I shoot an 870 that I drilled & tapped for a weaver base(429M). Cabela's 2.5 shotgun scope with diamond retical. I tried a Red Dot & didn't like it, I think they sit too high and this does't let you get a good cheek weld. B-Square & Weaver sell saddle mounts and again this makes the optics sit too high. If you decide to get the weaver base and drill and tap your shotgun only a standard length scope will fit, the cross slots on the base are spaced too far apart for a red dot, and a compact scope like a tru-glo 4X will not have the proper eye relief. If you put a piece of 1/2 inch foam under a uncle mikes or allen black elastic buttstock shell holder the comb of the stockis the perfect height. Badger Ordinance and others make a picatanny rail for the 870 but these cost about 50 bucks. If you are set on a red dot this would be the way to go. or you could get additional 5/32'' cross slots milled on the weaver. I was going to do this but I don't have access to an end mill.

weaver 429M base -R&R arms - about 5 bucks
6X48 high speed tap -Brownells- about 5 bucks
#33 drill bit -Brownells -2 or 3 bucks