? Reference/Datum points of various optic mount standards

SB_Pete

Newbie
Question is from what bottom point to what top point are all these new red dot optic standards? Ie for 1.42" 1.57", 1.73", 1.93", 2.04", 2.26" mounts, what is that measurement exactly?

Is it from the top flat of the Picatinny rail? The bottom flat? the bottom flat of the mount that interfaces w/ the top flat of the pic rail, something else?

Is that to a notional point above the mount that would correspond to the centerline of an Aimpoint T2? the top of the riser? Something else?

I'm trying to make a cheat sheet (for purposes of inputting height over bore into ballistic calculators), and I can't find any info actually giving the spec of these things. I know that old school it was a " 1" riser and a .83" riser " that got you to "lower 1/3 co-witness" (~2.8" above bore) and "absolute co-witness" (~2.6" above bore) when using an M68, but those were 30mm tubes and there wasn't a whole lot of variety back then. With the new convention that has popped up over the last few years, a 1.42" yields absolute and a 1.57" yields lower 1/3 co-witness, but is that only with a T2 form factor? If I understood exactly how it was measured it would be easy to convert, but that info doesn't seem to be listed anywhere. Thanks for any responses.
 

shoobe01

Established
Top of pic rail to center of optic. AFAIK, 100% of the time, it is the standard back to weaver rails, 22 rails, and other such.

Yes, that means the same height of mount for a smaller optic will have more metal in it. And many you can't measure as in this image, as they are bolted to the bottom of the mount so there's no clear reference line.
scope-ring-height.jpg


I wish scopes had a little tick on the side for vertical center, like how cameras have the film plane marked on the top.
 

SB_Pete

Newbie
That's what I was thinking it was. As you say, the problem with that is how alot of these are now footprint standards which may or may not correspond to the actual optic on top of the mount. Is center of the objective for your Holosun or Vortex red dot the same as for a T2 even though it bolts to a T2 footprint? I don't know. Is the difference enough to matter? Maybe, maybe not, but I just don't know.

Can anybody confirm that that is how they are measured (from top flat of rail to center of obj for the reference optic used in a given footprint)?
Thanks
 
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