I am unaware of any off-the-shelf tags designed for use on pistols. Several companies offer small, metal-mount tags that would work on a handgun, though these tags could be damaged during normal handling.
An appropriate solution would really depend on what you are trying to achieve. Do you need to read the tags on the guns while the weapons are stored in a case? Do you need to read the pistols as they enter or leave a room? The proper tag might be different in each case, but I would recommend, in either scenario, placing the tag inside the grip so that the tag won’t be damaged. In addition, there is often room for a thin RFID label to be placed within a plastic handle, which means a variety of tags might work.
Given that guns are mostly composed of metal, however, there might be some difficulty in reading the tag. Handgun manufacturer Sig Sauer utilized a special type of RFID technology called RuBee that works well in the presence of metal. The company placed small RuBee tags inside the handles of some of its pistols, thereby enabling the weapons to be tracked (see Gunning for Change).
—Mark Roberti, Founder and Editor, RFID Journal
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