While at the range with my C93 I found a new way to adjust the HK rear drum sight without the use of the $50-60 tool from HK. I used two small allen wrenches (1.5mm and 1/16") stuck them in sight drum as in the pic and used the sides of the drum as leverage to depress the two tabs that let it spin for aperature selection. You only get it started with the wrenches in, take them out and complete the quarter turn. Righty-tighty (moves sight down); Lefty-loosey (moves sight up).
Every pair of needle nose pliars I have are too big to fit in the small recess. Hemostats would work to, as they are smaller, for any doctor shooters, or if you steal them from a doctor.