I'm not so sure about this mythical 4,000 fps, although there is no one whose opinion I respect more on all things firearm related than P.O. Ackley. The fact is a fast moving .224 caliber bullet is wicked good at penetrating metal. A bullet going 3,500 fps, for example, won't do as good as one going 4,000 fps...but it WILL penetrate a hell of a lot. And so on down the line. As I say, I don't think there is any magic velocity and I've spent a lifetime trying to shoot holes in crazy things.

But before anyone gets the idea a very fast .224 bullet will shoot through anything, let me hasten to say it's not so. There is a big difference between setting up various types of steel plate and shooting it head on with different projectiles than what happens in the real world. A very fast .224 bullet is capable of tremendous penetration...but only once. After the bullet has penetrated the initial object, there is usually nothing left of it, i.e. it comes apart.

The best way to get some real world appreciation of what works and what doesn't may be to find an abandoned old car. These make wonderful "targets" for the curious. Here a hot .22 rifle is only fair at going completely through the car body side to side. Most times it will penetrate side A and blow up so there is little or nothing to penetrate side B. More conventional high powered rifles like .308 or 30/06 do better at the thru and thru penetrations but still hardly any guarantee. Frequently their bullets will hit something and get deflected. A .30 caliber bullet is about as likely to exit the roof of floor as the opposite side of the car. Tests have shown that about the only things that will reliably punch in one side and out the other are the 50BMG and believe it or not a 12 ga slug.

Keep in mind this is not a pure AP test but more of a "real world" test of what goes through what and how. A .220 swift may go through more plate steel but for general uses you'll be better off with a clip full of .308 AP for smoking people out of cars and houses and even woods.

This is a difficult subject to compare apples to apples.