This is from The BATFE NFA Handbook on the M2 Carbine:
M2 Carbine conversion kit
The above parts consisting of an M2 selector lever, selector lever spring, disconnector lever assembly,
M2 disconnector, disconnector spring, disconnector plunger and M2 hammer are classified as a
machinegun. These parts are used specifically for fully automatic fire and have no application in a
semiautomatic carbine. While other parts such as an M2 sear, operating slide, trigger housing and stock
are used in the fully automatic carbine, these parts are also appropriate for use in semiautomatic M1icon
carbines.16
Therefore, the M2 sear, operating slide, trigger housing and stock are not a combination of parts
designed and intended for use in converting a weapon into a machinegun.