Is this due to the limited read range?
If you are thinking that guns could be tagged and then tracked as they cross into Mexico for use in the drug wars that country is currently enduring, there are several issues involved. First, it's unlikely that gun manufacturers would voluntarily agree to put tags on their weapons. It would incur an extra cost, and some gun owners might feel that Big Brother was requiring the tagging in order to track them, which I suspect they would not like.
Another reason, as you suggest, involves read range. Passive tags can be read from a distance of only about 30 feet or so. Active tags can be read from 1,000 feet away or more, but that would still make it expensive and difficult to cover the entire border. Moreover, RFID signals are blocked by metal, so placing the weapons into a steel box and then storing that box on a truck would render the transponders useless.
The solution to the problem of gun smuggling, unfortunately, is not radio frequency identification.
—Mark Roberti, Founder and Editor, RFID Journal