There are a couple of versions of what parts constitute a "combination of parts" for a carbine sufficient to legally become a machinegun in and by themselves.
A list I obtained some years ago has the following: 1) M2 slide, 2) M2 hammer, 3) M2 sear, 4) selector (switch), 5) disconnector lever, and 6) disconnector. The latter three parts have no function in an M1 carbine and the lever, as noted above, cannot even be installed in a carbine with only M1 parts. I have seen another version that adds the disconnector plunger and the 9 spring, but those are so insignificant, I question the validity of including them. Also note that the round bolt, M2 stock, and other upgrade parts are NOT listed as they were commonly used as replacement parts on M1 carbines.
The "six magic parts" as they have been known for years, are as listed, and ALL are required to make a carbine selective fire. No single part or combination of parts is enough - all must be in the possession or under the control of one person. Note that the person does NOT have to have a carbine; the parts themselves are legally a machinegun and illegal if not registered. I have seen reports that BATFE agents have stated that even one M2 part is a machinegun, but that appears to be totally wrong, and a misstatement by an overzealous individual agent. I have never heard of an arrest or conviction for having anything less than the full kit.
Many M1 carbines sold by the DCM and CMP have M2 slides or M2 sears. The M2 hammer will work in an M1 carbine but requires a shim to take up the space normally occupied by the disconnector.
HTH
Jim
