Depends on the base. I'm stationed in ND and you can drive on base with a pistol but you CANNOT conceal carry! you must have the gun stored in the furthermost rear compartment of the vehicle, unloaded, and ammo stored in a different compartment of your vehicle. also ammo cannot be in the magazine when stored.
every base is different tho. i go hunting almost every day after work and have my shotgun in my vehicle at all times. its in plain view of security forces when going through the gate and never had a problem.
best bet it don't even attempt to carry on base before you know the rules unless you want an M4 pressed against your skull and face in the mud!!!
what base do you plan on visiting?