well... with a bong, you pull what you want and when you want. a shotgun usually consists of someone blowing the smoke into the chamber, while you pull from the other end or them coming directly up to you and blowing the smoke directly in your mouth... there's also the cupping hand method for those that's aren't comfortable with their sexuality.
My personal favourite is giving yourself a shotgun... by cupping your hands around your nose/mouth area, reversing the joint in your mouth and blowing lightly while inhaling through your nose. for some this is tricky.. but if you had a good music teacher in school and payed attention, then it should be pretty easy..
The solo shotgun is not for the lite toker though, i do warn you.