Great addition to gen4 21 - LaserMax Guide Rod Red Laser Sight for Glock 20/21/41

This is super easy to install. The switch is a little hard to engage at first, but after a little break-in it gets better. I personally like that it takes a little effort to switch it on because it helps prevent it switching on in the holster and you ending up with a dead laser. The pulsing green dot is easy to see in any conditions. After a shooting session make sure to clean off the lens. I haven't put many rounds through it yet so I can't speak to durability, but with the testing that Lasermax does I imagine it can handle anything I can dish out. Another great thing is you don't have to have a special holster.