You can absolutely get mud or dirt there and prevent either the dot from being seen and/or cover the entire window. I got no issues with people running MRDS on their pistols, but typically the same people that are anal and squared away about their defense pistol being 100% reliable want to just throw out the fact that you could pull up that optic and see...nothing. It's a strange justification for something that really only starts to show it's legs at distances that are beyond standard SD engagements anyway.Having done a few RDS classes in the rain (one of which was at night), most of the "issues" folks have with open emitters are completely unfounded.