I am looking at the Picatinny specs on this page.

What I don't get, is that it looks like some measurements are missing to my untrained eye. For instance, in the second picture it looks like there isn't a distance given from the top of the rail to where the lower 45 degree face meets the vertical face. I see some measurements that look like they are measuring from the middle of the top angled face to the middle of the lower angled face, but not the top (horizontal) face, to where the 45 degree face meets the vertical face.

In other words, I want to know the thickness, top to bottom, of this part of the rail:     <        >

Does that make sense? Sorry for the complete lack of technical terminology, I wasn't formally trained as an engineer. If someone could give me a nice breakdown of what everything means on this diagram I would really appreciate it!

If my calculations are correct, and there's a good chance they're not, it's .22"

Figure .230"

The given dimension from the top is .164. The horizontal dimension in from there is .0655" (half of .748-.617). because the angle is 45 degrees, the vertical dimension is the same. .164+.066=.230

This may vary, as the dimension isn't controlled.

The "C" in the box defines the given dimension as a "datum". The "target" symbol is "true position", the boxes mean that the other referenced features need to be within .006 or .010 of the "true position" of the "datum" at maximum material condition (the "M").

To truly explain this particular drawing would require a half day lesson in geometric tolerancing.

Oddly enough, this tolerancing scheme is very useful with mass produced parts.  It can be a bit difficult to learn, and I've heard some loud arguements about how specific units are actually measured, but I use it when appropriate. In this case, when a lot of parts from a lot of manufacturers need to interchange, it looks pretty good to me.

Lurking Engineer, at your service!