I'm sorry that I did not see this post sooner. Congratulations on your new gun. I'm a true 870 fan. You also have a fine choke in that Kick's .650. I like that choke a lot and I also like that e.d.
I have barrels for 870's that range from 21" to 30". I have 1 barrel that's 21" long that shoots good patterns. I always find that barrels 26" or longer shoot better patterns but that does not mean that a shorter barrel won't shoot good patterns.
With that 21" barrel and the Kick's .650 choke, I'd shoot either 3" Winchester Xtended Range Hi-Density shotshells with 1.75 oz. of #6 shot or the 3" Winchester HV (Black Shells) with 1.75 oz. of #6 lead shot.
If that barrel and choke are "deep cleaned", I'm sure you'll be very pleased with the patterns you get.
"If he's out of range, it just means that he has another day and so do you."