I'm going to assume we qualitatively follow Newton's Laws, even if we don't crunch numbers to prove so.
Just so we're on the same page, here are Newton's laws or Motion:
- First law: If an object experiences no net force, then its velocity
is constant: the object is either at rest (if its velocity is zero),
or it moves in a straight line with constant speed (if its velocity
- Second law: The acceleration a of a body is parallel and directly
proportional to the net force F acting on the body, is in the
direction of the net force, and is inversely proportional to the mass
m of the body, i.e., F = ma.
- Third law: When a first body exerts a force F1 on a second body, the
second body simultaneously exerts a force F2 = −F1 on the first body.
This means that F1 and F2 are equal in magnitude and opposite in
So from the first law we see that we need a constant acceleration towards the centre of rotation (Swinging the arm when firing only affects the starting velocity, not the acceleration), this requires a force to act on the bullet originating from the same direction, and this would then push against the originator of the force.
So we have two options, as I see it: A force external to the system acts on the bullet, or the forces already acting on the bullet, i.e. air resistance.
So we can see the velocity the bullet would go before the extra force was applied, the new 'curved' velocity and the force required...
An external force would have to be some electromagnetic force, but I can't imagine that being exerted by an assassin, who would need powers like Magneto.
I've wracked my brains for about 10 minutes thinking of a way to get a force towards a point so we have a curved trajectory, and the solution is the lame looking spoilers/wings on the back of the bullet, and no rifling. It's the only solution (I could think of) that would cause the trajectory we see in the film.
But really, it's not possible. The spoilers would be unreliable at best I'd imagine. A ball could be fired in such a way that the spin could cause it to curve, but making it spin in the barrel would be another challenge, for another day.