It's my understanding that since BATF considers only the machined lower receiver casting of the AR series to be the firearm, all other parts of an AR pistol or rifle (which are not an NFA item) are not a firearm and may be sold, gifted, or transferred without regulatory complication. One could legitimately transfer the upper assembly, the buttstock, and all fire control components which ARE NOT the machined/finished lower receiver casting. A recipient could simply buy a commercially finished lower receiver and build a complete AR firearm with those parts. Please correct me if I am mistaken.
Tom