Though this is (at least to me) "everyone knows" information, I have failed to find the actual ruling using Google and various search terms. Perhaps my Google-fu is weak today. The original ATF ruling on open bolt firearms, here: http://www.google.com/url?q=http://www.atf.gov/regulations-rulings/rulings/atf-rulings/atf-ruling-82-8.pdf&sa=U&ei=_n2OUN2oHKiCyQHvqIGoAQ&ved=0CBQQFjAA&usg=AFQjCNHHueG8I0L8XX8M9VA09lrguFAL4Q, specific to the Ingram MAC platform. A further rule here, http://www.google.com/url?q=http://www.atf.gov/publications/download/p/atf-p-5320-8/atf-p-5320-8-appendix-b.pdf&sa=U&ei=_n2OUN2oHKiCyQHvqIGoAQ&ved=0CBYQFjAB&usg=AFQjCNFeL8RMngOhz4Dv1eeX38TmWsn_Wg specific to the KG-9 and Sten. These are reproduced in the "NFA Handbook" http://www.google.com/url?q=http://www.atf.gov/publications/download/p/atf-p-5320-8/atf-p-5320-8.pdf&sa=U&ei=U3-OUNCVKcT7ygH7g4C4Dw&ved=0CBQQFjAA&usg=AFQjCNH3O7fgJfKX70fI65p-H2IXnZHHBQ Searching "open bolt" in that document returns only the MAC, Sten, and KG-9 from the earlier documents. Hmm, I need to do some more investigating on this. The ATF says it banned open bolt on those specific platforms because they were "easily convertible" to full-auto operation. Same with the "ATF Rulings and Articles" link, http://www.google.com/url?q=http://www.atf.gov/publications/download/p/atf-p-5320-8/atf-p-5320-8-appendix-b.pdf&sa=U&ei=U3-OUNCVKcT7ygH7g4C4Dw&ved=0CBcQFjAB&usg=AFQjCNGT9YVmhBCWPBkJtlWX-FJOcD67BA I'd always thought the open bolt ban was absolute, possibly because I'd just bought a MAC when the first one went through. Can anyone point me to a .gov link saying open bolt guns are categorically banned, as opposed to those three specific platforms?