Magnetic inducer

Or maybe it's a laser. Lasers are the same as shells, right?

Amongst many things, Wheeljack's shoulder-mounted cannon can fire magnetic inducer shells. On detonation, they cover the target in a special magnetic dust. The victim, now magnetized, will find every metal object within 4000 feet is suddenly flying towards them. The dust may also include force the victim to change mode and/or disable them.

Fiction

Generation 1 cartoon

Following the construction of the Dinobots, the Autobots found themselves under attack by their own creations. Grimlock, Slag and Sludge rampaged through Headquarters until Optimus Prime ordered they be destroyed. Wheeljack instead suggested the use of his magnetic inducer, which covered the three Dinobots in shimmering yellow dust and forced them to return to robot mode, successfully haulting their ruckus. S.O.S. Dinobots