Gunpowder, the explosive used a typical bullet, undergoes a chemical reaction and causes the release of gases. The gases are released in very high quantities in a small span of time. This release of large amounts gases cause an increase in pressure and push the core of the bullet. Thus the core of the bullet derives its kinetic energy from the explosion. The explosion of the gunpowder takes place in a controlled manner. The controlled explosion avoids any damage to the gun and also the person shooting the bullet. The explosion takes place in such a way the initial amounts of energy released is just enough to move the bullet core and then slowly the energy released rises rapidly accelerating the bullet down the barrel.
The structure of the bullet and the barrel are such that the explosion initiated behind the bullet does not reach the front part of the core which is achieved by sealing the gases released efficiently behind the core of the bullet. If the gases go past the core, there is a pressure leak and the pressure released by explosion cannot efficiently accelerate the bullet. Frictional forces between the barrel and the bullet should not be to great to hamper the motion of the bullet. If the bullet’s motion is hampered by the friction, it confines the large amounts of gas released to a very tiny volume which is sealed by the core. This confinement of gases can cause explosion and wound (or worse kill) the user. The shape of the core should be preserved in the whole process. Any damage to the shape can drastically vary the aerodynamics of the bullet affecting its efficiency and accuracy.
Once the bullet leaves the core, it disturbs the air around it causing the loud noise generally heard. The bullet, generally, moves in the air with high speeds (around the speed of sound or 3-4 times the speed of sound). Just like a supersonic jet, bullets moving above the speed of sound also create shock waves. Bullets that travel small distances generally do so in straight lines but the ones that travel distances of kilometers do so in a curved path due to the affect of gravity. Bullets are also affected, mid-air, by crosswinds. The effect of this crosswinds is curved motion and is dependent on various factors like humidity and temperature. This explains why sniping from long distances is a challenging task and requires not just the skill but also rigorous knowledge of the concepts we learnt during our Physics tuition classes.