I read Alaska Dept. Fish and Game literature on bear protection means and methods, but by and large, they're no more expert in this field of study than experienced hunters. They're typically biologists. Not big game guides.
Almost any field shell from a 12ga shotgun will stop a bear at 5yds with a shot to their skull. Even birdshot. F&G's study with #00 buckshot was conducted at greater yardage, where the penetration of individual pellets could be examined. They found individual pellet penetration to be lacking against brown bear skulls. If they had tested a load of #00 buckshot at only 5yds, they would have reported their brown bear skulls were destroyed.
Another consideration on the effectiveness of bear spray, which I believe is not openly discussed, is its effectiveness on bears intent on fighting to the death, versus ----- bears that are testing their opponents the same as kids do on the playground. When I read instances of bear spray being dispensed, yet the person/people involved are still mauled and killed after dispensing their bear spray, I reach the conclusion all bears are not created equal, and bears come with differing intent and commitment, under different circumstances. By and large, F&G departments, and especially USF&WS and National Parks staff would prefer that people not harm "their" bears. Many individuals working for such Government agencies are bunny huggers, and sympathetically to the wildlife, recommend non-lethal bear defense over firearms, IMO. Same with National Parks fighting firearm possession in our parks. And even employees that may personally feel differently bow and defer to the company line when discussing the subject 'on the record'. They know what's good or bad for their personal careers. Furthermore, no published papers leave the printing presses from these agencies without internal review and the blessing of the top brass. So what's published has been filtered to align with the preference of top level administrators in agencies that hug trees whenever possible.
The true test of bear spray effectiveness on bears that are playing for keeps can only be completed in real life instances of fight to the death attacks. And we've now had examples where the bear spray has failed to deter, and victims are killed after spray is dispensed. The only bear that's a safe bear is a dead bear. And bear spray is designed to keep them alive.