Weapon of choice?

We don't know anything about the store layout or the clerk area. We don't know if there is room for a shotgun, even a 18.5" barreled shotgun. We don't know what kind of obstructions may or may not be present to make a shotgun of any kind a viable weapon for this particular usage.

We also don't know if the store owner is willing to get trained with a weapon (today, not 20 years ago) and to keep his skills up to date at the range.

We don't know if the store owner is a larger individual, or a smaller person with small hands. We don't know their comfort level with different styles of weapons. We don't know the legalities of owning a firearm in the location proposed by the store owner.

In short, there are not nearly enough details to make a responsible recommendation to the OP of this thread.

I know someone whom is purchasing a convenience store. It is not in a bad neighborhood but close to one. We were talking about the need (or not) to have a hand gun. He is not a gun owner but was in the military long ago and apparently a good shot with a rifle. To gun experts the questions are this.

1/ Good idea or bad idea to own a gun for this purpose?
2/ Weapon to buy and reason for this choice?
3/ Anything else to consider?