Originally Posted by
rossmum
It serves three purposes. First of all, it's a sunshade. Second, it's a rain cover. Third, it gives you the correct eye relief, since most or all of the scopes with them fitted are fixed-focus marksman optics rather than genuine sniper scopes. Cheap, easy-to-build optics with mid-range magnification and a reticle calibrated for easy ranging and use of hold-over rather than pissing about with adjustments. The PSO is a direct descendant of the PU, and the 1P29 is basically a Russian copy of the Trilux which led to the SUSAT. You are comparing them to the wrong thing, they are counterparts to the ACOG rather than a S&B PM II.
Speaking of ACOGs, fuck ACOGs. I cannot fucking believe the UK is moving to them when they already had the SUSAT, which is a far, far superior sight in my opinion.