Have an extra lower, and was considering setting it up for bipod/bagged pistol shooting. 5.56 or Grendel in probably 18" length, so I can use for both rifle and "pistol" configuration. What lower parts are others using for a bench pistol?
You are correct, a rifle converted to a pistol needs a stamp. A "undesignated action", or lower not marked, can be either. If it starts as a pistol, it can be converted to a rifle, and back, or so says the ATF. As the lower is nothing right now, and will "start" as a pistol, I am safe.