I've got a CZ527 that I moved the front sight back to thread the barrel and I need to drill and tap for the sight screw. The screw measures 0.135" diameter with a 40 pitch (or maybe a 0.6 metric) using calipers and a thread gauge. Everything else on the rifle is metric, and the specs for a 3.5x0.6 metric screw are really close to 6-40.

Which do you think it more likely is or does it not really matter which tap since they're so close in spec?