Pattern it? What's that mean?
Seriously, I know what it means. But unless you're getting into hardcore, competition trap, don't bother. The only people I know that "pattern" a shotgun are turkey hunters.
Stick with a modified choke and some good target loads, and you'll be fine. An 870 is an "out-of-the-box" shooter. It requires no prep work.
Victory rewards not the army that fires the most rounds, but who is the more accurate shot. ---Unknown
You can't go wrong with the 870. I use mine mainly for home defense, but it likes the clays too.
My mind is a raging torrent, flooded with rivulets of thought cascading into a waterfall of creative alternatives.