The court found Narongsak Plaiaram and Pirapong Sinthusonthichart guilty of collaborating in firing an M79 grenade at a PDRC rally in front of the Shinawatra III building on March 7, 2014.
The grenade missed the rally site but hit a pillar and a tree, causing damage worth Bt20,000.
The two were security guards of the United Front of Democracy against Dictatorship when they were arrested.
The two defendants confessed during the investigation, the court was told.
Narongsak confessed to picking up the M79 grenade and launcher from Imperial Lat Phrao’s car park, the court was told.
The court also heard that Narongsak drove Yongyut Boondee to fire the grenade. Yongyut was arrested last year and is facing trial in another case.
Pirapong confessed to calling Narongsak to pick up the grenade and the launcher.
The court ruled that the statements of state witnesses were convincing enough to substantiate the charges that the two suspects had illegal firearms in their possession.
The court initially sentenced each of the two to 12 years in jail for possessing illegal firearms, three more years for carrying a weapon in public place and to life on a premediated murder attempt.
But the court commuted the sentence by one third because the two confessed so they were given 43 years and four months in jail.