What distinguishes it from the original M16 was the addition of the forward assist button and a chrome bolt carrier (which was later dropped with the M16A1).
However, an easier way was for commercially sold semiautomatic Colt AR-15 SP1 rifles to be converted to full auto fire in the 1970s and 1980s and these were in movie armories for years.
Mockup of an XM16E1 rifle with 20 round magazine - 5.56x45mm.
Yoshinon's personality only comes out while Yoshino is wearing the puppet and both personalities consider the puppet to be Yoshinon.
Yoshino is always the human shaped spirit Yoshino, Yoshinon is always the puppet so there are never any cases where Yoshinon takes over Yoshino's body, other than the arm holding the puppet, and Yoshino never becomes the puppet.
In April 2013 an anime adaptation began which covered the events of volumes 1-4 of the light novel which ended in June 2013.