To curve a bullet:

You need to spin the bullet in a manner other than the traditional rotational effect you get from rifling the barrel—that being a rotational angle 90 degrees to the path of the bullet.

To do this, you need to remove the rifling from the barrel first. Second, you much use a round bullet and probably one with some sort of dimpling (like a golf ball) to increase air friction. Third, and most important is you need to characterize one side of the barrel so that it generates more friction against one side of the bullet thus creating the bullet to spin sideways by being “drug” slower on one side as it propels down the rifling. You could characterize the barrel any number of ways, you could have half the barrel lined with a different materal or you could simply roughen one side.

If you wanted even more curvature you could make the barrel and bullet slightly flattened to ensure the spin angle (however this could make the weapon very dangerous to fire and a remote firing mechanism should definitely be utilized).

If a man can curve a golf ball with a club, he can darn well curve a bullet if he can control the forces involved and understands the physics involved. The big question is how much curvature is achievable. If golf balls are curving roughly up to ~30+ degrees off of the original trajectory I suspect a bullet can achieve similar results, but as it travels so much faster the distance to measure the curvature needs to be significant and a 50 yard firing range in a police basement (like the show) is likely insufficient to support or disprove the trick if someone wanted to try my approach.

Flinging your arm to participate in the curvature is ridiculous.