Not exactly sure what we have here, but this is looking like a modified 100-150 year old cannon, mounted to a truck and used by the Free Syrian Army. It's possible that the cannon was homemade, partially homemade, or is an Ottoman relic stolen from Syria's National Museum of Aleppo. If true, this highlights an unusual source of weaponry in times of national chaos: museums. If the excrement ever hits the air conditioning in America, I guess I'd be tempted to pay a visit to the National Air and Space Museum.
Wikipedia has a decent page on improvised Syrian artillery, specifically the famous Hell Cannon: Improvised artillery in the Syrian war
Homemade ISIS fighting vehicles
Homemade Syrian weapons