[int_19h]

My point is that it's not even hard, much less superhard. Sure, you won't have a couple of pipes of the right size just lying around - but all you need is a measuring tape to get the diameter of a shotgun shell, and then walking into the nearest Ace or Home Depot to get those. So I would argue that they are, effectively, "common possessions", in a sense that it is something that can be obtained with very little time and effort - comparable to grocery shopping.

And it doesn't require any specialized knowledge or skills at all - I couldn't fix my own sink, but I could make a working zip gun.

The reason why you don't see this happening a lot in practice is because it's easier to get a proper gun in US, and it's much more functional. But if you make that harder, you'll have to deal with this workaround suddenly becoming a lot more popular.

Speed bumps are a poor analogy, I think. They work, because their intended effect is localized - you put them where you want to avoid speeding because it's too dangerous. There's no similar locality in gun control.