It's not that bad. At the very least, a destructive device must be "designed for use as a weapon" or else it doesn't count. I'm still not sure why these things (the definition seems to include most guns, although I'm not sure what the bore measurements imply) are called "weapons of mass destruction", though...
The bore measurement requirement excludes any guns of .50 caliber or under (or around 12.7 mm in metric) from the "destructive device" category for legal purposes, which covers most modern small arms. Aside from a handful of experimental or exotic weapons, the only real exceptions are a few Eastern Bloc heavy machine guns and anti-materiel rifles, which you'd have a hard time getting ahold of in the States anyway.
It's common for black powder weapons to have larger bores -- .5 to .8 inches were typical calibers for colonial-era muskets -- but they're excluded from the "destructive device" category by a separate provision.
There's a lot of background mess in our mental pictures of the world. We try and be accurate on important issues, but a whole lot of the less important stuff we pick up from the media, the movies, and random impressions. And once these impressions are in our mental pictures, they just don't go away - until we find a fact that causes us to say "huh", and reassess.
Here are three facts that have caused that "huh" in me, recently, and completely rearranged minor parts of my mental map. I'm sharing them here, because that experience is a valuable one.