[Yo-Yo]

http://www.atf.treas.gov/firearms/nfa/nfa_handbook/appendix_b.pdf

Basically the fire control group. Nothing said about milled receivers.

[ctdonath2]

The link refers more to aftermarket "drop-in" auto sears. The standard M16/M4 auto sear requires the receiver be milled differently. And as the link says: "In order to avoid violations of the NFA, M16, hammers, triggers, disconnectors, selectors and bolt carriers must not be used in assembly of AR-15 type semiautomatic rifles, unless the M16 parts have been modified to AR-15 Model SP1 configuration", to wit you can't just swap sears and turn an M16/M4 into a legit semi-auto.