Scout. i own the Marlin 45-70 guide gun in lever action.It has a 18.5" barrel with ports.I have never shot a black bear,but have shot deer with it.I shot a doe at 15yds from my treestand on nov-23 WI gun season.I used a 350gr hardcast bullet.The doe smacked the ground right in front of me.The 45-70 is a perfect black bear round.It has alot of power.I reload for my 45-70 and like the Hornady 350gr flat pt bullet at around 1950fps.Its a real thumper on the shoulder but alot of fun to shoot.Its my favorite caliber to shoot.I have never shot a encore but i bet its alot of fun!Another good bullet is the REM 405gr along with the Nosler 300gr Protected pt.If you ever need loads i have the Nosler #5,Hodgdons #27,Lymans #47
Good Luck black bear hunting!