Bullets are dumb. Really dumb. This is a problem, because bullets spend a significant portion of their actively useful life in very rapid transit from one place to another without paying the least amount of attention to what’s going on around them while they do so. If you or I behaved in such an ignorant manner while traveling from place to place, we’d almost certainly get run over by a bus.
In order to not get run over by a bus, those of us who are cleverer than bullets do our best to be aware of our surroundings while we travel, compensating for changes in both our environment and our destination. DARPA has managed to imbue bullets with a similar level of intelligence, allowing them to steer themselves to a moving target while dynamically adjusting for whatever sorts of things might send them off-target, such as crosswinds or the lousy aim of whoever pulled the trigger.
DARPA’s Extreme Accuracy Tasked Ordnance (EXACTO) program has produced a bullet that can somehow steer itself after it’s fired, tracking what looks like a laser (or other optical designator) towards a target that can be “moving and evading.” This allows an experienced shooter to consistently and reliably hit very difficult targets, and as the video shows, even a novice can hit a moving target at long range on the first try.
The key differentiator here is that since the bullets are self-steering, they can be controlled after they leave the barrel of the gun, which hasn’t been possible until very recently. Systems like TrackingPoint’s precision-guided rifle and scope use lasers and computer vision to autonomously lock on to targets and will even fire the rifle for you, but no matter how fancy your aiming system is, a downrange crosswind or a target that changes direction after you fire is impossible to compensate for: once you’ve pulled the trigger, all you can do is watch what happens. But with EXACTO, as long as you keep a laser dot on whatever you’re aiming at, the bullet will steer itself there.
At this point, it’s not entirely clear just how, exactly, DARPA’s bullets do what they do, probably because it’s highly classified. Sandia National Labs demonstrated an optically guided steerable bullet a few years ago that uses fins, but it had to be fired from a special smooth bore rifle. DARPA's bullets can be fired from a standard .50 caliber weapon, suggesting that they can steer themselves while spinning, which is quite a trick. The rendering that DARPA provided of the bullet doesn't reveal much at all:
The implication here is that the actuation mechanism is all internal, which could mean (and I’m just making an educated guess) some sort of system that alters the bullet's center of mass in resonance with its spin to alter its flightpath. Or black magic, which DARPA specializes in. We’re not sure, and we’re also not sure when (or if) we’ll find out. We’re also not sure how much the EXACTO bullets are able to alter their trajectories over a given distance, but we’re guessing that it’s not as much as the “Replay” button on the Zorg ZF-1:
It’s interesting to speculate about what else might be possible with this technology, like bullets that swerve around objects to hit targets behind cover. It’s also interesting to speculate about how a smart bullet might be countered: if you can blind the optical tracking system in the bullet itself somehow, would that make you effectively bulletproof? As this is the very definition of an arms race, I’m sure we’ll eventually find out.