A gun shop is just like any other business- if you are asking people on the internet about what your markup is going to be you are 100% sure to fail. If you put a business plan together, you would know the answer. Don't put the cart way in front of the horse.

I'm always excited to see new gun shops open, but most people who open them do it as an extension of their hobby and have little business sense and end up with a lot of money stuck into their business with very little income.