You reference NATO A2 spec flashhiders on all your gate lok documentation. Can I get  ref to that spec? Or just the critical dimensions?

I have two specimens that I want to qualify as keep or ditch.

The mount was designed around USGI prime vendor specs in a 2008 market reality where the parts from 8-12 large name brands of rifles used those parts.

Those create a.124+.004 -.000 rear groove width. They create a .8615-.8633” od tolerance in general.

The nato print allows .13”-.02 for groove width, and .864-.004 for diameter, but the USGI parts don’t typically fall all over this broad range.  The european Nato parts typically imported are the parts that have thin grooves and large and small diameters less suitable for use as mounts.

There are also cheap look alike parts on market, that have different looking slots, or that have 9mm compatible bores rather than approximately .338” minor ID like an actual A2. Unfortunately there are a lot of companies that have gotten into the market and the quality of parts and their dimensions varies a lot more than it did in 2008.

Thanks for this info, it's super helpful. I have a bag full of take off A2's from various manufacturers I'm going to go through and measure.

If I'm reading this right as long as the rear groove width measures within 0.124 - 0.128" and the OD measures 0.8615 - 0.8633" it should work with the gate-lok mount? Are those the only critical dimensions?

Correct.  That and having a wrench flat that communicates with the rear groove but all A2’s have that.