Easily one of the most sophisticated Syrian hell cannons. It's breach-loading, it has a recoil damper and it's mounted on a strong enough truck chassis. They are even seemingly hitting what they are actually intending to hit, which is far from guaranteed with these kinds of improvised artillery pieces.
This is built from scratch. There is no reason to use an antique cannon barrel for this - and it's not like those are lying around in Syria, whereas material and equipment to manufacture this is far easier to come by.
Judging from the amount of ISIS soldiers I've seen blow up the person behind them when firing heavy ordnance like manpads or bazookas, there must've been some first-hand experience first.