Wars have been won and lost purely on the ability of one side to hide really well. Even a superior enemy can't take you out if they can't find you. Give ancient Rome barbarian-seeking arrows, and we'd still be wearing sandals to the bath house to wipe ourselves with sticks.
OK, bad example.
Finally, in the 21st century, technology is catching up to the problem with weapons that almost seem like cheating. So we're not sure if we should be impressed or terrified by...
6Bullets That Will Outright Chase Your Ass Down
Really the only thing there is to know about bullets is that they travel in a straight line. If a dude is shooting at you, you're fine as long as there's something good and solid between you and him. A car door, a sandbag, another less fortunate person you're using like a bullet proof vest. This has been the rule for the last 700 years or so.
Until somebody came up with a way to develop guided bullets.
The building full of crazed geniuses known as the Defense Advanced Research Projects Agency (DARPA), have developed the EXACTO program, which involves using "smart" bullets complete with processors and steering vanes to allow the bullet to correct its trajectory in mid-fucking-air.
So even if you duck around a corner as some asshole is shooting at you, his bullets will supposedly be able to just swing around and follow your ass. We aren't talking about some wimpy small-caliber bullets either; the only size they are building these in (for now) are .50 caliber BMG, huge rounds which are intended either to kill trucks, or explode a guy's head from several miles away.
Just in case the word "overkill" hasn't entered your mind yet, they've got a much more explodey version of that idea, too. It's the XM-25 "smart" grenade launcher. It has a laser range-finder built in that can detect the distance to the wall or trench that you're hiding behind. That way the shell knows not to explode until it's right next to your fucking head. That's right; it won't detonate until after it punches through the wall you're hiding behind, but just before it hits you in the nose.
The XM-25 has a range of different munitions it can use, from high explosive to thermobaric, all of which are designed to make your trip to the other side as sudden and unexpected as it is horrifying.
5BOSS Will Find You
But they can't shoot at you if they can't find you, right? After all, two can play the sniper game, and maybe you're hidden a hundred yards away in some bushes, or peeking through a small hole in a building.
Ah, wait. You have suddenly exploded. It was your eyeballs that gave you away.
You can thank the Battlefield Optical Surveillance System, a device that can be mounted on a truck that scans the landscape with lasers and sensors. When it sees the glint of a rifle scope, or your freaking retinas, it lights you up with a laser beam, at which point a bunch of soldiers turn you into salsa.
It will feel way less awesome than it looks.
If you do avoid detection, god help you if you actually fire a shot. That's where the Boomerang system comes in, which uses an array of microphones to instantly judge the speed and direction of the shot, based on the muzzle report and the shockwave of the bullet as it zips past. Immediately it knows where the shot came from, and even what kind of weapon it is.
So if you only get one shot, you'd better have something pretty badass, right? Like a rocket propelled grenade? Ah, that won't do you much good with the goddamned invisible force field protecting the vehicle you shot at.
DARPA calls it the Iron Curtain, and it uses an invisible pulse to detonate shells before they can hit the vehicle. Our only question is how long until they have a beam that just makes the enemy's own rocket explode right in his face.