"Consider maximum effective range to be about 60 yards."
I'd view 60 yards as a real "stretch" for my 870 Special Purpose, even with super full choke that came in it and 3" premium loads. Have done quite a bit of testing with the buffered, copper plated turkey loads on Shoot'n See turkey heads. With the high-dollar rounds, it's consistently good for at least one good CNS hit out to 50 yards, but 55 yards can begin to get "iffy".
FWIW, the 3" premium turkey loads mentioned above have worked great for me from 21" bbl. and modified choke on Texas Panhandle pheasant. We work hard to find 'em and when my teenager misses or wings a cock, I like being able to drop 'em clean at up to 50 to 55 yards with the 4's or 6's. Again tho, even with the most expensive loads, I would not feel good about intentionally taking a 60 yard shot unless the bird was already wounded. Speaking of expensive loads, I popped for a box of Remington's "Heavy Shot" and am looking forward to see how they work out from 870 under similar circumstances this weekend.