A Mossberg 500 or Remington 870 will take interchangeable barrels of 18" for home defense, 26" for Skeet, and 30" for Trap.
If you are talking about shooting either or both of the regulation sports...
Skeet is a short range game, a 26" cylinder, skeet, or improved cylinder barrel is good.
Trap is a longer range game, a 28 or 30" modified or full choke barrel is best. Also, you should be shooting at the clays before they have quit rising off the trap machine, so a strap on cheekpiece to raise your line of sight would be good.
Of course one barrel, anything from 26-30 inches, with interchangeable screw chokes would cover a lot of ground.
Shooting regulation Skeet calls for four pairs of doubles per round of 25. You will have to get good pumping the gun for the second target but it is not impossible. It used to be common in the days before over-unders and reliable autos.
If you will be shooting over a portable trap in the corn field, get a screw choke barrel so you can go along with the gang and the way the machine is set up.
