BAD IDEA. I can only imagine the outcome of an untrained employee pulling a gun in a crowded store against two armed idiots. Bett er to give the thieves what they want. After all, it's only money, and it's not even theirs.
I second that! This is a horrible idea and the best way to end up with a dead employee or customer.
The best thing to do is install a top quality digital surveillance system that records everything and backs up to a computer. The cameras should be damn near concealled and of course, color images. If you do not already have the height measurement tape on the exit doors, think 7-11, install that to be able to get a height on the criminals.
Then I would install a cheap black and white camera that is visible and obvious as a decoy.
Then, God forbid, you get robbed again, police will have excellent quality color pictures to identify the culprits and prosecute them. Make sure your cameras cover you entire store, you never know it could also have the collateral benefit of preventing any internal problems one might experience.
Believe me I have looked at a lot of surveillance camera images from stores like the one you may have and 90% of the time they are useless because the quality is so bad-worthless. Very frustrating when you need to ID the bad guy.