only thing i worry about with the 870 full is bulging the barrel with bigger shot. i have used 4's ok but would like to try 2's and 3's. this old 870 i got from a dear friend since passed and don't want to harm it.
Remington has always said their fixed full choke single barrel shotguns built after 1960 where safe for standard steel loads up to #2 Your 870 will be perfectly safe as long as you stick with the Remington 3" 1 1/8oz 1550fps loads in #2 or 1 1/4oz 1375fps #2.
