I have a M500 8-shot 12ga shotgun backing up my G17 here in Hartford, VT. I have it loaded with a special load that has a 12ga slug as well as three 00-buck pellets. I was thinking of No. 1 buckshot because I read where it uses the shell volume most efficiently, but I am concerned about the smaller buckshot's penetration ability, especially through apartment doors and windows. I'll bet you know which buckshot is best to use.