Most steel target manufacturers and retailers have a chart with some guidelines showing what hardness and thickness of plate is appropriate for what cartridges. This is a fairly crude way of doing it, but easy to use, and conservative. The long-range shooter may want to select a thinner target than these charts show, in order to save money, have a lighter target to haul into remote locations, and get much better audible feedback at long range. A thinner target rings much better than a thicker one. And, because at very long range the bullets are moving rather slowly, we can get away with less steel than up close.
Making an appropriate selection is very easy, if you have the ability to calculate remaining velocity and energy for your bullet at the range the target will be used. If you’re shooting long range, you almost certainly have this ability with whatever drop calculator you are using.
First, a primer on why targets come in various hardness and thickness:
Target hardness, achieved through heat-treating, is given by the AR rating. There are many steel alloys and treatments available in the general materials market, but the only three that you normally run into for targets: mild steel, AR400, and AR500. These typically come in thicknesses from 3/16" to 1".
Bullets damage targets in two ways. When the bullet strikes, it splatters itself out, and this process turns a lot of kinetic energy (KE) into heat and pressure. This heat and fast-moving bullet material can cut the target surface, like a torch. If the steel is harder, it will resist this cutting better. The heat and pressure generated is directly related to velocity, so a .223 Rem with a 40gr bullet at 3,700 fps will actually do more damage to a 1" AR500 target than a 300 Win Mag (210gr bullet at 2900fps), by putting small craters in the surface. These pits will grow with successive shots, and eventually act as half-pipes to shoot lead and copper back at you. Think about washing a ladle in the sink and having the water shoot back up at you.
The other method of damage is a bowing or denting. This is from the mass of the bullet deforming the target backwards, away from the impact, just like hitting it with a hammer. AR steel is a little more resistant to this, but thickness is the dominant factor by far. There's a very strong correlation between impact energy and the bending or denting damage the plate will see. This will also create a concavity in the plate, but over a much larger area. If you see this happening in a gentle curve over the entire plate, you can just flip the plate around. If it’s a localized dent, you’re going to need to replace it.
Now, the easy, fool-proof steel target selection method: pull up your ballistic app, and run the velocity and energy numbers at the ranges of interest to you. Then pick your targets based on these ranges:
First, select hardness based on velocity:
· <1200fps - mild steel is fine.
· 1200-2500fps: AR400
· 2500-3000fps: AR500 (might get pitting with FMJ at 3k fps)
AR550 buys slightly more velocity, and can be a good idea for 5.56 at closer ranges (3000-3200fps), but is really past the point of diminishing returns for most uses.
Then, select thickness based on energy:
· <500 ft-lb: 3/16"
· 500-1500 ft-lb: 1/4"
· 1500-3000ft-lb: 3/8"
· 3000-6000ft-lb: 1/2"
· 6000-8000ft-lb: 3/4"
· 8000-10,000ft-lb: 1"
For long-range shooters, two things become apparent quickly: AR400 steel is normally just fine, because we rarely have velocities over 2500fps at extended ranges. Also, at very long range, even really heavy-hitting rounds can take a pretty light piece of steel. Lighter steel not only saves money, is easier to hang (and haul to temporary ranges), but also rings louder. Hearing impacts at 1200yd with a 6.5mm bullet can be tough! 1/4" steel is plenty thick for just about any .30 cal magnum at 1,000yd, and can even take .338LM hits at 1500yd or so.
As usual, caveat emptor. Hang your targets with a little forward lean to deflect the bullets at an angle, and hang them so they can move when struck. This information is only valid for lead and copper projectiles. Solid brass or copper, or even thick-jacketed FMJ rounds are harder on targets. So are very long bullets - that cutting-torch action is prolonged when a 90-gr VLD hammers into a target, vs a 40gr varmint bullet. Steel-cored or steel-jacketed bullets are NOT to be fired at steel targets, without substantial extra range margin! So, if you are on the hairy edge of the tables above, consider other factors like bullet type, volume of shooting, and if your buddy with a .243 WSSM or .338 Lapua Magnum might end up using the range some day.