I have a lot of experience with buckshot, not for deer but coyotes, we patterned and tried many different choke combinations and brands of buckshot, I watched a guy blow a choke right out of his barrel one day shooting at a coyote, it was obviously way to tight lol, anyways Remington 870 supermag 3.5” gets the vote around here amongst us 20 or 30 coyote hunters, winchester brand 00 buckshot, 18 pellets per shell, and our best tested choke was remingtons factory super full turkey choke, it sticks out of the barrel about an inch, I could take a picture if you need one, I have one of the first 870 supermags made, I live 30
minutes from where they used to make them, I spent a lot of time and money testing stuff and this was far the best I could come up with, I have zero experience for deer and buckshot.