I would suggest a M68A1 120 mm cannon, however, it is only affixed the the M1A2 Abrams tank as of now.
I have personally killed 4 black bear, my brother 1, brother-in-law 2, and couple friends 1 each.
I find it simply amazing the number of people who recommend a .44 magnum handgun for bears but will recoil in horror over the thought of using a
30-30.
Even with the hot Buffalo Bore +P+ 340 grain out of a 7.5 inch barreled pistol you generate only 1478 fps and 1649 muzzle energy. A Federal 170 grain 30-30 out of a carbine has 2200 fps and 1800 muzzle energy.