I have 3 Rem 870's, for all of them I use a .665 choke. As far as a shell if you want a tight pattern, use the Winchester Longbeard's XR in a #5 shot. I can guarantee you you will not have to worry about getting another brand or two of shells to get a good pattern. If you don't like a tight pattern I would go with winchester Double X #5 shot. The pattern will be a little more open than the longbeards. As far as your undertaker choke I don't know what size it is, so I couldn't give an opinion on it. I wouldn't choke the #5's down very much more, You could probably use a more open choke if you wanted to open the longbeard pattern up.