He realised that he was completely surrounded and that the Taliban were about to launch a well-planned attempt to overrun the compound.
The enemy opened fire from all sides, destroying the sentry position where Acting Sgt Pun had been on duty minutes before.
Defending the base from the roof, the Gurkha remained under continuous attack from rocket-propelled grenades and AK47s for over quarter of an hour.
Most of the militants were about 50ft away from him, but at one point he turned around to see a 'huge' Taliban fighter looming over him.
Acting Sgt Pun picked up his machine gun and fired a long burst at the man until he fell off the roof.
When another insurgent tried to climb up to his position, the Gurkha attempted to shoot him with his SA80 rifle. But it did not work, either because it had jammed or because the magazine was empty.
Acting Sgt Pun first grabbed a sandbag but it had not been tied up and the contents fell to the floor.
Then he seized the metal tripod of his machine gun and threw it at the approaching Taliban militant, shouting in Nepali 'Marchu talai' ('I will kill you') and knocking him down.
Two insurgents were still attacking by the time the heroic Gurkha had used up all his ammunition, but he set off a Claymore mine to repel them.