OK, Badger Arms, I'll do the full explanation this time...
Min, you can get a barrel chambered for 3" shells and install it on your receiver for use with 2 3/4" shells only. To use 3" shells takes the mods mentioned.
BA hinted at this. Waterfowl and Turkey require big loads. I shot a mess of geese and some ducks with dinky ol' 2 3/4" loads back in the days of lead shot, but A, lead's a nono these days and B, we hunted with experts and rarely had to shoot more than 35 yards.
In the present era, non-toxic loads oft lack the moxie of yesteryear. And, shot opps run a little longer,even for those of us who can really set decoys and call birds.
The 3" maggie numb compensates a bit, and the 3 1/2" shell teeters on wretched excess, IMO. Some people like it that are not sky busters nor Magnumistas.
Still, the short cases do the job most of the time.
I do a fair amount of shotgunning. Out of 7 shotguns here, only Frankenstein gets Mags on a semi regular basis. Naturally, it's what I use for Waterfowl and Turkey.
The cost of a new 3" capable barrel and changing out the ejector on your present 870 would make a nice downpayment on another 870.
Need an excuse for another shotgun?....
However, if that's not in the cards, yes, it can be done...