Fiber Optic Sights ?

[schoots]

I have got the F.O.S made by Star Dot for my 870 Rem Exp. but I dont know where to mount the rear sight on the gun. Can anyone let me know how far up the barrel I should be mounting it. Thanks

 

[beretta boy]

Generally, just behind the third rib post ahead of the receiver.

 

[Mike Oxbig]

I put a rear bead on my Nova for shooting slugs at Deer last season. I put it right above the trigger (farther back the better) using mill shims (one at a time) to bring it up until it shot straight at 50 yards.
Shots at 50 yards with slugs gets me in a 4" circle (offhand smooth bore) without trying too hard.