My mom worked at a bank that installed what they called a "man trap". Essentially there's a large magnetometer between two sets of heavy doors that locks the doors if it detects someone with a firearm (or any large block of magnetic material) has tried to enter the bank.

On any given week, they'd catch about a half dozen police officers, a maintenance person or two who still had their tools on them, and the occasional delivery person.

After a few weeks they disabled the system entirely.