How A Gun Works
A gun is a simple yet complex device of gears and levers that cause a round of ammunition to detonate with in a chamber, thus propelling, with heated gas, a solid metal projectile down a rifled barrel.
A tension spring pulls back on a part that looks like a nail called a 'firing pin' and once released by a user operated lever called a 'trigger' it will hit the back center area of a round of ammunition, striking a round component called a 'primer' or 'percussion cap.' This ammunition component or primer, filled with a soft, burning explosive material, when detonated, will shoot a stream of flames into a tight opening at the back of a shell casing called a 'flash hole.' The flash hole forms the flame into a tight stream of focused flame similar in fashion when you insert your thumb into the opening of a garden hose, which causes the stream or water to shoot far in a focused beam. The flame travels through the internals of the shell casing igniting the gun powder contained therein. The gun powder burns and super-heats the air causing an immense amount of pressure to build up.
This pressure causes the round of ammunition shell casing to expand. The expansion is contained by the internals of a 'chamber' in the firearm. The rearward expansion of the round is controlled by parts of the shell casing coming in contact with other areas of the chamber, preventing the round from traveling in unsafe directions.
With nowhere to go, the super-heated gasses push and propel a solid metal projectile (bullet) which is then forced into an area between the chamber and rifled barrel called a 'forcing cone.' The forcing cone aligns the bullet to travel safely and accurately down the barrel or a firearm and out of the barrel crown on its way to its target.
While the bullet is traveling down the barrel, the hot shell casing cools down and shrinks allowing it to be extracted with ease as not to cause any extraction jams. This is why soft, yet strong, metals are chosen to be used as shell casings for rounds of ammunition. However, when not in a strong metal chamber, the shell casing is usually not strong enough to contain the expanding gasses and can rupture causing a catastrophic failure.
Now knowing all of this, a young US Army personnel used a .50BMG round of ammunition... as a hammer. He used the back or 'primer' side of the round of ammunition to strike... a nail, which is shaped like a firing pin. What happened?
Have you ever seen a frozen chicken after it was blown up with a M80? Meat all over the place and the pink flesh in a mangled mess? Well, imagine that flesh being not pink, but bright red. Also, imagine that tiny bit of chicken wing hanging off the side was the top part of a human thumb. That's the 'amazing picture' I can't show you here that's now traveling around the internet after some US Army soldier used a 50BMG round as a hammer. I'm sure a Google search will help you find it.
That is all.