Here's our "game" with the clam shells. No prizes, but challenges for shooters of all skill levels:
Set up a row of shells for each shooter- five, three, or one, your pick. Start shooting at a comfortable distance for everyone. Maybe 20 yards or so.
Hit all of yours in as many shots? Set up another row of shells 5 yards further back and do it again.
Pretty quick everyone is handicapped for their own shooting abilities, reaching a point that they can almost get all of them, but not quite. You might end up with clam shells at 25, 40 and 60 yards, but everyone is challenged equal to their skill levels. If matching skill levels isn't an issue and you need to announce a winner, whoever's clams are furthest back is the winner. Bring lots of powder, balls and patches cuzz these shoots can go on for hours and never get boring.