I now have 1000 registered targets under my NSCA belt, worked my way up to C class, and everyone of them was shot with my Remington 870. That's not overly impressive, but I have learned a few things. First, shooting a pump will handicap you, but not much. Second, make sure the gun works properly, before you shoot your first hundred and nearly get a DQ due to malfunctions. Third, and the point of this post, make sure the gun fits you!
On a sporting clays course you will get some strange looks if you use anything other than blued steel and fine walnut. So I purchased a nice stock/forend set. B-E-A-utiful wood if I do say so myself. But, it's made for trap guns.
It's significantly too long for me, but I wasn't worried because the forend comes all the way back to the receiver, meaning I could hold it farther back. Bad news there. The empty shells coming out would clip my fingertips and sometimes fail to eject. It would also cause me to torque the forend against the receiver and make shucking very difficult, at best. So now I have come full circle, back to my Magpul furniture.
It's not pretty, but it's beyond functional. Adjusting the length of pull is a snap and the forearm is M-Lok compatible so add anything you want to it. The Magpul covers I use are abrasive enough you need to wear gloves. But that's ok too, keeps sweat from being an issue.
Gun fit is subjective for everyone. I'm sure there are minute details about it that are over my head, but in large you will know when it fits just by how it feels. It's more important than I originally gave it credit for. True gun fitters are not common and their services expensive. Wood is better for them to use. Polymer on the other hand is typically easier to configure with parts to get "close enough". If you are serious about breaking more clays, or birds, or general accuracy, fit the gun to you, don't try to fit yourself to the gun. I'm looking forward to the knowledge that comes with the next 1K targets and I have every intention of working my way to M class with this very 870. Maybe I'll paint it brown to better blend in!