Standardized Spacing For Rail Taps?

[Studer]

Sorry if I'm not wording this right. I have a 870 clone shotgun that already has been tapped on the top of the receiver, for what I believe to be a rail for an optic. There are 2 screw holes, spaced approx 1-7/16 inches apart. Is this some sort of standard sizing/spacing for the taps? I don't want to buy a rail just to have to re-tap the receiver to make it fit. Thanks for the help. Hopefully I explained that right

 

[Potashminer]

I am quite sure that you won't have much luck just buying "a rail" at random. The mounting dimensions are often specific to the fire-arm - there is no "standard" that covers all applications. The top cross section of your receiver will have a contour - might be flat, might be a radius of a certain diameter. A flat bottom rail will just wobble side to side on a rounded top receiver. A rounded bottom rail cut to a larger radius will do the same. There must be a dozen or more one piece "rails" listed on Weaver charts - differing bottom shapes, differing hole spacings. I am not familiar with Picatinny makers, but I expect they would make similar. As well, it might be totally normal for shotguns, but I would have expected 4 holes - standard mounting screws are only #6-48, although I believe many "tactical" type arms use larger #8-40 screws. I do believe that you would be way ahead to actually see a listing for a specific "rail" for your make and model; next best might be to take your receiver to a gun shop that will let you try to find a matching set-up.