26" on a game pump
Back in the days BEFORE choke tubes, I ended up going w/ a 26" IC tube and a 26" full tube for my 870 and found it an ideal practice.
Long enough to offer a sight radius, but not cumbersome in cover. Noticably more handy and practical than 28 or 30" guns. I put a mid-bead identically on both,and the gun swung and pointed the same no matter the choke.
If I wanted to improve on the IC tube, I shot premium ammo, sometimes short mags for decoyed ducks (and we shot LEAD, heaven forbid), and I felt like I got "modified" results. The full barrel was reserved for pass shooting , usually high doves, and and busting called turkeys.
I haven't had either on the 870 in years, having bought other shotguns, and not wing shooting near as much. But back then it was a good combo.
A twenty-six inch tube on a pump worked fine for me.