(Gazing sternly at Chris over reading glasses, what the kids call the Daddy Look)
Thanks,Chris, nice to know the message is getting out.
Buzz, Chris nailed it,hubris aside.
The time and place for an accessory or modification here is when performance CANNOT be improved by practice and training. That's farther up the learning curve than most go.
Of course there's exceptions, in the real world few absolutes rule.
The shotgun HAS to fit, tho not down to the right 1/32". That's the whole ergonomic bundle and not jsut the fit of the stock.
The trigger HAS to be clean,safe and of a weight the shooter finds controllable.
The sights NEED to be easy and fast to acquire.
Buy an 870, new or used. Make sure these three things are OK, then go shoot. Use good form and burn up a few cases of lighter loads. After maybe 200 rounds,note how much better you're shooting. Buy more ammo, and when you stop improving, figure out if it's form, fit, or the lack of gadgets that's blocking you.If that's hard,post rat cheer, there's a lot of helpful folks around this BB.
HTH....