Check the taper of your barrels-most have a taper that will allow shot to intersect at a given range, so keep your slug shots at that range for reliable hits. If you need really long range, check your local regulations on what you are hunting, as well as if your gun is proofed for steel, then have someone load you some flechettes (steel darts, Vietnam era sniper suprises) with some dehydrated habenero pepper seeds in the hull with the darts. These will kill light skinned game (humans down) up to 300 yards away, and because they are built off of speed and not weight, you can let fly with both barrels at the same time if you have too. The pepper seeds (they flake and attach their meaness to the darts) allow you to track the wounded animal if it comes down to it, since there is little blood.
