itsasmallfarm, the Mare's Leg is a pistol in the US, because both the barrel length and over all length would make it a short barrel rifle under US legal definition per the National Firearms Act. If Henry's, Rossi, or anyone else, sold them here as rifles, instead of going to an FFL and going through the usual background check process, you would have to submit paperwork with the ATF, pay up front for the $200 tax stamp, get a long form background check and the current processing wait time is 6 months for approval to buy and own a short-barrel rifle, then you have to pick it up from an FFL licensed to deal in NFA items.
It's less hassle to classify them as pistols.