The court found Narongsak Plai-aram 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 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 pre-mediated 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.