AGAIN, it's NOT a registration (doesn't matter how it's used, its what it is legally). All the confusion comes from having the wrong mindset and treating it like a registration.
And I would not recommend lying on a gov form. Say it's 16" when you know it's not. You'll get in more trouble for that.
More misunderstanding. Under MA law it becomes a gun when it can fire. So you assemble an AR as a rifle, it is now a gun and you just came into possession of the gun, and it has a serial number. Changing a part does not change possession. Did you buy a gun? Did you come into possession of a gun? These require an eFA10. But changing a part on a gun does not change possession.
Think of it this way, for possession to change, it has to become possessed by someone else, and them maybe come back to you, that's 2 transactions. If it's yours at the begining and the end and none elses in between, there has been no change in posession, so no transaction, so no eFA10.