Paul Macdonald, who is a historic weapons expert from Edinburgh, found the object at the ruins of Highbridge near Spean Bridge in Lochaber.
He discovered the projectile during an investigation of the site with his family.
Macdonald clansmen ambushed two companies of Royal Scots at Highbridge, killing at least two of them.
The Battle of Highbridge in August 1745 was the first armed encounter of Bonnie Prince Charlie’s campaign.
The remaining 85 Royal Scots - who were marching from Fort Augustus to Fort William - were then forced to make a retreat from Highbridge.
The troops were pursued by another group of Macdonalds and forced to surrender, said Mr Macdonald.
Mr Macdonald, who runs Macdonald Armouries and makes period weaponry, including swords for TV show Game of Thrones, believes the last time the musket ball was touched was when it was being loaded into a firearm.
He said: “It is a musket ball of one of the first shots fired in the 1745 campaign. This is really quite exciting.”