In February 1917 the Northern Front sent the 1st and 4th Don Cossack regiments to Petrograd from the 1st and 4th cavalry division respectively. They proved unreliable in riot control and joined the mutiny
Page 394 in Nicholas and Alexandra also has Protopopov having the Petrograd city police training in the use of machine guns. Other accounts also say this was done. More than a few people in Petrograd report police machine guns firing on protesters. This appears to be a Urban legend. It started because in mid-January 1917 at a session of the Special conference of the Duma Rodzianko accused Protpopov of diverting machine guns from England, which were intended for the front , to police units for quelling expected domestic disturbances. During the post march 1917 revolution provisional government Extraordinary Investigation Commission all who testified including Protopopov, General balk , the Petrograd Police chief, General Khabalov the Petrograd Military district commander, General Beliaev , the War minister and a number of police and okhrana officials said no police detachments were trained in the use or machine guns or had plans for their use. Also as a result of the post revolution investigation a inventory was done of all machine guns in the Petrograd area and all of them belonged to military units.