This photo was posted on what I am told is the Syrian Revolution facebook page. The machine gun pictured is the 12.7×108mm DShK.
The DShK was the Soviet WWII equivalent of the Browning M2. Its 12.7×108mm cartridge is very similar in size and power to the .50 BMG. During WWII the Soviets mounted the DShK on trailers with two wheels which could be hitched to a truck or tractor, which seems like it would be easier to maneuver than a one-wheels wheelbarrow.
[ Many thanks to Nizar for emailing us the photo. ]