Re: Vietnam 10 ga
Just as a general comment, it should be quite possible (though perhaps not really feasible) to convert a pump shotgun to semi-auto; that is what manufacturers have done by using a piston surrounding the magazine tube. And any semi-auto gun can be made full auto with more or less difficulty. But such contraptions were not issued to American troops.
As for records, I was a company and battalion armorer and my weapon inventory and assignment records and cards were "ephemera" which is a fancy word for "trash when obsolete". When a soldier transferred out, I trashed his weapons card. When weapons were turned in for repair or replacement, the inventory list was thrown away and a new one prepared (no computers in those days). Nothing went in the soldier's 201 file about the weapon his was issued.