im going to go black bear hunting next year and was wanting to use a shot gun and i was going to sight it in but will a 20 gauge slug work for a black bear?

Someone is probably going to tell you to go bigger. I was wondering the same thing, my wife wants to go next year and I'm contemplating getting her a 20 for the job.

A modern 20ga with a rifled barrel using one of today's 3" slugs will kill a black bear with no problems. It is the same as with all game where you need to study the vitals area and then place a slug there and the bear won't go more than 50 yards or less.

PS: Of course this is keeping your shots 75 yards and under. Will you be hunting over bait from a treestand ?

I agree with Big Dawg, no problem on Black Bear at closer range, There are a lot of different slugs on the market, try a few and see which one is more accurate in your gun. The 20 gauge Buck hammer load has a MV of 1500 fps and ME of 2236 ft. lbs. The 100 yard figures are 995 fps and 1074 ft. lbs. Zeroed at 50 yards, the 1 ounce slug should hit 4.6" low at 100 yards.

20 guage will do the trick with good slugs and a steady hand