I prefer to use the pistols whenever possible to conserve ammo for when the Tank knocks a hole in the wall and munches on my head. Basically any time that 'Oh no zombies' music isn't playing I try to stick with the pistol.

I'm in the air on the M16/Hunting Rifle thing. On the one hand the hunting rifle has massive penetration, which means that you can occasionally drop six zombies in one shot if they line up running towards you. The scope let's you reach out and touch someone, and there is nothing better when you need to drop a witch without getting frapped.

Oh the other hand the M16 is much better when you're swarmed and seems to do better against the tank. It doesn't have the same shot for shot damage, but it's still very accurate and two round bursts can usually drop any zombie you can see. Also, you can wade into a mess of zombies that are munching a downed buddy and unload the whole magazine to very quickly clear twenty or thirty Zack.

I almost never use the auto shotgun. I always seem to run out of ammo, and I don't like the spread. It's okay for hitting zombies, but it's wonderful for hitting your teammates. Also, it's in theory possible to get fifty kills in one magazine with the m16, but with the auto shotty you are going to have to reload at some point.