Hi, flcjinflorida!
You might want to look at a Lasermax unit for your Glock 40.
They operate using an ON/OFF switch, so that unless you actually need to use the red dot, the laser stays off. (A lot of people don’t like this---they much prefer to have the red dot come on as soon as they grip their handguns.)
I have had one on the Glock 23 I normally carry for the past 6 months, and it has worked reliably. The unit replaces the recoil spring assembly on the Glock, and unless you are looking for it, you don’t see it. It doesn’t alter the lines or the appearance of the gun.
The downsides are:
1) Lasermax recommends that the recoil spring component of the unit be replaced every 4.000 rounds. (I get around this by dropping the original Glock recoil spring assembly back in for practice, unless I am training on laser-pointing techniques.)
2) The unit is non-adjustable. It has no provisions for changing the red dot’s orientation in relation to the bullet’s point of impact. (Lasermax guarantees that it should be within 2” of point of impact at 25 yards, however, and I have found this to be correct.)
3) The unit does not automatically switch on. You have to do it manually. This means that in a fast-moving scenario, your laser comes into play a bit slower than say, an instinctively-activated Crimson Trace unit.
4) The spring on the Lasermax unit is heavier. It takes a bit more effort to rack the slide.
I have a Crimson Trace unit on the Ruger LCP that I carry as a back-up gun, but prefer the Lasermax on my primary. I like the selectivity it gives me on my primary, but figure that by the time I need to use the BUG, things are really going to be hairy---very up close and personal. The laser on the limited-capacity BUG will allow every shot from the LCP to count.
Hope this information helps. If you have any further questions, just ask.
Cheers!