Although this may be stating the obvious to this crowd, the ATF rule of rifles-to-pistols is unless it was done before the National Firearms Act, and preferably before 1898, you need to have a tax stamp for it.
Remember the old series Wanted Dead or Alive? That show premiered in 1958. The producers chose to give Josh Randall a very different firearm to distinguish him from all the other Western heroes. They had a gunsmith cut down three Winchester Model 1892s into a lever-action "pistol" chambered in .44-40. The ATF was not amused. The producers were fined $1,100 dollars (equivalent to $11,300 today) for manufacturing short-barreled rifles, and had to get tax stamps for the cut-down Mare's Laig rifles.
The modern production Mare's Leg pistols like the Rossi Ranch Hand and the Chiappa Puma do not have this problem because they are manufactured from the start as pistols. They are not cut-down rifles. Even the barrels and the magazine tube are not cut down, but made in pistol lengths. You may have problems if you live in a state like New York that has specifically outlawed them. As a rule, most anti-gun states (hereafter Peoples Democratic Republic states) regard the Mare's Leg pistols as short-barreled rifles, despite the ATF ruling that if they are manufactured as pistols, they are pistols, not short-barreled rifles. As far as I know, the state vs federal definition of SBR vs pistol has never been challenged in court. Might be interesting if it were.