Colt M-16A1
The Colt M-16 is the Assault Rifle of choice for the Resistance against the Visitors in the original series. The weapon was used by many, including Mike Donovan.
The weapon fires a 5.56x45mm NATO rifle round that has been made out of Teflon, which can pierce the Visitor's armor. In the original series, this weapon was seen with either a 20 or 30 round detachable box magazine and the model mainly used was the Colt M-16A1.
Many of the M-16A1 rifles in the hands of the Resistance were taken from US Army and California National Guard armories that were under Visitor guard after the Visitors placed the US military (and the armed forces of Earth's countries) under house arrest.