An earlier trial saw video footage of the lead-up to the raid in Newton Mearns, Glasgow, when masked raiders armed with hammers and a machete smashed display cases.
The robbers left behind jewellery worth thousands of pounds -which might have proved easier to sell than the Rolex watches, which all have readily traceable serial numbers.
Cousins Alan Mitchell, 34, from Cambuslang and Gary Mitchell, 20, from Shettleston, Glasgow, denied the assault and robbery in Eric N Smith's shop in Ayr Road on April 13 but a jury found them guilty.
A third raider is still at large.
At the High Court in Edinburgh yesterday Alan Mitchell was jailed for eight years.
Judge Lord Jones noted that his criminal record includes convictions for the use of weapons - among them a five and a half year sentence for using a firearm during a bank robbery.
Gary Mitchell was sentenced to six years' detention.
Lord Jones told them: "This must have been a terrifying experience and the raid was carefully planned."
The trial heard that the raiders were in the shop for only 33 seconds. But during that time they caused damage estimated at more than £210,000.
Eric Smith and his wife, Yvonne, were threatened with a machete by one of the masked and gloved robbers who stood guard while the other two smashed display cases with hammers.
Mrs Smith, 52, said: "The first thing I was aware of was a really loud bang," she said. "It was like a gun going off."
In court she said she thought Gary Mitchell was the man with the machete.
Afterwards passers by chased the robbers' getaway car and noted its registration.
Defence QC Gordon Jackson said father-of-three Alan Mitchell maintained his denial that he had participated in the raid.
Solicitor advocate John Carroll, for Gary Mitchell, said he had been trying to turn his life around.
"It is clear that those efforts have been fruitless," said Lord Jones.