How does a gun have the energy to fire a bullet near the speed of sound? What is the physics behind that?

the gun doesn't. the shell does. the shell is a hollow tube closed at one end. in this closed end is placed a measured amount of gunpowder. on top of this is placed the bullet. when you pull the trigger, it sets off the gunpowder. the explosion from the powder in the enclosed pipe forces the blast in the direction of the bullet which gets pushed down the barrel by the blast. this is a kind of simplified version but basically how it happens. hope I helped!

It's from the gunpowder stored in the bullet . when the powder is ignited it forces the bullet out of the barrel. It is ignited by a primer cap that explodes when it is struck by the hammer

Light weight projectile, pressure build up due to the rapid expansion of gas from the igniting of the powder..... ergo, high speeds.

This is kinda basic...... I thought everyone knew what was behind it.