Department of defense policy is that guns may be carried on military facilities "when there is a reasonable expectation that life or DoD assets will be jeopardized if firearms are not carried." Rather than making military bases "gun-free zones," the 1992 directive issued by the Department of Defense instead describes the circumstances under which guns can be carried on military bases.
Under the circumstances, it would appear that this is an easy fix. DoD only needs to rule that recruiting stations are potential targets for terrorists and arm responsible personnel. The sign only warns civilians entering the facility that they are subject to a search for firearms and makes it a crime to do so. This allows someone caught with a gun there to be convicted, even if they have a carry permit.
To limit and control the carrying of firearms by DoD military and civilian personnel. The authorization to carry firearms shall be issued only to qualified personnel when there is a reasonable expectation that life or DoD assets will be jeopardized if firearms are not carried. Evaluation of the necessity to carry a firearm shall be made considering this expectation weighed against the possible consequences of accidental or indiscriminate use of firearms. DoD personnel regularly engaged in law enforcement or security duties shall be armed. [Department of Defense, 2/25/92]