A normal laserpen. I wanted to use a laser because of the trigger, it's easy to push with the gun trigger and it only lights up as long as you press it. you don't have to press twice to turn it on and off. I cant explain it better than that.
I guess you have a point in regards to the trigger aspect, but if you know how to, it could be possible to convert a torch to having a push switch. But then its getting to be too much to do, so its more hassle than it's worth.