Full auto conversion.

[MT2008]

Even if you do manage to purchase a transferable auto sear (good luck!), you've barely scratched the surface. Just about all of the military-style rifles sold in the U.S. are designed specifically so that they cannot be converted back to auto very easily. If you compare the receivers of an AR-15 and an M16, for example, you'll find that the AR-15's lower receiver is filled in the space below the buffer tube, which would prevent the installation of an M16's full-auto selector. In the upper receiver, the bolt carrier channel is narrower than that of an M16, and the bolt carriers themselves are a bit different in design (the AR-15's bolt carrier lacks a certain tab that is responsible for engaging the auto sear in an M16). You'd need to do some serious work on both the upper and lower before you could even begin to think about installing the auto sear.

Quite a few of the transferable machine guns within the chain of commerce, particularly those of non-U.S. manufacture, are actually conversions of semi-automatic weapons. For instance, all of the transferable MP5s on the market are converted from HK94s or other semi-auto MP5 clones. However, most of these were done by Class III manufacturers/dealers who specialized in such conversions - La France, F.J. Vollmer, etc. I'm sure are plenty of people who can and have done their own conversion work after getting squared away with BATF (my Dad, who is a way better gunsmith than I am, is currently trying to learn how to convert my Chinese Poly Tech AK to auto). But most people just send their semi-auto rifle and the registered auto sear to one of these specialists, and pay them a ridiculous amount to do the conversion work for them.