This needs repeating DO NOT USE BUCKSHOT. Any size or load does not give anywhere near the needed penetration on a bear. Many places have outlawed its use against even 75 lb deer because it does not kill quickly enough. Buckshot is a fine choice for SD against humans, but anyone who would suggest it for bears simply does not know what they are talking about. At best you may deter the animal, but either way you are putting someone else in danger. You, or someone is now going to have to go into the bears backyard and track down a hurt, angry bear and finish him with a proper gun.
The gun you have with slugs is fine. Any shooting will be at very close range anyway. Most any hunting rifle .30 or larger would work as well. A good leveraction in 30-30, 44 mag, or 35 is probably as good as it gets. If I had a 45-70 or 444 I'd use it, but you don't have to have that much power and wouldn't suggest going out and buy one just for this purpose. 30-30's are everywhere and cheap. They have been killing black bear quite reliably for 106 years.
A proper handgun is an option. A 357 mag or 10mm with good loads is the minimum. Both 357 and 10mm have loads pushing 200gr hardcast bullets at 1300 fps. If I were using handgun I'd feel OK with either of these. Nothing wrong with 44 mag either, if you can shoot one well. I thing the really big 460 and 500 handguns are better suited for hunting and are a bit much for this. At the close ranges you would be shooting you don't need the extra power of these rounds.