There are acoustic targets somewhat within the means of hobbiests. Ken Oehler, who posts on this board in the "Ask the Gunwriter" sub board produces one that I believe is an option to his PBL package. As 1minute has mentioned, they use mic's to triagulate the passage of the bullet. Where it differ's from 1minute's account is that the shock wave is picked up by the mics and paper is not necessary.
I had a conversation with a noted Ballistic Engineer a few years back, and he proposed using very thin paper on the midrange target (ie onion skin etc) that would not disturb the flight of the bullet as it travelled to the terminal target. It's arguable whether that midrange target would appreciably alter the natural flight of the bullet. I'll have to go back and re-read Dr Franklin Mann's book too.
