RE: max distance with a smoothbore shotgun
I have a couple 870's, one has a 26" improved cylinder barrel and the other has a 30" full choke and the rule of thumb is the shorter the barrel, the better it shoots slugs. The 26" that I have can spit a slug out and hit a pie plate every time at 100 yrds. I'm not sure what a modified can do but go down to the range and cheque it out.
Bobby