characters.
Oh yea, if you count arrows I guess that's another type of weapon you can add onto the list.

the best weapon is the chicago typewriter even though it costs alot