AR500 Steel – Pitting Vs Warping
When you think of a target taking damage you're more than likely thinking of pitting. It’s the damage that occurs, to some extent, every time a high velocity (rifle) round strikes the target and is immediately visible. Many people have the misconception that the larger the caliber, the more damage it will cause to the target when in fact that is not true. During testing we shot a 3 .50 BMG rounds at our close range 1/2" AC zone target from 50 yards with no perceptable damage. Of course this is certainly not recommended and it wouldn't take long for that large of a round to warp the target but we simply had to know what it would do. Velocity is the true enemy of your AR500 target. At 100 yards I have seen .22-250 rounds go through 3/8" thick AR500 steel. The fast velocities (above 3,000 fps at impact) concentrated on a small caliber round create a lot of heat and pressure where it strikes the target. This in turn creates the pits that slowly (or quickly) chip away at our targets. In the picture below you can see the difference 50 yards makes to the target. I highlighted the diameter of the actual pitting because at 100 yards the pit is so shallow that the lead splatters outside of the actual "crater" and removes some paint making the pit look larger than it is in the photo. At 50 yards the pit is deeper with more defined edges resulting in no paint being removed. In most cases, the thickness of the target has no bearing on the amount of damage that it will sustain, the benefit to going thicker is that it won't warp from larger caliber rounds. One exception being .22-250 wherein it will poke holes in 3/8" thick AR500 and will only leave a large crater in 1/2" thick AR500 leaving the other side of the target in good shape.
tl;dr... Keeping the velocity of whatever round you're using to a minimum will increase the lifespan of your target. Anything about 3,000 fps at impact will drastically reduce the the lifespan of your target.
Warping is caused by using a large caliber on a thin plate. Using a 250 grain .300 Win Mag cartridge won't cause much pitting but repeated use on a 3/8" thick target at 100 yards will bend the plate and can eventually result in bullet fragments bein deflected in an unsafe direction. You can get by using a 3/8" thick plate for larger calibers but you have to make sure to keep the distance to the targert further away and regularly inspect your target to make sure it's not warping. If it does slightly warp you can flip the target around and shoot at the other side but you need to make sure you keep on top of it so it doesn't warp too far in either direction. That's why you're better off paying a little more for thicker targets if you're going to be shooting larger caliber rifles.
tl;dr... Large calibers can cause your target to warp (bend). Thicker plates or longer ranges are the key to making your target last as long as possible.