I think he has a lot of hands-on time with firearms and he's a better shot than most/many of his viewers. He (like many youtubers/influencers) are following a model set by gear magazines decades ago: receive product to test (or buy it yourself) and report your experience. By demonstrating proficiency and experience over time you gain people's trust. He's basically a Guns and Ammo or Firearm blog (though FB does more instrumented testing)
I enjoy his content because he talks about ergonomics, feature functions, fitment, and malfunctions.
Ultimately people watch HO because he provides information, entertainment (similar to PewView), vicarious living, and has a wealth of testing videos. His content is digestible (meaning light on data, charts, paper targets, FPS), fun, and gives you an idea of what holding that firearm is like. Millions of people clearly find value in at least one of those categories.
Ain't for everyone but the great thing about YT is there's at least 1 channel making every kind of possible content you could want.