Brass, I didn't even bother with the pressure switch, and aligned my surefire to my forward grip. That way I can use my thumb to toggle the light on/off.
(I.E. I am a lefty, so i threw my light on my Right side rail just a mark or two forward of where my grip is located on my lower rail.)
...we had an issue of people accidentally triggering their light when I was deployed, so I started doing it this way as did others.
M&P 15, XD:M .40cal, Mossberg 500 with Knox tactical stock and now also a XD:M 3.8 in .40cal...