The Mirror Gun is used by Mirror Master for several purposes.
History
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".[1]
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.[2] Whenever something leaves the dimension via the Mirror Lands, a soot of glass and silver residue is left behind.