M16 rifle
The M16 was a very 'futuristic' rifle shooting a light caliber for it's era, but that is considered a medium one by nowadays standards. It was an 'assault rifle' with respect to it's heavier and more powerful counterpart, the M14, which was considered a 'battle rifle' (a distinction that was never defined very well, and that nowadays doesn't exist any more).
Used since the very first start of the conflict, the M16 was issued with twenty rounds magazines. It used both a caliber and working system completely new, but in the beginning of it's story it proved itself a very bad design, jamming too often and with other problems that continued until it's barrel was modified and soldiers trained to clean it much often than they used to do.
SOG soldiers – who received better training the average soldier – had less issues while using it since it's beginning, and they liked it against the opinion of the most because of it's lightness, which was a very important feature during long-range missions.
Near the end of the conflict the army started using thirty-rounds magazines, and it's original too-fast rate of fire (20 rounds per second) was slowed down.
After these modifications, it finally became a very fine weapon.