Shotgun choice is simple...Remington Model 870 with 18 1/2' barrel with either rifle sights or ghost ring sights (Tritium front sight best option for low light conditions). Tactical or Police doesn't matter too much, just no front bead only. Prefer extended magazine for extra capacity.
Reasons:
Number one choice of military and law enforcement due to RELIABILITY! Dual steel feed rails as opposed to single aluminum feed rails in other makes.
Extended magazine tubes allow for up to 7 shots in firearm.
Rifle or ghost ring sights most accurate and durable. Good designs will not cause hang up issues.
Only model allowed by most companies hiring Wildlife Monitors (bear watch/problem wildlife) in BC.
Slugs only! 2 3/4 or 3" Buckshot not allowed for Wildlife Monitor work in BC. 000 buckshot will NOT reliably penetrate 1" plywood at 15 yards...do you want to trust your life on this during a charge?
Rubber bullets/non-lethal ammo not allowed in BC.
If choked, Cylinder only...or you may damage your shotgun using slugs. (Bulged barrel or worse)
Regardless of presentation, aim for big bone! Head, shoulder, spine...
A solid hit to big bone will drop the animal, kill shots can be given after that.
As stated earlier...practice! With what you have. Muscle memory and mental preparedness biggest factors to success in such a situation.
Better yet, take a course where you have to be tested for firearms proficiency. You are tested on time, accuracy and ability to hit a moving target (charging bear).
Have had to shoot a charging black bear, years ago, at work.
Luckily, I had a rifle in my hands at the time, as I had seen it enter the worksite, before it saw me, and I had gotten the rifle out and loaded, before it charged, unprovoked, upon seeing me.
I shot it with my lever action in 375 Win (as I did not have a shotgun at the time) at 5 yards, and again as it turned away. A finishing shot was given when it stopped 15 yards away. 3 shots in the vitals with 200 gr bullets. But I had not hit big bone. First shot was in the chest, under the chin. I did not have the training then, that I do now.
I have shot many black bears and a grizzly while hunting over the years. This has added to my experience on this topic.
Have also taken the training several times over the years, and helped develop our new Wildlife Monitoring policy a few years ago.