Ion blaster

The ion blaster is Optimus Prime's primary weapon. Only used by the leader of the Autobots when all attempts a peace have failed, the ion blaster ensures that freedom is the right of all sentient beings, and strikes fear in the sparks of the Decepticons who are foolish enough to challenge him.

Except Megatron. Only Primus knows how many times Optimus has shot Megatron and failed to kill him. Starscream knows that frustration all too well.