My dept's training includes shooting skeet with our 18" and 20" 870.'s. I am no skeet shooter but I have managed 21 out of 25. My current duty shotgun is an 18" police magnum. Yes it has a choke and has been ported. YES it is loud...but oh so sweet to shoot. I use a modified choke due to most of my shooting invloves
buckshot. I would probably go to an improved for skeet.
BTW over at www.ShotgunWorld.com, one of their moderators is a gunsmith. He gives substantial discount to members. If I remember right he charges like $45 for installing a choke. I have been thinking about sending a gun to him. I figured choke, forcing cone lengthend, polish, and shipping was going to be less than
$100. Check the gunsmithing forum over there.