The applicable definition in law is in 26USC5845(b):
"(b) Machinegun
The term "machinegun'' means any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. The term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person."
The key word is "designed"
Both the M14 rifle and M2 carbine were designed to shoot full auto. They can't be un-designed to semi-auto so they will always be "machineguns."
The M1 carbine wasn't designed to shoot full auto so in the absence of "...any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun..." it's not a "machinegun"
Now that I think about it I was talking to Larry Ruth about the M1A1 carbines and at that time he related his experience of the CMP sorting process. He indicated that if a M1 was over-stamped with an M2 designation they were stripped down and sent back to the Army to be scrapped.
He is no fool who gives what he cannot keep to gain that which he cannot lose
There are no great men, only great challenges that ordinary men are forced by circumstances to meet.
The "M1-M2" Carbine debate is fairly academic....very few M2's were ever marked as such, fewer still were actually built as such, which explains the BATFE's major antipathy toward the genre as a whole! There are over a million of them sitting in Vietnam (mostly M2's), that they'd like to sell and Americans would like to buy....but the Feds get a case of the vapors if it's even mentioned!
I had an M2 (M1 with kit) carbine for many years and sold it a year or so ago. It was pretty neat and sort of controllable with the recoil check mounted. It recoils up and to the right. I could see it only if it were used against a close-in mass attack. The idea of hitting enemies at 100 yds firing FA with a carbine is silly; the first shot may hit, but the rest won't be anywhere near the target. A fun gun, though, and I had a lot of ammo bought at $2 a hundred, so it didn't cost a fortune.
Just FYI, if you acquire a kit and decide to put it on an M1 carbine, make sure you install the M2 stock, the round bolt, the 4-rivet handguard, and the Type 3 recoil shield. Mine was a QHMC with the separate recoil spring tube and I had no problems, but I suggest the use of a receiver with the drilled hole. And make sure you get GI parts, especially the disconnector lever; I have seen several of the repros break and if it is the numbered part, that is the end for the machinegun.
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 M1
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.
FWIW, I know of no ruling that an M2 trigger housing, either by itself or on an M1 carbine, is a machinegun or that possession is illegal. Can anyone cite something official? (No, not the cousin of a friend said that someone told him....)
It says, in part, "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 insemiautomatic M1 carbines. 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."
I disagree about the hammer. Plainfield used to ship their M-1 carbines with M-2 hammers, using a washer to take up the space occupied by the M-2 disconnector. Proceed at your own risk.
Calif Dept of Justice(!) has spent considerable time and effort telling dealers to alter M2 trigger housings. I know of many dealers in Calif who have sent their housings to machine shops to have them altered. I am not aware of any laws requiring this, but in Calif everyone knows the Bud Evers Story and all are afraid of DOJ.