I think that one reason the 12 is so prevalent for HD over the 20 is that you can find a lot more loadings/specialty ammo for the 12. I've read that a lot of larger departments are going to the 20 for women and smaller statured men and not having any problems.
When I first started hunting, I used a 20 but I switched to a 12 because of the ammo situation that you mentioned. I' ve read that the Air Force Security Specialists are transitioning from the 20 gauge to the .410 because the heavy recoil of the 20 was messing up their hair.
