I have an 870 with a Improved Cylinder choke that I use with buckshot and rifled slugs for deer. I just got it recently and have only shot out to 50 yards, with good results. I'll hit the 100 yard range on New Year's Day, but I'm not expecting anything great with my setup.

If you want to make accurate coyote shots at 75-100 yards, getting a rifled barrel and using sabot slugs is probably your best bet. I've heard that using rifled slugs in a fully-rifled barrel is not a good idea, but don't quote me on that. Better ask the resident shotgun gurus to be certain.

Cabela's sells Remington replacement barrels in the $200 and up range, including 20" full-rifled deer barrels. Hope that gets you pointed in the right direction!