Gun Store Tactics
First, there are any number of ways to strike at a retail establishment. It is generally accepted not to draw on a drawn gun so the first rule for the loaded shotgun or carbine with the different-colored tag is to intrust it to whoever is not likely to be the first employee/owner confronted by a robber.
Smash and grab with vehicle: Bollards etc, completely around the store. Doors and street displays: do away with plate glass and replace with blast proof glass or one of several different polymers used on armored cars or within banks. All exit doors, including emergency exits should be commercial all-metal doors with metal frames.
Robbers must gain immediate control of the store if they are to be successful. All separate offices, classrooms, gunsmithing areas, storage, management offices should place the employee within behind a locked door. Yes, its a pain but if the robbers cannot gain immediate control, they can't prevent 911 calls or armed resistance through a shooting port within the upper part of the door. In the larger emporiums, I would opt for a 9mm carbine within these separate areas besides a sidearm.
Stealth Attack: It isn't especially difficult to case an establishment to establish what time someone opens or closes the store and shuts off the alarm system. Is he alone? How much time before the next employee arrives? Is there sufficient cover/concealment near the entry point to immediately take control of the first employee? To counter a stealth attack, you need to be aware of anyone who may be casing you. There should be nothing near the employee entrance that may be used for cover/concealment by an armed individual. Make use of of convex mirrors, the type used to overcome blind corners for drivers, so that the employee opening the store can more readily observe what's going on behind him. Particularly in rural areas where the gun shop is free-standing and does not share common walls with neighboring establishments, consider doing a 360 degree walk-around, at distance, prior to opening the store.
Regardless of your state laws, all handguns should be returned to a monster safe at closing. Assault type firearms should be cabled to a fixed object. The more obstacles you put in a burglar's/robber's path, the less likely he is to select your store or if he strikes anyway, it may reduce the amount of loss.
Just some suggestions