It's probably not dangerous, but as the .32 Long casing is somewhat smaller diameter than the .32-20, it will expand and could split upon firing. I wouldn't do it except in an emergency situation. PS - I once accidentally loaded and fired some .32-20 rounds in a .38 Special revolver. Nothing bad happened, but the .32-20 cases all split. In fact I didn't even notice anything amiss when I did that until I ejected the fired .32-20 cases and saw what happened.