I would just like to point out that I have a degree in Mechanical and Aerospace Engineering with an emphasis in heat transfer and thermodynamics, and I am working on a PhD in a similar subject.
I recently did a project for my last heat transfer class, and the intent of that project was to estimate the temperature of an AK barrel as the gun was fired (30 rounds, 1/second), allowed to cool, then fired again (another 30 rounds, 1/second). My analysis included the convective heating of the barrel (and bullet) from hot gas, the convective and radiation cooling of the barrel, and of course the heat generated by friction between the barrel and the bullet.
As it turns out, bullets can get fairly hot in the barrel. However, due to the respective hardnesses of the steel and the bullet jacket, most of this friction goes into the steel barrel as the heated jacket rubs off onto the barrel. This is similar to ablative cooling (which I will discuss in a moment). In summary, bullets can get fairly hot, but not hot enough to melt lead. My analysis concluded that bullets can get up to about half the melting temperature of lead in most rifle calibers. The super-fast rifle calibers can get up to around 80% Tm where the lead is weakened enough to fly apart when the jacket is scored by the rifling (a room-temperature lead bullet would be strong enough to spin that fast without flying apart).
As to heating by air friction, that's actually a misnomer. The air is technically heated by compression. Moving air has energy, which can be exchanged between pressure, velocity, and temperature. Pressure and temperature are related by the ideal gas law (which, though it doesn't apply exactly, can be used in a corrected form). The term for this "energy" is the "total pressure." "Total pressure" or "stagnation pressure" is the pressure that a gas flow would have if it was suddenly stopped without any loss of energy (adiabatically) so that all the velocity was exchanged for pressure. Likewise you have the stagnation temperature (the two are not independent).
As the air near the surface of, say, the SR-71 blackbird, is slowed down by friction (which generates a tiny amount of heat) the pressure and temperature increase (remember Bernoulli's law? As the speed of the gas increases, the pressure decreases? Bernouilli does not apply at supersonic speeds, but similar in principle). It is this temperature and pressure increase that heats the surface of the Blackbird and other supersonic aircraft. However, this effect does not occur until very high velocities are achieved - at lower speeds the convective cooling has a greater effect than the compression heating.
As to meteorites entering Earth's atmosphere... They are usually found covered in frost. In the first place, they were in outer space to begin with - and outer space is friggin cold. When the hit the atmosphere, the heat actually melts the outer layer of the meteorite and "blows" it away. This is what is called "ablative cooling" and is used on all the intercontinental ballistic missiles to prevent them from burning up. Most of the heat from flying through the air so fast is used to melt the surface material, and very little is used to heat the interior of the object. (Of note is that the space shuttle and capsules such as Mercury are not protected this way - the shock wave from the blunt surface actually acts as a shield from the hot gases.)
Well, now that I've thoroughly bored/confused everyone, it's off to class!
