In the first X-Men movie, Magneto is shown to stop a bullet in mid-flight. How can this be possible? Magneto's power is the control of magnetic forces but normal bullets are made from nonferrous (non-magnetic) materials; lead core encased within a copper jacket. He should have no more control of those projectiles than the plastic darts from X-Men: The Last Stand.
Copper and lead are actually magnetic -- more accurately, they're diamagnetic. Thus, in theory, a magnetic field in front of the copper-jacketed lead bullet could resist the bullet's motion and, along with a field around it, suspend it in the air.
In reality, the strength of a magnetic field capable of stopping a supersonic bullet1 at point-blank range is astronomically staggering, and would have all kinds of effects on ferromagnetic materials all around (except that Magneto is quite clearly able to shape and limit his projected magnetic fields such that they only affect what he wants them to, as pointed out by a few commenters below); someone better at the math than I could probably tell you how much energy would be required to bring a typical FMJ bullet to rest from a velocity of at least 360m/s in the space of no more than half a meter. Even one strong enough to suspend a bullet in mid-air is mind-boggling. (Diamagnetic forces are generally very weak, and both lead and copper have very high resistances.)
Further, once stopped, the bullet simply has no more momentum, period -- completely negating Magneto's implied threat by leaving it suspended in the poor cop's forehead (as well as showing that the slow forward motion of the bullet during his little chat with Xavier's proxies was his deliberate doing, not an effect of him holding the bullet back).
Basically, the whole scene is lazy writing (bullet = metal = magnetic!) done solely for the Rule of Cool (WARNING: TV Tropes link!). Even granting the existence of the controlled manipulation of magnetic fields (such as Magneto's powers), this scene is just not possible, and you're exactly right -- Magneto should have effectively no more control of lead-and-copper bullets than he does of the plastic darts encountered later. (But see next section below.)
On the other hand, it is conceivable (albeit unlikely) that the cops here were loaded up with armor-piercing rounds. These are generally steel- or brass-core bullets (the heavier, harder metals resisting deformation on impact, thus resisting the dispersion of their inertia), the former of which is more conceivable to be affected by Magneto's powers (I'm unsure about brass, however). On the third appendage, bringing a heavy steel bullet from supersonic velocity to a complete stop that quickly would, still, require a magnetic field of such staggering power it defies belief even in a world where shape-shifters and metal-clawed mutants are commonplace.
All of that being said, though, as pointed out by several commenters below Magneto's powers seem to be less reliant upon magnetic forces, and behave more like telekinesis that is (usually) limited to metal objects (perhaps because he believes his powers are magnetic, and thus he subconsciously limits himself to only those materials he believes can be affected by such). If that's the case, then stopping a bullet is merely an act of will, and magnetic forces per se don't really play any part in it. To be sure, he does seem to have at least some form of limited telepathic powers, in that even without his telepathy-blocking helmet he does seem to be able to resist Prof. X's telepathy to some degree; in a world where telepathy and telekinesis seem to be linked (if not outright confused), that could be a sign of a telekinetic basis to his magnetic personality (nyuk nyuk!).
1While I'm no firearms expert, it looks to me like a 9mm handgun firing a standard FMJ round, which makes sense as the venerable 9mm is the caliber of choice among most police departments in this country; from what I've found that generally has a lower-end muzzle velocity of 360m/s, which is of course greater than the speed of sound of ~340m/s and, thus, is supersonic.
Most of the answers have been pretty good, but let me add a few points. Sci-fi culturally speaking, Adamantium is worked from meteoric iron, with slightly different properties. Most notably, it is much easier to work into an amorphous metal structure, making for a virtually indestructible metal. There are a few other amorphous metals, most notably silica glass. Most of us don't typically think of glass as metal though it actually is.
If Magneto had direct manipulative powers over all metals, this would include glass as well as most crystalline solids. Mystique had to inject a guard with high quantities of iron in order for Magneto to sense its presence and begin to manipulate it. Following this line of thinking leads credence to Magneto's power being mastery over magnetic fields, and not over metals in general.
The copper jackets on most bullets are actually gilding metal, which is mostly copper with small amounts of tin, if I remember correctly. Virtually all commercially made pistol bullets are some combination of copper and lead, or just one of the two. Occasionally a coating is present, or tin and/or antimony in the lead core. None are magnetic enough for Magneto to exploit.
As an aside, while it does appear to be a .355" bullet fired from a 9x19 or 9mm Parabellum, it would have a velocity of 1000-1200 feet/sec, depending on bullet weight. This is not hypersonic, it is supersonic. I can hand load them a bit hotter, but nowhere near hypersonic velocity. As a gun owner and hand loader of ammunition, my fastest velocities are from my 300 RUM. I have clocked a 165grain bullet at 3600fps. Thats still not hypersonic.
Also, steel core bullets are most commonly found in .224 and .308/.311 diameter bullets used by militaries around the world, though seldom by Americans. Their use is based more on economics than any armor piercing qualities. They aren't usually considered armor piercing at all, though any center-fire rifle will punch through basic bullet resistant fabrics commonly found in BP vests. While possible, I am not aware of any pistol bullets using steel cores. Someone mentioned steel cores being heavier... check yourself on that one. Lead is by far heavier than steel, though steel is harder. To be AP, bullets will typically have a hardened steel core or tip designed to shatter armor plating. Commonly found on .224 bullets, it doesn't take a lot of plate to stop such a small round. While called AP, they are far from anything serious. Typical pistol caliber bullets are never going to be true armor piercing, despite the claims made by nefarious political machinery. Please be aware that most BP vests will stop a 44 magnum and a 12ga slug. What kind of bullet can you make that will make a 9mm anywhere near as potent as 12ga?
In the meantime, X-Men is a good movie. Do remember it is based on comic books and is fantasy. These guys and gals get a lot of leeway and creative license with their story lines.
Magneto has control over all metals according to Marvel. He is just called "Magneto", but his power extends beyond magnetic abilities. He cannot control organic materials, only metals.
This is why, in X-Men: Apocalypse, he was
defeated by Apocalypse (whom is made up of celestial metals) because the organic make up and Apocalypse has much more control over molecules especially his own.
Please consider, that it was Magneto who fired that gun. We cannot assume, that the bullet actually had normal speed to start with. The energy propelling the bullet was present, but he had the bullet under control from the very begining.
The actual counter force wasnt stopping the bullet from high velocity to 0 at point blank range. I think Magneto used his power to not let the bullet accelerate in the first place.
if bullet encounters resitance in barrel, would the recoil of weapon be greater? meaning greater portion of propelling energy would be transfered to the gun?
Magneto is probably using the dualism of electricity and magnetism in relativity theory. So by thinking relativily he is also able to create electric potentials inside the bullet and in the air which cause the bullet to slow down and move around. Simple applied physics, no voodoo here.
Thanks industry.
Air pollution aspects of iron and its compounds
As you can see air has a lot of magnetic potential. He thus could create a capsule to stop each of the bullets that are in the air which is filled with magnetic particles.
Also a less complex answer may be a magnetic field was induced in the bullet. That is it was turned into a magnet through induction.
