In the US, short-barreled rifles and shotguns are strictly controlled at the federal level, making them an expensive headache to own even in the states that don't ban them outright. Yes, this makes very little sense in a nation where the people have a Constitutional right to own handguns. If I understand correctly, the argument in favor of the restriction consists mostly of jumping up and down shouting "SAWED OFF SHOTGUNS!" while waving around pictures of doe-eyed children.
I bring this up because at the NRA Annual Meeting, I had an opportunity to fall in love with the single-shot guns made by Thompson Center. The TC guns are wonderful little sporters that use a break-action mechanism with interchangeable barrels: you buy one gun, and can get additional barrels for any cartridge you want, from .22lr to .357 magnum to rifle cartridges*. They can be used in the standard pistol configuration, or fitted with a stock and long barrel to make a rifle.
You can see where this is going. The conversion kit's stock is stamped with a warning instructing the user not to attach the stock before attaching the long barrel, lest the Nanny Gods wax wroth. But as far as the ATF was concerned, owning that conversion kit at all constituted "constructive possession"--evidence per se that the owner intended to assemble a short-barreled rifle.
The case went all the way to the Supreme Court before the ATF was forced to back off.
Still, the Court didn't rule against the law itself, so to sum up:
--Rifle-caliber pistol: Perfectly legal
--Assembling rifle kit barrel-first: Perfectly legal
--Assembling rifle kit stock-first: FEDERAL FELONY YEARS IN PRISON LOSS OF WEAPONS AND VOTING RIGHTS CRAZY RIGHT WING DOMESTIC TERRORIST MILITIA WACKO THINK OF THE CHILDREN WHAAARGARBLE!
If you ever hear anti-gun advocates talking about how we need common-sense gun laws, understand that we hardcore gun nuts agree completely.
[* - I want a .45-70 shoggoth-pistol.]