I agree with the idea of being familiar with the handgun types found in an area, but I don’t see basing what I carry on what the police or military carry as being huge. I mean I am not very likely to share ammo with someone I don’t know. Would you? The idea of scrounging ammo during a firefight isn’t being put forth by very many.
So you got an AR and 3 mags, after a brief encounter with gang _____ you manage to break contact and beat feet – after all there was 20 of them and just you. A mile down the road you see a couple of cop cars, so you sling your rifle and walk up and ask for more ammo? I would be a little hesitant on that. As an individual, the only way that I can see of resupplying with ammo in a crisis is a) you ambush another individual and take theirs or b) you happen across someone else that all parties involved in a shoot out are now dead and you can secure whatever gear that is there. From a purely tactical stand point, both ideas are less than ideal. Ideally you want to move from where you happen to be to where you need to be. If you can do that without firing any shots, that is best – less chance of injury and death.
Yes I do own rifles in 5.56, 7.62×39, and 7.62 NATO. But I have them because the ammo was common when I bought them and easy to get. Now in my area of the US, no caliber is in abundance at any store. Today, perhaps I would buy a rifle in another caliber if I needed an new rifle.