You of course make a good point. They were banned at our gun club because people using them sent projectiles into areas outside of the range they were using that put others at risk.
The incident that brought these stocks to the publics attention was a terrorist attack. The sound of what seemed to be machine gun fire added to the terror of the public, which was the point. The “bump stock” was used more as a terror inducing device than as a tool to make the firearm more effective. Would you argue that a “bump stock” makes a semiautomatic weapon more effective? If not, then what does your argument have to do with the 2nd amendment?
2) A bump-stock, or any other gun part, falls into the general category of "arms". See tagline.
Conceding ANY ground to the gun-grabbers is ALWAYS a bad idea.