I don't own one, but worked in one as a "fun" (and it was fun!) job for about 4 years. Kind of a "biggest little store" around.
Markup isn't high if you want to stay competitive.
Not sure what the future holds, but the owner told me that all this summer through now, literally every gun and all ammunition is allocated (I believe this is pretty much unpreceded, but may be wrong. He could tell you, he's been in business for 48 years this year!). Meaning, he can't go online and order what he wants (or his customers want), his distributors call him and say "I have 3 boxes of 9mm" (sad as that is, I'm not making it up. Obviously it's not like that every call, but when they call you and tell you that you can only have 3 boxes, it's not a good situation). Same for guns. The demand is just insane right now.
Now, the market will equalize at some point, like it always does (barring drastic political changes/regulations, etc). The question is: can you make it long enough on basically no inventory to survive?
Obviously you will sell just about anything you get in stock right now, but will it be enough? And if you raise your markup due to supply shortages, will you retain your reputation and market base?
Is all up to you, and I 100% wish you the best of luck if you go forward with it. I'd proceed with caution though. Very, very tough time to get into the game.