You can't be serious. As others have pointed out, a firearm
on the premises (including a locked vehicle but available to the owner) is a risk to others when that owner is involved in disputes, including with the employer. I recall
the shootings in Sunnyvale (Silicon Valley), at DEC in
Hudson Mass, the various incidents at post offices (genesis
of the "going postal" syndrome)..
My wife carries out terminations as part of her job. Its
stressful enough without the danger of a fired employee coming into mahogany row and offing the management. Ultimately this issue will lose on the grounds of common sense and liability.
I'd be willing to bet that your company had more lost days from comunicable diseases and from the tools at work than from firearm incidents between co workers.
But let's not let facts get in the way of fearful perceptions!
You'll also have to consider that people with GUNS IN CARS have helped stop school shooting sprees.
But you act as if policies against guns in the parking lots keep people from returning after a termination to commit crimes.