It comes down to two main points:

1. To deflect the trajectory of the bullet, there needs to be some sort of force applied to the bullet while it is in flight. The Magnus Force is one such factor (the same thing that makes curveballs curve) but has negligible effect on the bullet in real life (a few millimeters).

2. Screwing around with the GUN itself will NOT affect the bullet's path after it has left the gun.

It's a fun idea brought to life in a movie, but like so many things in movies, it isn't realistic at all.

Recently on the Discovery channel's show Mythbusters they busted a simular myth that you can stop or even alter a bullets path with magnets. They rigged a path of high powered magnets under the bullets path and than fired a bullet out of a 9mm pistol (Im not sure on the gun), point being the most that they altered the bullets path was 4 or 5 mm. Hardley enough to be considered a drastic change. So curving a bullet need it be with magnets or useing a calculated recoche it is nearly impossible to reproduce in the way the way that the movie does the bullet curve. Even if you do move the bullet enough to avoid someone it will be slim to no chance of putting the bullet back around on its initial course and still hit the target with the speed and rotation that the bullet is moving at.

-Rach

My two cents:

Bullets have a rotational motion, which causes a bullet to "bend." Unfortunately for Hollywood, this bending does not occur at close distances. At ranges approaching and exceeding one mile, bullets will bend slightly. They bend enough that you cannot accurately plot them within a certain radius (a matter of inches).
But you never hear about 2-mile shots. Aside from having to arc your shot severely, the parabolic rotation of the bullet becomes so much that the grouping radius is now a matter of feet. In other words, EXTREME long-shots are near-impossible due to the "bending" of a bullet.
Now, for pistols, the limitations of the weapon (short barrel, less rifling, bullet dynamics) make it so that the accuracy and effective range come into question long before parabolic rotation ever does.

What this movie should've done to be more believable is issue the assassins guns that fire flat disks similar to a Frisbee, and then they would have all of the aerodynamic tricks of a Frisbee at their disposal.

There is a way humans can force a bullet into a curved trajectory thereby impacting a target blocked by an obstacle. Below are detailed steps that will produce a successful outcome every time.

1. Remove a bullet of any caliber or grain from its casing
2. Grip said bullet between thumb and forefinger
3. Walk with bullet-in-hand around obstacle
4. Throw bullet very quickly at target.

Note: If point 4 does not achieve penetration of target, load a new, unspent (fully assembled) cartridge into a firearm. Proceed to fire at target from new location.

the thing that most people dont realize is that the rotation of a bullet completely different than a baseball curve. the baseball has a 3/4 over the top rotation, with the rotation spinning forward.

The disc comment above would seem the only logical way to make this curve work because the rotation is the correct direction to get the shown curve. a bullet does not have the correct axis of rotation to give it the desired effect shown in the movie.

All that taken into account with a grain of salt cause it was a movie, that was a fun ride nonetheless.

With the right steering gear on the projectile, it might turn, but I doubt if aerodynamic forces would be enough to turn it very much. Also, pistols don't spin the bullet.

what about shaping the bullet in the way that air pressure will affect it to curl to one side? similar to the lift a plane gets.

what if the bullet didn't exist , could you bend it
of course you could , there wouldn't be anything to bend

bend in what relation?

would a jagged , or otherwise reformed surface of a bullet change it's trajectory , of course it would

but at what point , meaning speed or force or energy (whatever you wanna call it)
would the bullet cease to fall victim(muhahahaha, lol) to it's surface , and therefore it's trajectory

i don't know for sure but i would imagine ( i use that word lightly)
that at a certain speed , no matter the surface of the bullet , it would overcome all except gravity

by the way , the wicked curve ball example is slightly off , as to say , what if you could change the way a bullet is fired , because while it is the rotation of the ball that determines how much it moves (not how far , just to make my point) isn't or rather , can the way the ball is gripped (it's rotation) and the sufficient force and angle applied to release the ball determine how the ball moves , and therefore the way a bullet is fired (increased rotation) plus , per say the angle and the amount of force/energy applied would seem to make the bullet move more.
I don't know if it would bend meaning leaving one plane to another plane then returning that plane , but it definitely would move , to perhaps miss the target beyond just not shooting in the proposed direction..

blah blah blah i know the ball and the bullet aren't the same blah blah ugh i don't even feel like continuing but if i get response i will further elaborate , but until then

bigger bullet + reformed surface + different firing mechanism (rotation)
+ reduced or increased energy (depending on angle) =

ha! , there you have it , a bullet that bends , wherever you want it to

but remember , if laws determine how you can make the board , i will smash it creating a new science that determines what can't break the board

Some of the comments on this board are amazing, I can't help but chime in.

1 - For those of you that have studied physics or dynamics you know that an object in motion will continue in motion at constant speed AND DIRECTION unless acted upon by an external force. That means that right after the bullet has left the barrel, it will want to move in a straight line at constant speed (in a vacuum in space, more on that later).

2 - We live on earth and not in a vacuum in space therefore there are ALWAYS two forces acting upon any moving object. First you have gravity pulling the object toward the earths center, and second you have fluid (wind) resistance slowing the object.

3 - That being said, in a perfect environment (does not exist) a bullet will slow down and fall toward the center of the earth every time no matter what, end of story. However we do not live in a perfect environment and there is this stuff called wind that will cause a bullet to move to the side. Therefore, if you want to hit an object at a great distance, you need to account for gravity, decrease in speed, and wind. Not easy, try it.

4 - Any imperfections in a bullets geometry will change it's trajectory, but not in any way that is predictable, sorry.

5 - Every pistol, rifle, (and some shotguns designed for slug hunting) manufactured after the civil war have what is called rifling which induces spin into the bullet for better accuracy. So yes pistols "spin" the bullet.

So, if you want to slow a bullet down, curve it toward the ground, and in the direction of the wind, than you are in luck because it will happen every time no matter what!