Yup, illegal since the very first Federal gun law in 1934.

Thompson Center used to make a slick little carbine conversion for their Contender switch-barrel single shot pistol. Even though it was legal length for a rifle, the ATF bureaucrats decided a receiver is a long-gun or handgun and must remain such forever. Otherwise it's classified same as a machine-gun.

C'mon, who besides a criminal would want a single-shot convertible !