The rotary bolt opens due to pressure on the slopping surface camming it around, sort of like squirting a watermelon seed. Different makers used different means to prevent this occurring in their designs, but this seems to have been a major problem with the otherwise excellent rotary bolt.
Winchester used a very shallow taper on its underbolt so the friction preventing it moving was greater than the force trying to move it. To keep it from sticking making it difficult to open they placed the stop screw in the notch. With their underbolt Parker Bros used a steeper taper which eliminated sticking, but to prevent the gun opening they used little side rails with parallel surfaces. If te bolt opened even the slightest these parallel rails on the bolt & luig came in contact thus preventing further opening. I have never owned oine but from what I have heard the Ithaca NID rotary bolt seems to have been more prone to opening with ordinary loads than was the Smith, not sure how the Fox fared on this aspect.
