20-Year Prospect
America's relationship with firearms has been as tumultuous as its relationship with free speech, alcohol, and civil rights. In the past twenty years alone, we've seen the passing and repealing of bans on assault weapons, Supreme Court decisions on self-defense laws, and a gallery of high-profile gun-related news stories.
All of these things have the potential to affect the gun store ownership business, and a particularly tumultuous event could do away with the industry altogether (though that's pretty unlikely).
As employment in the gun industry grows, the backlash against guns grows to match (source). The one thing that won't change in the next twenty years is that gun laws will always be changing. But as long as the Second Amendment remains the law of the land, people will want to buy guns and probably be able to readily do so.
Luckily, this isn't a job that can be outsourced. At least not legally. You should be able to hold onto it as long as you can keep your shop open.