Assad explosive barrels engineering 
The explosive barrel is thrown, and the helicopter pilot waits in the air .. The time from launching the barrel from the helicopter until it touches the ground is approximately one minute .. Some barrels have something like fins to fall down perpendicularly for sure explosion to avoid not exploding .. The height of helicopter is not less than 7.5 Km , this is why all the anti aircraft weapons owned by the FSA or the available missiles cannot do anything, because the available missiles range is 3 Km in the best cases..
The helicopter waits until barrel is exploded, but if not then drops another one in the same spot to be exploded together and make a huge destruction ..!
There are 4 helicopters that start shelling at 9:00 AM until 5:00 PM , with the aid of one SUKHOY warplane for missiles shelling and navigation