I believe the actually reason for the 21 foot rule was to make a convincing argument in court. There is a grain of truth to it, but the rule aspect is for the benefit of the people in the 12 cheap seats.
The flip side is when you do get a bunch of shots, which has happened. But also, when the shooting is in defense of a remote person. So there was a case in Canada, but the couple was not able to get to their guns, and one of them died. But the situation was roughly a triangle. Bear, husband, wife. If one of them had been armed they could have shot repeatedly at the bear, which was after the other person. There was some distance between them, so you would not want to gamble the whole thing on one or two shots. And you want an accurate enough shooter/gun proposition, not one that is mostly effective when the assailant has taken up residence in your lap. Just as a numbers game, I want a weapon that is capable of defending others, as well as myself. And with some capability at a distance. Really yells rifle. But at least something modern would help. Of course, in certain situations, you might have all three. Say armoring your jeep. People like to yelp about great white north gun laws, but we can legally get shotguns with 12 inch barrels up here. Personally I like longer barrels, but I think I have seen 8 or 10 inches, if one wants something that is very compact, but has the capability to carry optics, and to be shoulder fired.