You can certainly make your own silencer, but something that has only been touched on is what PTK said, you only get one try.
If you get your stamp, you start working on it, and halfway through, you screw it up, if what you made can be called a silencer, you can not legally make another one. If you finish, and it doesn't work well, you can't just scrap it and try again.
If you could legally experiment with it, ie, out of the country, it wouldn't be a problem, but when you have to put up the 200 dollars for each experiment, it becomes a whole other animal.
Not saying don't do it, just saying, think about that.