Well, it's obviously a close-range weapon so you need to practice efficiently closing in on an opponent. If you have the chance, use cover as you approach and try to fake your opponent out with the direction you're going to go. The other way is to try and approach from a blindside. Most folks have their camera facing forward only so even just approaching from the side will allow you to get the jump on them.
Once you get in close, use auto-aim. Close-range firing is really difficult in the game and I don't know why folks don't use Auto-Aim for it, it was built specifically for that. Anyway, as soon as you get close enough for a lock-on, fire and keep circling them as you fire. The auto-aim will keep you locked on and pressing sideways will just circle ya. You'll need to compensate in cases where they roll away so rotate the camera in the direction they're going and retap L1 to face that way. You should be able to kill someone in 2-3 shots if they don't get one of the crappy headshots that are far too abundant. You also get stun (or knockdowns with Slugs, as I recall) so that can help quite a bit.
Also, Slugs work as a long-range bullet if need be.