You may want to read http://glocktalk.com/forums/showthre...m#post14353933
. The loose fit of a Glock slide to its frame poses a problem for frame mounted lasers. Don't be surprised if your groups aren't as tight as with the original sights.
I just returned my GLK because it suddenly refused to stay on. After having had it for 4-5 months, they allowed me to opt for a brand new X5L as my replacement, by paying the difference between the MSRP prices. I'll have it in a few days. Hope I'm not throwing good money after bad, but they are really easy to work with and I love the green laser (for these old progressive lens eyes).