No matter what, people will get their guns illegitimately. Some use these in an effort to get legal guns banned, often lying about the legality of their guns to do this. They go in and shoot up a place, often with stolen or otherwise illegally obtained guns, and then they jump on this as a reason why guns should be banned. When they succeed, it will be ridiculously easy for the government to enforce Noahide Law without worry.
I remember a line on the All in the Family show trailer they used for an advertisement for that show. The daughter discusses the number of people killed by guns. Archie Bunker retorted "Would it make you feel better if they were pushed out of windows?" Guns are not the only weapons that can be used to kill mass numbers of people--it is actually more efficient to use bombs or flammable liquids than guns. (Not that I recommend that, either--you will probably pay with your life.) But, they use guns so they can disarm the public, making it easier for criminals and tyrants (who will get their guns regardless) to safely do their jobs.