Al Jazeera tweeted a photo of an interesting mounted weapon built by a Syrian rebel. This single shot rifle (or cannon if its chambered in a small caliber cannon round) has a floating barrel action that recoils against a spring attached to a very heavy tripod weighed down by concrete blocks. It looks like it was originally supposed to be fired from the shoulder, note the original pistol grip and trigger, but now is fired using a cable, possibly a bicycle brake, that connects its “handle bars” at the rear to the original trigger.  he muzzle brake looks like it is much to big. It is at least twice as large as the muzzle brake on the Anzio Ironworks 20mm rifle. The loading/ejection port looks slightly larger than the Anzio.

Anzio Ironworks 20mm Rifle

I think this is either chambered in 14.5x114mm or one of the many Russian 23mm or 25mm rounds. The barrels of anti aircraft guns, such as the old ZU-23-2, that are chambered in the 23×152mm round look approximately to be approximately the same dimensions as the Syrian gun. Syria is a known operator of the ZU-23-2.

ZU-23-2 chambered in 23×152mm

Thanks to David for the tip.