Marines have suffered from dehydration
The 2,000 rounds of bullets are the first discovery in Operation Snipe and are the kind used in Kalashnikov rifles.
Intelligence reports suggest the area, in the south-east of the country, has been an al-Qaeda stronghold, and it is thought the ammunition was hidden by Osama bin Laden's fighters.
In a separate development, an anti-aircraft gun was seized from residents in a village.
Meanwhile, British peacekeepers in the country's capital Kabul are destroying a cache of old Soviet AS-10 rockets.
They have leafleted local residents to reassure them that the explosions are intentional and controlled.
The leaflets read: "The soldiers that are on duty in your area are from Britain.
"They have arrived here for the purpose of capturing individuals belonging to al-Qaeda and their supporters.
"They have arrived here at the invitation of the temporary government. If you cooperate with these soldiers they will let no harm come to you."
Operation Snipe involves 1,000 British troops, including Royal Marines from 45 Commando.
It began on Monday after four days of secret deployment and preparation and focuses on an area about 40 miles from the Pakistan border.
It is hoped this could be the last phase of the mission to flush out al-Qaeda terrorists from Afghanistan.
Although there has been no combat yet, the men are under instructions to "capture or destroy".
'Duress'
The physical danger is compounded by the threat of dehydration and altitude sickness, because the troops are operating at heights of 8,000ft and above.
A Royal Marines spokesman, Lieutenant Colonel Paul Harradine, said the marines had covered about 20 per cent of the chosen region.
He added the villagers had been friendly and were being instructed by the troops how they could help.
But he was treating with scepticism the belief of locals that no al-Qaeda fighters were in the area.
Lt Col Harradine said: "They could be under duress to say that, they could be saying that for a decoy or indeed they could be friendly and there are none there."
In a separate operation in the south of Afghanistan, special forces and Afghan soldiers have disarmed the residents of a village.
The American military say the villagers were al-Qaeda sympathisers, and that an anti-aircraft gun was among the weapons seized there.