Solid laser - Crimson Trace LG-499 Laserguard, Red Laser Sight for Heckler and Koch

At first, it felt a bit flimsy as I installed it, but once it's on, it feels really solid. The switch activates very easily. The elevation and windage are very sensitive to adjustment, so you aren't turning the set screws several times around to get where you need to be. Just a few degrees here and there, and you are all lined up.