I really could use some help with my 11 year olds new turkey gun. It is a Remington 870 mag 20 gauge with 21" barrel. It came with a modified choke tube. I need to buy a good tube for turkeys and have found some but don' t know which restriction or brand to try. I know it' s just a guess but any suggestions would be appricated. I also could use some suggestions on shot size. Thanks in advance. Don