Trying to make an 870 Express into a Trap gun is ... well... it just ain't worth it !
An 870 Classic Trap on the other hand, while a fair bit more expensive, is a pretty decent Trap gun.... but it's NOT
a Sporting Clays gun, Skeet gun, Waterfowl or Upland gun ! Kinda like trying to use your 5 iron for a complete round
of golf ... you can, but most will never do well with it. It ain't a driver and it ain't a putter !!!
Trap guns are specifically made for the game ... they aren't a "one gun will do everything" deal ... they are made for
one purpose ... and one purpose only.
Your 870 Express is a pretty decent gun for waterfowling ... I have one I use for almost nothing else, and it works very well
for that purpose. I also have the aforementioned DT-10 ... albeit in a 32" Sporter configuration. It's O.K. for shooting Trap,
but it's not a Trap gun either ! Yes I know, you can shoot Trap with almost anything ... and some do well even with their
18" Riot guns, 23" Upland guns, 26" Skeet guns and so on, but a Trap gun - made for Trap - will give you a leg up.
I'll concur with what another gent has recommended. If the budget allows, I'd look for a used (but not abused) Beretta 391
TRAP gun. They are as reliable as semi's come, easy on the shoulder and feature a stock design with shims that you can
easily change the drop at heel/comb and cast off/on to get a pretty good fit. Even an older Remington 1100 Trap gun -if well maintained -can be found for a reasonable sum. They are easy to maintain and make a decent gun for the game. They are not that hard to modify to get a better stock fit, although they take some judicious wood work as opposed to Beretta's shims.
Remington's pro (Industry) Trap shooters D. Lee Braun and #### Baldwin both made their bones with 1100's !
You will no doubt get a ton of opinions and all kinds of brand preferences and prejudices, but after owning some 60 or 70 shotguns over the years, you may take my 2 cents worth at no charge ! ... and yes, stick to "Target" loads !