Will rifling a a gun with threading in the opposite direction cancel bullet twist?

well no because bullets dont twist. they are normally flat on the back and rifling is what makes the bullet spin. if you smoothed out the rifling the bullets would become very innacurate because the spinning(due to the rifling) makes it muich more accurate. if you want to see what happens to an unrifled bullet go find the mythbusters episode "bend a bullet" the bullet goes sideways and spins randomly

well no because bullets dont twist. they are normally flat on the back and rifling is what makes the bullet spin. if you smoothed out the rifling the bullets would become very innacurate because the spinning(due to the rifling) makes it muich more accurate. if you want to see what happens to an unrifled bullet go find the mythbusters episode "bend a bullet" the bullet goes sideways and spins randomly

Zac is correct --- it is the rifling that makes the bullet spin. Were you to make a barrel with the rifling in the opposite direction, the bullet would spin in the opposite direction. The bullet would not spin in either direction if there were no rifling grooves in the barrel. ADDITIONAL INFORMATION: Were you to make a barrel with two sets of equal but opposite rifling, the bullet would not develop any spin. Furthermore, the grooves would soon fill with metal from which the bullet is made.