I have a couple of the Glock saf-t-trigger doohickeys. Both fit either the G42-size or larger. Both of them have a little screw to tighten the fit should it be loose. It takes about a half second to push out in the event of an emergency. I mostly carry - incl walking before dawn this morning - a 637 but I use the safe-trigger thing if I carry a G42. I've mentioned before (sorry!) that I like the 637 b/c the first round up is a shot cartridge for what I think is the most likely thing to happen - a dog encounter.