First of all, that's not what they're protesting for at the moment. All they want is to maintain their autonomy without interference from China, which was promised for 50 years. Most people know that Independence isn't feasible. (Both politically and geographically)
The moment people start using guns, will also be the moment when HK is completely over. China knows how important their international image is, which is why they won't send in the army any time soon. But the moment things get seriously out of hand, I'm fairly certain they'll send in everything they got, and end HK as we know it once and for all.