I had friends who would scour the produce isle to find potatoes they could cut down to fit their potato gun with a rifled barrel.

The could if lawmakers wanted them to. Here in Sweden potato guns are actually illegal if the potato achieves 10+ joule.

That's effectively a complete ban as a thrown potato would have considerably more energy than that. A quick web search suggests professional baseball pitchers achieve ~130J, and a potato is roughly comparable to a baseball in mass.

Yep.

I'm not saying that I'm for it, just that writing a law that bans them isn't all that hard.