1) http://www.youtube.com/watch?v=b3r6oCH1 ... r_embedded
Missile vs Tank

2) http://www.youtube.com/watch?v=-EVqT3XE ... r_embedded
Rifle - Hows that arm of yours.

That cannon moped is actually a real weapon. It was used in WW2, but quickly went out of fashion because of its practical obstacles. Leave it to the Italians to design something that looks sweet but doesn't work for shit.

The second one is actually pretty serious, itś an idea to create a heli without a rear rotor (which are always very vulnerable).

You mean vulnerable to enemy fire or just in general (elements, technical faults)? Because I would imagine it being easier to just target the huge top rotor instead of the smaller rear one if I was shooting at a chopper.

But I'm not engineer so I have no clue anyway.

Loss of the rear rotor results in a uncontrollable spin. Shooting from the ground with rifles it is difficult to hit the main rotor because the shooter is aiming at an upwards angle, it is much easier to target the rear rotor. Not sure about the russian copters but the brittish ones will send a guided rocket from more than a km away.

Shooting at a rotor isn't the best thing you can do. They're hard to see, the bullet may travel between the rotors, and even if you hit it, the damage may be insufficient to cause any problems. You're better off shooting at the body of the helicopter. In reality, if you're in combat, the chopper won't be sitting still, it'll be buzzing around at low level. Just hitting it at all may prove a challenge.

I think most engagements on helicopters occur behind it. Also, I think the bulk of the heat sig is near the tail.

It's not just vulnerable in combat situations. The rear rotor is on a long tail and can hit something by accident. If the two top rotor concept works, you can do away with the entire tail part and make the heli a lot more compact.
Here's a video that shows what happens when the tail fails: