A lot of states are starting to do it that way. Nevada does but their kicker is that if you want a bonus point you have to pay, if you don't want a point then you don't have to. In Utah you have to purchase a license but if you work it right you only need to purchase one ever two years since the life of the license is 365 days from when you purchase it.
I go to Arizona every year to hunt javelina so I have the license and then I have been buying a bonus point for deer every year for $7.50 but that looks like it is going to change next year since they are revamping the system.
What does "gauge" mean anyway? As used here gauge means the number of round lead balls the same diameter as the inside of the barrel (aka the bore) that it would take to weigh one pound. It takes 12 lead balls the same diameter as a 12 gauge barrel to weigh a pound. The smaller 20 gauge would require 20 balls of that barrel diameter. The larger bore 12 gauge would require 12 lead balls of the bore diameter to weigh one pound.
In general the number of pellets in a shotgun shell...