A 20 ga can kill a turkey just as far as a 10ga, provided you put a pellet in the right spot. Obviously, pattern density is the variable. Pattern your gun and shoot as far as you have a dense, consistent pattern. My son shot his first two turkeys with a .410, and quite a few after that with a 20. Longest shot with the 20 with factory full choke and 6's was about 35 yds, which is a lot farther than it looks..