This is a follow up to my previous post How to choose the correct steel for your shooting needs. There are two primary ways that steel is affected by bullet strikes, pitting (cratering in the extreme) and denting. Pitting is material removal from the strike face, caused by super-heating the strike face. Think of an asteroid hitting the earth.
The asteroid is insignificant in size related to the earth but it’s moving pretty fast. That kinetic energy is converted largely into thermal energy when the strike occurs. The thermal energy weakens steel surface enough to eject some of the steel. It also affects the temper of the steel, weakening it to allow future damage. High velocity, low sectional density (small diameter), and hardened projectiles such as steel core and armor piercing all tend to cause pitting of steel targets. Varmint rounds tend to be small diameter and very high velocity, and can cause pitting and cratering far exceeding what you’d expect for something used to shoot rodents.
Harder steel target surfaces resist pitting better than softer surfaces. For our purposes the Brinell Hardness scale (HB) is used for relative hardness. Per wikipedia, pine wood is about 1.6 HB, lead is 5-22 HB, depending on alloy, copper is 35 HB. Mild steel is 120-180 HB in my experience. Hardened tool steel, similar to what is used for AP core ammo, is 600-900 HB. Pure tungsten is 2570 HB.
For target purposes, I use 400 HB (pistol) and 500 HB (rifle) abrasion resistant (AR) steel. Why abrasion resistant? Because at the 400+ HB hardness, typically the material is sold as industrial plate steel for uses that need abrasion resistance. It doesn’t hurt anything for our use, and it’s what’s available at a reasonable price.
While the difference between mild steel and AR400 is pretty significant, the difference between AR400 and AR500 is less so, but still notable. It’s enough that you don’t want to use your AR400 pistol targets for rifles until you get several hundred yards out. The difference between AR500 and AR550 is barely perceptible. At 2570 HB, tungsten targets might last you forever, but it has a scrap price of about $15/lb (steel scrap is about $0.10/lb right now). Tungsten is about twice as dense as steel (0.54 lb/cubic inch as opposed to .28 lb/cubic inch), so an eight inch diameter, 3/8″ thick tungsten target would weight over 10 lbs, and probably retail for over $600.
Denting or deformation is caused by exceeding the strength of the material. Total energy is useful if you want to cause denting or deformation. Heavy projectiles tend to dent targets, as opposed to pitting them. They tend to be slower than their light projectile cousins, the 168 grain .308 (2700 fps) as opposed to the 55 grain .223 (3200 fps). Thicker targets resist denting and deformation better than thin targets.
Impact velocity, projectile hardness, projectile mass, and angle of impact are your factors that affect your targets, assuming the target isn’t the variable. The softer the projectile (no steel jacket, steel core, or AP), and the further away you are from the target, the longer that target will last. Total energy of impact decreases as you get further away from the target, due to drag in the air reducing your impact velocity. Angling the target reduces the energy absorbed as well. Make sure you always angle the target in a safe direction.
In a practical sense, here’s what all this information means to you. If you’re getting pitting on your target, going to a thicker steel won’t help. 5.56×45 M855 is going to pit AR500 at 100 yards, even if you upgrade to 1/2″ or 1″ steel from your current 3/8″. The only way you’re going to stop the pitting is to stop using steel core M855, move target further away, or go to a harder target.
If you’re denting your target, you’re in luck. You can go to a harder target (assuming you’re using mild steel), a thicker target (if you’re using a rifle), or simply move further away. It’s unlikely that you’re using a projectile that’s too hard or too fast if you’re denting instead of pitting the target. It is, however, possible to dent and pit (crater) a target at the same time. If you manage that, you’re doing something terribly wrong. That, or you need a medal for your creativity.
tl;dr – Hard targets resist pitting, thick targets resist denting