I am a very avid hunter,beyond 30 yards or so a 12 gauge just isnt going to kill a coon all the time...escpesially if you dont get a good clean shot..Even if you did use a 12 gauge,you would have to be close(most likely you wont be able to get 50 yards maybee)and you would have to have a full choke, and heavier loads,(turkey,duck, or buckshot loads)
bird shot would just wound the coon,and the coon will most likely come back after a few weeks.
A .22 LR is the BEST way to go for coons.
It will kill a coon at 70 yards or better,if a good head shot is present.
Take safety precautions ALWAYS while shooting rifles..and shot guns, though rifles have longer range damage.
If your aiming at a downward angle(shoulder height- to a coons ground level,if you miss, your bullet will get submurged in the dirt.
If you live out in the country and are shooting up in a tree,ALWAYS make sure your shooting in a safe direction
.22 LR can travel a mile or so, if shot up into a tree.
So, if you want to really take care of the coons, a .22 is your best bet... SAFETY FIRST...