I also have this setup. I hunt everything I can with my slug guns. First what range. My 870 is set up for short 100 yard shots with buckhammers these are no longer available thus I have moved to federal fusion. Fusions are huge slugs same slug as there 12 gauge and only 8 dollars a box. If you want the most accurate longer range slug and don't mind 15 dollars a box than Accutips are far and above the best slug I have ever shot out of any of my slug guns. Good luck.
I had a 870 super slug. The only one I could find to shoot under two inches at 100yds was the federal barnes expander 1oz slugs. Everything else it sprayed. I don't know what twist Remington's have in their barrels but they don't seem to stabilize anything with velocity higher than 1500fps.
I have gotten rid of that barrel and am looking for a badger barrel. They have 1:26twist and shoot exceptionally well. I've read where some are getting moa passed 100yds with them. But I can't seem to get one on order. I've called them before and they said they would call me when they have them ready but still waiting two years later. So if anyone knows what the deal is with them let me know please.
I second that buy a H&R if you only need one shot. Keep the rem for fast shooting. On my 870 I have a Hastings 24" with muzzle break 1 1/2 groups at 100 yds. with Barnes non-tipped expanders. They no longer make those and switched to a copper solids. I should sell that barrel I paid more for the Hastings barrel than I did a H&R combo! I use my Rem bird barrel with im.cyl for under 100 yards and driving deer. I can get on a deer real fast with out a scope. And it makes 3" groups all day with cheap winchester rifled slugs. The H&R will stack em on top of another at 100 with Accutips. So, how often did you need a second shot?