born: Brautarholt at Kjalarnes in Iceland
died: Esjuberg at Kjalarnes in Iceland
married: Helga Þorgrímsdóttir
Fighting men in the Viking age improvised weapons, using whatever tools were available to help them succeed in their fight. Búi threw stones to kill his ambushers before they got close enough to use conventional weapons.
Búi was travelling to a ship with a few companions when he saw an ambushing party coming up on him led by the brothers Helgi and Vakur. Búi told his companions to wait and see to the horses while he dismounted and climbed a nearby hill.
He climbed the hill and carried some stones up with him. All his weapons were good and he was wearing the shirt which his foster-mother had given him. The brothers soon arrived. They jumped from their horses and launched a fierce attack on Bui. Bui defended himself manfully. At first, he let fly with stones. By the time he had used up his stones, four of the attackers lay dead. Then he took up his sword and shield. The brothers attacked with great force, since they were both courageous men. Two more of their band were killed. The brothers and all their men were wounded. Bui was still not wounded then, but he was extremely battle-weary.
Kjalnesinga saga, ch. 11
The fight was broken up by a farmer who separated the two sides. The two brothers were both wounded and gave up the fight. Búi returned to his companions and traveled to the ship. The farmer saw to the burial of the dead men and to binding the wounds of the injured. The two brothers were not pleased with the outcome of the fight.
Búi took advantage of the stones lying all around him. He used them as missiles to improve his odds before the fight with more conventional weapons began. This kind of improvisation seems to have been a regular part of Viking-age fights.