A Fired .38 caibre Hollow Point round, completely expanded and deformed after striking its target. From Wikipedia, by user Rickochet

Welcome back! So far we’ve talked about the flight of projectiles, and how things change as our projectiles and the atmosphere around them change.  Now lets talk about what happens to our projectiles when they actually impact with something!

These projectiles that we’ve thrown have now flown from their projector to the other end of their ballistic trajectory.  Eventually, they’ll hit something other than the medium they are flying in.  Wether it’s the intended target, something that happens to be in the way, or one of those weird bubblegum asteroids those Sci-Fi writers keep dreaming up, something will happen.

So what happens?  Well, I guess in the simplest explanation, Physics happens.  As I wrote that, I realized it was wrong because physics didn’t just happen.  It’s been happening the whole time.  There are many factors and many different types of projectile we could be discussing, and I may delve into some of those later, but for now, let’s stick to simple bullets, and what happens when they strike human (or human-like) targets.

The instant the bullet leaves the barrel, it has been imparted with all of the Kinetic energy it’s going to get from the gun.  That is of course, unless it’s a self-propelled projectile like a rocket or missile.  That kinetic energy imparted gets slowly bled off by aerodynamic (or hydrodynamic, or bubblegum-dynamic all depending on the chosen medium of travel of course) drag until the projectile impacts its target.  The remaining kinetic energy then gets transferred to the target.

What occurs during that transfer is based on the interaction between the target and the projectile.  If the projectile has enough density and kinetic energy, it will pass through the target.  If the target has enough density and mass, it will stop the projectile.  Either one could be the intended result, depending on the design of the projectile. In modern military small arms ammunition, this is generally divided into two broad categories:

Full Metal Jacket ammunition, from Wikipedia, by user Major Tom

Full Metal Jacket (FMJ) ammunition has a dense lead core surrounded by a harder metal jacket, usually copper.  This shell prevents the bullet from deforming as it transfers its kinetic energy, allowing it to retain more energy as it travels through the target, hopefully emerging on the other side.  By the laws of armed conflict, several Geneva Conventions and other agreements this is the type of ammunition most civilized nations use in their military small arms.  The reasoning is two-fold.  A through and through wound is easier to patch up.  There is less risk of fragment contamination, it makes a smaller wound tract, and the target has a higher chance of survival.  Also, if the target is killed outright, they are taken out of the fight.  If the target is wounded, it takes two other people to carry them out of the fight.  One bullet just removed three people from the conflict.  We can all see the realistic problems with this idealistic view of a battlefield, but that is a discussion for another time.

Jacketed Hollow point ammunition, from Wikipedia, by user OlegVolk

Hollow point (JHP): or soft point ammunition is designed using a mix of a dense inner core, a harder outer jacket, and some sort of soft plastic (usually) tip or hollow space at the nose of the bullet.  When this type of ammo strikes a dense enough medium, they immediately begin to deform during the transfer of kinetic energy.  This deformation usually takes place in the form of some sort of expansion, increasing the surface area of the bullet against the target, increasing the rate of kinetic energy transfer.  This type of bullet leaves wounds typically seen, as short, massive wound channels with the bullet still in the wound.  This has several advantages, the first and foremost being the higher chance of a one shot incapacitation.  Of course, if the shot is a graze, or impacts in a non life threatening area, the wounds tend to be much more severe than with a full metal jacket round.  Police and Hunting ammunition are normally of this type.  Police services like to use it as it helps guarantee a quicker incapacitation of a target if they have to take a shot at someone.  Also, because of its tendency to quickly deform and transfer kinetic energy it has a lower chance of over penetration into say, someone else or into the next room through the wall.  Hunters use deforming ammunition to help guarantee a first shot kill by creating as powerful an impact as possible in their chosen calibre.  Hunters are also usually forced to choose this type as most jurisdictions have outlawed full metal jacket ammo to reduce the instances of wounded prey animals running through the woods with through and through gunshot wounds.

And now a visual comparison of the Terminal effect of these two types of ammunition, fired at Melons.  From Youtube user TrainASDI:

You might have noticed the pace of our journey through the world of Ballistics has slowed down a bit… a whole blog entry devoted to FMJ and JHP? Well, we have a lot to cover here folks.  Tune in next week for a look into a few more ammunition types, and we’ll tie it back into the Sci-Fi stuff with a look as to what our Martian Soldiers would be carrying.

Stay tuned!