The Mirror Gun is used by Mirror Master for several purposes. He can even use it to cause a reflective surface to replay the events it has witnessed in the past day by using "reflective surfaces to take photon impressions and expand them into the visible spectrum".
Mirror Master is able to turn people into flat shards of glass with the gun, as well as reverse the process and returning the person to a normal state. If this mirror is destroyed while people are in the reflective mirror, the people inside of the mirror die. A perfect reflection of the mirror must be made first before smashing it, in order to release people trapped in the mirror world. Whenever something leaves the dimension via the Mirror Lands, a soot of glass and silver residue is left behind.
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