# eli5 – How do Projectiles that are made to shatter on impact not shatter in the barrel?

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I allways wondered, because the momentum that pushes them out of the barrel must be stronger than the one they have when they impact a target? so how do barrel launched grenades or paintsballs or other similar projectiles work?

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Because the full force of the projectile isn’t hitting the sides of the barrel.

When you shoot a paintball out of a paintball gun, the paintball is just barely skimming the barrel, so there’s just a bit of friction going on.

Compared to when it hits someone and suddenly comes to a dead stop.

It’s like a car accident where the car just scraps alongside the road barrier, vs a car accident where the car plows straight into a solid wall or big tree and stops instantly.

Time.

A projectile is accelerated down a barrel at *approximately* a constant rate. Let’s say the barrel is 1m long, and the projectile reaches 100m/s.

It will take 0.02 seconds to accelerate down the barrel, at 5,000m/s^2

Now, impact. This is against a person who’s skin yields 1cm.

Deceleration will take 0.0002 seconds, with an acceleration of *500,000m/s^2*

Obviously the deceleration is a lot more likely to deform the projectile.

This is the difference between kinetic energy; or the energy shared between two objects in related motion; and potential energy. That of the stored energy of the propellent.

Imagine a rocket. It will not disintegrate under the pressures of being introduced to atmospheric resistance and friction because the structural integrity accounts for those forces. However, if that same rocket were to exchange kinetic energy with an object; say a bird. The consequences of which would be very detrimental; to at least the bird and perhaps the rocket depending on the speeds and the structural integrity where the exchange occurs.

Of course the eagle eyed reader might accurately suggest that potential energy gets converted to kinetic energy. This is true. That conversion however takes *time*. We can use that time as a force of continuous propulsion in which the total potential energy is divided by the amount of time it takes to be expended.

Imagine a rocket powered by TNT. TNT is potential energy, but its conversion to kinetic energy takes place in a very short time, releasing tremendous amounts of kinetic energy in the process.

Rocket fuel on the other hand, its conversion from potential energy (fuel) to kinetic (propulsion) is regulated by many factors; the chemical nature of the propellent itself, but *also* mechanical separation. We don’t ignite the entire source of fuel at once, only as required to provide the propulsion needed in that moment.