It's small. That's a good thing if you bought a small Sig and want to keep the whole thing manageable. This does the trick. The kit comes with two different size buttons you can swap out. It's a little fiddly to work it as a momentary light rather than "on" but with practice, you can. High stress situation... hmmm, gonna be easier to just go with ON. Read the manual! This comes from factory with strobe disabled, but it's easy to turn on and use (double tap button). It has a super easy "safety" to keep the light off; just twist the end of the light and it will not power on (think range bag).
It's very bright for indoors. Outdoors beyond 40ft, it's bright enough to see what you need with no problem. But for personal defense on a small frame gun, this is the ticket.