As for the shotgun, I suggest insta-gib with ridicoclously slow bullets, so it's like you can spit further than the bullets travel, but if ya'll catch a chance from above, you can have a nice multikill with a shot.
And the M79 slot - well, ya can give it damage 0, if it hits directly, it's still an instant-kill, if you barely miss, you at least get the explosion effect thing.