P250, P320, P320 X-5, M17, & M18 Pistols
I don't know if this is a known thing or not. I've recently acquired a Sig M18 and have mounted a Trijicon RMR sight. After firing several rounds, the front lens of the RMR is coated with what I assume is burnt powder.
I believe it's being ejected from the loaded chamber indicator on the top...