With clip-ons there is a need to check that the axis height of the scope off the picattiny rail is with in the margin of error on both. Most ACOG's are 40mm. Therefore most clip-ons are very close. My NV needs to be within 3mm of that axis height. My LWTS is the same. When I bought my scope mounts I chose ADM quick disconnect units and they are within 1mm of that 40mm above the rail.