...I have an 870 with a 14" Dlask barrel and no rear sight and I can hit within a the big orange "X" on a standard sight-in target at 50 yards consistently. I'm figuring that's about a 10" diameter circle.I practice regularly with slugs out to 100 yards and I know that if I had to take a big game animal in a survival situation with my shotgun I am confident I could score a vital hit [within a 14" diameter circle] at that distance almost everytime.
The key is to buy the 15 packs of 2 3/4" slugs at Canadian tire and just go out and practice. You will be amazed at how well you will be able to reference your front sight after you can actually see where the slug is hitting.
...BTW my Dlask barrel has a post type front sight not a bead, but FWIW I used to get similar results with slugs from my 1300 Defender using just the front bead.
Practice and Enjoy,
Mad Mikey