Ok, say I'm the guy with the gun. I'm outside and the thieves are within a short distance, they see my gun. Here are two possible scenarios:

A. If they were running away, I wouldn't have fired a shot. Would've just got back on the phone with the cops and would've just told them what direction the thieves were heading and what they looked like. (gender, approx. height/weight, skin color, clothing, etc.)

B. Had they been coming towards me, in a threatening manner, here's what I'd do:

1. I'd Warn them of your intentions if they continue their course of action, if they continued their course I'd fire a warning shot, to let the thieves know the gun is loaded and that I mean business.

2. Next I would point the shotgun at them and give out a final warning.

3. If they're still coming towards me, I'd fire, without the intention of killing. You know like hit them in the legs or something, disable the assailants. If this doesn't work, or a wound shot isn't possible, say they pull a out a gun or they attempt to attack me, lethal force may be necessary at this point.