Based on what the guy said, his clientele is mostly young men who play video games like Battlefield or Call of Duty, and want to see how the same guns there work in real life. So they literally come asking for "that gun that was in CoD", hence why he mainly stocks things that are trending in games and movies.

Apparently, it's also popular with foreign tourists; especially from Asian countries, with more extensive civilian gun control than in Europe, where many wouldn't even see a military (or military-looking) firearm outside of service. It got to the point where some ranges added signage in Chinese and hired Chinese-speaking instructors to cater to the new audience:

http://usa.chinadaily.com.cn/epaper/2015-04/17/content_20457...