I've built two triggers of the same simple design that I call the"butter knife trigger". The stainless steel trigger is cut from the handle of a solid sst butter knife which can be purchased for about a quarter from a local thrift stores collection of misc stainless flatware.
Here's a link to a web page that talks about these guns:
http://flshoredives.nexuswebs.net/biggun.html
The simple design makes it relatively easy to get the geometric and spatial relationships between the parts right. A safety can be incorporated into the design.
Is it safe? I'm one of those people that will never assume any trigger is absolutely safe. I do trust the two I've built. They've never failed to do what I expected them to do.
