I think all those saying shooting clays with shorter barrels is hard, are full of baloney.
Here is what I suggest, and it ties in with another poster. Get a Remington 870 Turkey which has a 21" barrel and rem chokes. Put a modified choke in it and add a +2 or +3 mag extension. Then, your good for defense or sporting use. You will not have any problem scoring hits with that 21" barrel. Another option would be a Mossberg 500 youth with 21" barrel and screw in chokes.