The 12 will work with a pheasant load, if the bear is close enough.
The course I took suggested spray is effective only if used accurately within 12 feet. A bear at 30mph covers that quite quickly.
A blast of bird-shot to the face from a choked gun at that same range, should pen. his frontal skull.
Yes - slugs are much better - buckshot will work at spray ranges, but not much beyond that 12 to 15 feet. It, like bird shot, has to hit as a solid slug would.
Best to pack slugs.
Here, there is no such thing as a 28 bore slug load, that I am aware of.
I would prefer something over an ounce.
-------------------- Daryl
"a gun without hammers is like a Spaniel without ears" King George V