Alan Mitchell, 34, from Cambuslang, Lanarkshire, and Gary Mitchell, 20, from Shettleston, Glasgow, carried out the raid at Eric N. Smith jewellers in Ayr Road, Newton Mearns, on April 13, with another unknown man.
The jury at the High Court in Glasgow was played CCTV footage that showed the masked robbers walking along Ayr Road with one carrying a large lump of concrete on his shoulder. This stone was used to hold open the shop door.
One of the robbers threatened the owners Eric Smith and his wife Yvonne with a machete while the other two smashed display cases containing Rolex watches with hammers.
Thirty-eight watches were taken, while other Rolexes worth £210,000 were damaged.
A mother and her son who saw the robbery chased the cousins in their car and a doctor and his wife saw them changing from their first getaway car into their second and noted the registration number.
The second getaway car, a Mazda owned by Alan Mitchell, was later searched by police and a scarf similar to the one worn by the robbers was found to have his DNA on it. A top, which was the same as one worn by the raiders, had Gary Mitchell's DNA on it.
In court, Mrs Smith identified Mitchell and Mitchell as the two men in the shop.
Both men, who have previous convictions, will be sentenced on November 13.