Toss the AK, IMO - The SIG is better. I don't even have any machine guns anymore. The handguns are powerful enough once upgraded. Pretty much all I ever carry is the rifle for some spot work, the shotgun for crowd pleasing and a handgun. For specific spots, the grenade launcher with situation specific ammo. Now that I have unlimited ammo unlocked for a few of the guns, I just use one of each for each character... You can conserve ammo by buying the Stun Stick - that thing is so much fun - one hit kill on most characters (not bosses), but you have to be up close and personal. Your accuracy goes through the roof.
Let me tell you about the unlockable guns:
M93 as mentioned previously - 3shot burst with a stalk and much more powerful than the M92, but only holds 30 bullets.
3 barrel shotgun called "Hydra" BANG. Not as focused as the M3, but a lot wider spray. I got a trophy with this one called "crowd control"
Chain gun - not accurate, but quantity over quality still gets the job done with this one - unlimited ammo.
Smith & Wesson M500 Magnum. It is almost twice as powerful as the other Mag when fully upgraded!! Blammo.
Only 6 shots, but you don't need many to kill pretty much anything.
Bow & Arrow
?? I haven't bought this yet. Unlimited ammo but manual aiming (no laser, duh).