The problem is that once legislation describes "what a legal firearm is," then we're limited to those things specifically in the description. Imagine that Congress had taken that step circa 1800. Then the Second Amendment really might apply only to muskets, just like the antis claim.
Of course, the point being to get them to say exactly where they truly want to go and then we can call it out.
Most do not want any guns, so, to be politically correct about not destroying the constitution they might say, "A single shot .22 LR long gun", or "..."
Then we can mount an attack against a real stance instead of "common sense" gun reform.