You can find designs for them right on the web
but they are primitive, and these "old fashioned" ones tend to be large and cumbersome. (If you look at some of the designs, the analogy to a car muffler is appropriate). Newer ones are much more sophisticated(very compact), probably one reason they're more expensive. Always wanted to experiment with making one, just to see if I could do it, but not legal in my state, and certainly didn't want the hassles/paperwork with ATF, just for an experiment.
but they are primitive, and these "old fashioned" ones tend to be large and cumbersome. (If you look at some of the designs, the analogy to a car muffler is appropriate). Newer ones are much more sophisticated(very compact), probably one reason they're more expensive. Always wanted to experiment with making one, just to see if I could do it, but not legal in my state, and certainly didn't want the hassles/paperwork with ATF, just for an experiment.