Christopher Dempsey and an accomplice raided Campbell's Jewellers in Milngavie, East Dunbartonshire, in February this year.
Dempsey was caught minutes later by police but insisted he was not involved.
However, the 36 year-old has now admitted to a charge of assault and robbery at the High Court in Glasgow.
Lady Rae said the incident must have been "terrifying" for the staff involved.
He was jailed for six years for the raid and ordered to serve 270 days from a previous sentence for assault and drugs offences.