This picatinny rail gauge is certified to be in picatinny spec. It is measured with gauge pins, gauge blocks, and a keyence microscope to ensure the tightest tolerances.

Instructions:
This gauge allows you to check if your picatinny is within +/-.002" of picatinny spec.
The -.002 cut out is .002" under the center of picatinny spec.
The 0 cut out is the center of picatinny spec.
The +.002 cut out is .002" over the center of picatinny spec.

Insert the rail in to the closest fitting cut out.
If the rail does not fit in to any of the cut outs, the rail is out of spec.
There should be a gap at the top of the rail.
Insert a .010" feeler gauge on top of the rail.
If a .010" gauge does not fit, the rail is over picatinny spec.
If a .010" or larger gauge does fit, the rail is within spec.

The cutout is made .01" bigger across the top so that you can easily see where the picatinny is tight.

With measurements for center (true picatinny spec), min and max, you can gauge if your picatinny is +2 or -2 thousandths.