Which manufacturer makes the best tags for this purpose?
It would be difficult to suggest a single tag that would work for all types of guns. Which one you should use would really depend on how and where you intend to place the tags. There are a variety of small, on-metal, passive ultrahigh-frequency (UHF) RFID tags available from such companies as Avery Dennison, Omni-ID and Xerafy that would work on guns (see Firearms Distributors to Track Guns Via RFID and U.S. State Dept.'s Security Bureau Checks Out Weapons-Tracking RFID App).
You might also want to look into a RuBee-based solution. RuBee is a kind of RFID system with some special characteristics enabling it to work well in the presence of metal, and it has been used in armories and elsewhere to track guns on racks (see Gunning for Change and Lockheed Martin Offers RuBee Solution for Monitoring Munitions).
—Mark Roberti, Founder and Editor, RFID Journal