Many turkeys have been killed with standard shells like #7's. You would want to be shooting at 20 yds or less if you could. Since you are not an expert caller, it would be better to use a "turkey" load, probably in the #5-6 range. You don't specify what choke you shoot, but be sure to pattern your gun to determine your effective range. I've had good luck with 3" #5s but switched to #6s last year. I shoot a standard full choke and the #6s have about 100 more pellets than the #5s.