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The Doctor used the matter conversion gun to defeat Omega (DW: Arc of Infinity).
It had originally been used by the Ergon to transport Colin Frazer, Robin Stuart and Tegan Jovanka into Omega's TARDIS. The beam from the weapon appeared to cause great pain or distress; those hit by it screamed as they vanished. The gun could be broken down into smaller pieces, allowing the Doctor to carry it in his pockets through the streets of Amsterdam. He then reluctantly fired it at Omega to stop him from willing his own destruction, which would have caused a massive explosion.