The Science of Shooting Steel Targets
You may have heard that velocity kills steel targets, but why is that? Shouldn’t the size of the round make a difference, too? The answer is that both are important, and affect the target in different ways. Let’s dive in.
Starting with the steel; AR500 is the most commonly used steel type used for commercial steel targets. It stands for ‘Abrasion Resistant’ steel with a nominal hardness rating of 500 on the Brinell hardness scale. Higher hardness is used to resist deformation from impacts and contact with abrasive materials, and in fact AR500 was designed originally for applications such as mining and construction equipment. The hardness is reached with a heat-treating process, however excessive amounts of heat will end up negatively affecting the material and making it weaker and brittle.
Moving on, a bullet flying through the air has Kinetic Energy (KE). That is: it has energy due to its motion. The amount of energy is defined as Since the velocity is squared, that’s the single thing that increases the kinetic energy the most. If we think back to those good old days of physics classes, we might remember that there are different types of energy, and that one can be converted to another. We’ll also remember there is a law called the conservation of energy which basically says energy never disappears, but it may transfer from one thing to another. That’s what’s happening when your bullet hits your target – the kinetic energy in the bullet gets split up into a few different places.
- The impact causes the bullet to fragment/deform and some of the kinetic energy sticks with those bits (spall) flying off into oblivion.
- Some of the energy also sticks around as kinetic energy that is transferred over to the target.
- Finally, some of the energy changes into thermal energy (heat), much of which is transferred into the target.
There are a lot of calculations that go into determining exactly how this gets split up, and we didn’t include some of the smaller things for simplicity, but just trust that most of the energy goes to item #3, thermal energy (Q).
Why we want to angle the target (and keep its surface smooth)
Item #1 is what happens when the round breaks up/disintegrates upon impact. Some of the total energy that the bullet had continues to move the remaining fragments (spall). Lots of testing done by many sources over the decades, most notably the US Army, has determined that when a round strikes the flat, steel target perpendicularly (at 90 degree angle), the resulting fragments of the round fly away in all directions around the impact area at about a 20 degree angle from the target surface. This has led to a typical recommendation that the target be hung with at least a 20-degree downward angle to deflect those fragments into the ground.
When there are divots/pitting on the target’s face, these small areas have angles that differ from the rest of the target face. It throws a monkey wrench into the deflection angle of the bullet fragments, and leads to a higher potential for a ricochet coming back towards the shooter.
Why we want the target to swing
Item #2 involves what is called “Work”. Mechanical work is done when one thing causes another to change location. Air is doing work on the round as it flies, slowing it down, and work is done when the bullet hits the target. Work done on the target face is defined as
Work is equal to kinetic energy. Why? Because science. But since this is the case, we are able to come up with the force that the bullet exerts on the target when we set W = KE and move the equation around. The angle from perpendicular at which the round strikes the target face changes the force as well, so we add in cos(θ) to account for that:
This shows that the force exerted on the target is a factor of the mass of the bullet, the impact angle, and its velocity, all divided by the distance traveled. The distance traveled is essentially the distance it takes the bullet to stop moving after its tip first contacts the target. That includes both the ‘crumple zone’ of the bullet (think of a car’s front end crumpling when it hits a wall) as well as the distance the target has moved once the impact is complete. This means that the less resistance to moving the target has, the lower the force will be that the target itself will need to handle.
What that force comes to is what causes bending of the target, and dents in the target face, and its why allowing the target to swing freely is ideal. It is also another reason to angle the target. A thicker plate can better resist these forces, but any target of a practical thickness will slowly bend over the course of thousands of rounds. Due to this, it’s a good idea to turn the target around occasionally to ‘even out’ the bending.
Why we want to limit velocity
Item #3 is essentially heating the target in a small area. The definition of Thermal Energy is
Since energy can change form, we can set Kinetic Energy equal to Thermal Energy, and using the equations for KE and Q, we come up with
Algebra the crap out of that and you’re left with:
Specific heat is a property of a material so it won’t change between two different lead bullets. Armed with this knowledge we see that the key difference in the amount of heat from an impact of two different rounds is how fast they’re going.
That’s why a 5.56 Federal XM193 with a 55 grain bullet and muzzle velocity of about 3300 ft/s (from a 20” barrel) can put 66% more heat energy into the target than a .308 Federal Gold Medal with a 168 grain bullet and muzzle velocity of ‘only’ 2560 ft/s (again from a 20” barrel).
Also consider that the cross sectional area of the 5.56 is about half that of the .308 and you have 132% MORE heat per square inch going into the target material. That’s a lot of heat in a small area which negatively affects the microstructure of the AR500 steel. This will progressively weaken the target over time, or all at once if you’re using particularly high-velocity loads. This change in the microstructure ends up making the material in that impact area very susceptible to pitting and/or chipping. This is the removal of material from the target rather than compressing it. This type of damage leads to particularly unpredictable ricochet patterns when bullets hit the area. Targets with this sort of damage can be referred to with the technical term “super frickin’ sketchy” (ie: dangerous) and should no longer be used.
Finally, he’s done talking!
So there you have it. There are two primary damage modes that wear out steel targets: bending, and pitting. Velocity is the biggest killer in both. Distance from the target helps reduce this, so back up when you can. Angling the target, and allowing it to swing also help increase life.