When dealing with an AXIS, what measurements break out a "long" action, from a "short" action?

I've heard a bunch of rumors, all divergent, would somebody here give me the straight dope please ?

Are the rail mounting holes different, front to back? what should the spacing be and how is it measured?

I'm trying to find a one piece picitinny rail, (0 deg. not 20 deg.) I would prefer steel, but after checking pricing, aluminum it is...

I ordered one from Brownells, it was too long, (by about 3/8th in. (c to c), front to back, on the mounting holes.

This really shouldn't be that difficult, should it?

Thanks for the help, Paul