How to tell the difference between a full-auto and semi-auto bolt carrier assembly.
This is a full-auto bolt carrier assembly. Notice that the rear (right side in the photo) of the bolt carrier reinforcements (top and bottom) are of equal length.
For a bolt carrier to be considered full-auto the lug on the under side of the carrier MUST be the SAME LENGTH as the top portion of the rear of the carrier.
These are both semi-auto bolt carrier assemblies. Notice the lugs on the under side of the bolt carriers are shorter in one instance and completely missing on the other. (Only Colt® manufacturing makes their carriers with the lug on the under side completely machined off.)
For a bolt carrier to be considered semi-auto the lug on the under side must be machined back only a short amount. This additional quarter inch or so is the portion that is used to trip the auto sear of a fully-automatic M16. The shorter length of the lug on the under side cannot trip an auto sear.
This view shows the lugs on the under sides of 3 different carriers. On the left is a full-auto carrier. It's under side lug is of equal length as the top side lug. The forward edge (bottom of the photo) is the portion that trips the auto sear of a fully automatic M16. Without this specific length of under side lug along with the timing and cooperation of other full-auto fire control components the M16 will not be capable of firing full-auto.
Timing is ultra critical with regard to full-auto fire of any machine gun. Without the critical location of the forward edge of the under side lug on the bolt carrier assembly (1/8" is enough to create the necessary timing difference) to prevent full-auto fire and function as a machine gun.
The length of this under side lug is the only critical dimension or feature that determines whether or not a bolt carrier is considered full-auto.
Although a fully covered firing pin area on the bottom of a bolt carrier assembly is a full-auto feature it has no bearing to whether a carrier is a full-auto model or not. On a technical note the mil-spec variance for a full-auto carrier's under side lug is only +/- .0005" and anything more does not qualify as a mil-spec full-auto carrier.
These bolt carriers are manufactured by Olympic Arms. To compy with regulations the under side lug of our sem-auto carrier is milled back approximately 1/3 of an inch. This length is enough to make the carrier semi-auto yet is considerably more than is required to make it not full-auto. Cut to this length the under side lug will not trip an auto sear at the proper time to allow full automatic fire but still maintians maximum strength and rigidity of the carrier.
In this photo as well you can see the differences in the lengths of the under side lugs when shown side by side.
Again, the semi-auto carrier is machined back just enough to make it a semi-auto carrier configuration.
This photo shows two red lines on the semi-auto carrier. These lines indicate the differences between the under side lug and the top side lug on the carriers.