That barrel houses the cable with the explosive charges, and it unravels out of it during flight.
Keen eyes may have seen the trailer used to move and launch this type weapon abandoned in Gaza a couple of weeks ago.
There’s even a video
This device was originally meant to clear a wide enough path through mine fields to let tanks and other armored vehicles pass safely. Its use in urban warfare means you can clear out an IED laden street in one fell swoop.
It’s a calculated military action during battle, not indiscriminate bombing of civilians from the air with unguided bombs.
Edit: Right on cue, another Zefa Shiryion trailer found, being described as an IDF vehicle:
— F. (@Palestinianism) August 5, 2014
Note that the images posted by Hamas are in the same general area, so this might in fact be the launcher.
— Palinfo (@PalinfoAr) August 5, 2014