I found this little nugget while perusing a copy of Guns Magazine from December 1964 about trying out Colt’s then-new AR pattern .223 sporting rifles. The article was titled, ‘The .223 Becomes a Civilian’ . . .
Converting a semi-automatic rifle of this kind would, for a great many skilled gun tinkerers, be child’s play. In order to forestall objections by the ATU [now known as the ATF] Colt engineers revamped the firing mechanism to such an extent that conversion of the AR-15 Sporter into full automatic is completely out of the question.
So much metal was removed that basement conversion to full auto is impossible, and making a completely new action would be the only way to convert the gun into a fully automatic rifle. The ATU men, according to Del Shorb, were “gratified with the work our engineers had done to forestall tampering with the gun.”
The world’s changed a little since 1964.
Note the serial number of the gun they had for testing: SP00118. I sure would love to know where it ended up!