I cannnot tell you what that particular 870 will shoot most accurately. And neiither can anyone else. Not until you put a good deal of EVERY slug out on the market thru it and see what kind of results you get.

But I can tell you this. The SMARTEST thing to do is just start out with your basic foster slugs and that would be Remington Sluggers. Don't beat yourself up with 3-inchers, 2-3/4 will do. I say Remington slugs first (cause the 870 is Remington, probobly designed & engineered the barrel using their own ammo, atleast I would think so). Then try some basic foster type slugs from Winchester, etc.... All these slugs, say maybe 4-5 boxes of each times 3-4 mfg's. @ $3/box will give you a pretty darn good low down at a total price of maybe $75 - 80. Then you can try some of the little bit of the basic offerings of "higher end" stuff like Brenneke and Lightfield. Now these are gonna run you more like $5 - 7/box so add another $40 - 70 to your total investigation.

Now, Brenneke has many offerings for your smooth bore barrel and Lghtfield have a good number, too. So adding the $40 - 70 can happen several times over.

Bringing you investigation up to anywhere from $3/350 - (could be) $500.

Getting the absolute best shooting ammo thru your individual gun can, easily cost more than the gun itself. No BS.

I would.................

try 4-5 boxes of sluggers

try 4-5 boxes of Win SuperX

at this point I think you should be shooting 50 yards maybe (for some control) and the difference won't be that great, maybe not be able to tell much or any difference.

try 4-5 boxes of the "plain jane" Lightfields

try the Brenneke Rotweils (the ones with the felt wad attached to the back of the slug by screw).

Something in there has got to put you in the race, my bet would be Rotweil (personally).