- Most folks are going with shorter barrels now a days, like 26" because many newer shot guns have barrels that are "back bored",which means that they shoot a tighter pattern with shorter barrels. I do not think your 870 is back bored though... The best choke tube for doves is the modified to improved modified.
-For waterfowl you can use your modified tube for steel shot, it will act like a "full" choke when you shoot steel through it and is generally considered the best for ducks and geese.
-Some folks are using the "extra full" tubes made for waterfowl and steel shot. The advantages are less cripples and greater shot distance and range.
-When I hunted with The Duck Commander - Phil Robertson, and asked him what type of chokes his bunch use in thier duck hunting videos. He said "we like extra full choke tubes, and tend shoot out shotguns more like a rifle,.. to decrease cripples and get better range on camera......"
- Hope that helps and happy hunting!!!!!