Now here's hoping that these are the "nice" Syrian rebels and not of the ISIS variety. What we have here is the use of an American made (Raytheon specifically) BGM-71 TOW missile which was designed to be an anti-tank/anti-armor weapon but can also be used as an anti-structure or in this case anti-artillery weapon. You can see that the operator of the missile has to stay with the launcher after firing as it needs to be guided in to its target--not the best of circumstances in a hot shooting environment but like the video I posted the other day of the helicopter shootdown, they do not seem to be under any pressure from an opposing force during these raids.
BGM-71 TOW in Afghanistan for comparo
especially frightening as the attacks come in what appears to be a "calm" environment when suddenly your helicopter is blown apart or your cannon is made into easily recyclable material. Attacks are very asymmetrical in this environment. The artillery piece in question looks to be a Russian 2S3 self propelled howitzer of the 152.4mm variety. The Syrians bought about 100 of these from Russia in recent years. Given their maximum armor of
Model of 2S3 for comparo with the video
30mm and the TOWs rating for armor penetration of at least 450mm (up to a reported 900mm in some varieties) it is safe to say that this howitzer is no longer in use.