Why is Rifling Twist Direction Usually Right-Handed?

The Question
Is rifling twist direction right-handed for a reason?

Have you ever noticed that most American gun makers make the rifling go in a right-handed—or clockwise—twist direction, whereas many British gun makers historically choose left-handed twists? Have you ever wondered why?

The Multiple Choice Test
Take your best guess as to the answer:

A) The Brits believed left-hand twist imparts slight leftward bullet drift that compensates for the tendency of right-handed shooters to pull the shot right when pulling the trigger.

B) The Coriolis effect is best mitigated by an opposite spin to the Earth’s rotational pull.

C) America’s traditional lathes were set up for right-handed tooling.

D) America is right, and the British are wrong.

The Answer
If you chose C, you are wrong. If you chose A, you are correct in the reasoning why the Brits initially gave their guns left-handed twist. But is the theory true?

The Real Reason For Twist Direction
There is no real reason for the twists of gun rifling being right or left handed these days—except for tradition. As for the British theory: According to the NRA Fact Book, “Any such advantage [in left hand twist compensating the right handed shooter] is only fanciful, and it makes no practical difference which direction of twist is chosen.”

The Answer
Because that’s how they’ve always done it.