He was probably not lying, nor using AP rounds. It seems counter-intuitive that a much smaller bullet would do so much more damage to the plate, but that is how it works. In fact, in the world of steel shooting, this is very common. Many ranges that have range-owned steel will not let you fire anything at it with a muzzle velocity over about 3000 fps. The smaller and faster they are, the more harmful they are to steel. Rounds like .221 Fireball, .204 Ruger, and stuff like that are the reasons for rules like this. They easily blow clean holes in AR500 even at several hundred yards. I can't find too much outside source information on WHY this is, so I'll go ahead and take a physics stab at it.

While I knew that this was the case, I never REALLY investigated why, so you asked a great question . I pondered a few different possible solutions. Impulse, power, sectional density, kinetic energy, just to name a few.

There is no question that the rifles you were shooting at the plate had a great deal more kinetic energy than his 5.56, even at 3700 fps. Kinetic energy is RELATED to my analysis, but it does not end there.

In the end, I came up with the conclusion that the power transferred to the plate is what is killing it. When I say power, I'm referring to energy per unit time, the physics power. (I say this because the word, "power," is thrown around quite a bit in the shooting community, with a very loose and often incorrect definition). Many materials have a power density limit, meaning you can only add so much energy, over so much time, before things start to break. For example, if you take a 100 W bulb and plug it in to a standard 110V fixture, you get 100W of power. Plug it in to a 440V outlet, you get 1600 W of power, but the filament will likely fail, and the bulb will burn out. Why? Because there was too much power.

If you leave a 100 W bulb on for an hour, you have expended significantly more total energy than the very short bright flash of 1600W, but the bulb will operate normally.

An additional example: Consider hammering a nail in to a board. In this case, we want the board to "fail," to allow the nail in. Trying to push the nail in with your hand is near impossible, right? Even if you're using WAY more energy than you would using a hammer, you're applying that energy over too long a time, and you have low power. A hammer, on the other hand, takes less energy, but imparts that energy on the nail and in to the board in a much shorter time, meaning you can deliver much more power with a hammer than you can by pushing with your hands. This "power" only lasts for a split second, but that's all you need

Power = Energy / Time

You can see there are two ways to increase power... Either increase the energy, or decrease the time.

The total amount of KE is just a part of the equation. The other important half is, "over what amount of time did this energy transfer occur?" In the above example, the normally operating bulb expended 360,000 J of energy, over the course of one hour. In the 440 outlet, Just 1600 J of energy were expended, but the filament failed. That is because the normally operating bulb was operating at 100 Joules per second, while the one that failed was operating at 1600 Joules per second. Somewhere in between 100 Joules per second and 1600 Joules per second is the filaments power density limit. Exceed it, and the filament fails.

To get the bullet power, you must first determine the KE of each round.

55 gr, 3700 fps = 1671 ft lb
150gr, 2700 fps = 2427 ft lb

Here's the kicker... Since the 55gr bullet is shorter, and traveling much faster, it dumps that energy in to the plate much faster than the .308 bullet.

To find the impact time, you take the bullet length in inches divided by the bullet speed in inches per second, to give an approximate amount of time the bullet is in contact with the plate.

Next, take your KE divided by impact time for each round, you'll get a power rating in ft lb / s

When you compare those two numbers, you find that the 5.56 round is delivering 1.6 times as much power in to the plate as the .308. Even though it is smaller, lighter, and has less KE, the bullet exceeded the plates power density rating, and punched a hole through it.

While .308 delivers more KE to the target, 5.56 delivers it's energy considerably faster.

For additional reading material about how speed kills armor, look up the SLAP rounds (Sabot light armor piercing) used in the military. Extreme muzzle velocities due to very low projectile weight, and can punch through armor very well.

I had heard of the phenomena of increasing yield strength with increasing strain rate, but never got far enough in to material science to quite understand why. I'm reading through that PDF now, and I'm learning quite a bit.

If I had to make a guess, I'd say the strain rate from a bullet impact would be smaller than that from a blast. Even "low" velocity blasts move in the 10's of thousands of feet per second.

From the holes I've seen made in AR500, it is brittle failure.