oh, I don't know the designation of it but my vote goes to the rocket launching pistol. That thing was a peice of crap, the ammo would corkscrew if you were lucky, it more than likely wouldn't detonate on impact, and you could stop the projectile but putting your thumb over the barrel (think muzzle velocity slower than a bottle rocket)
You're thinking of the Gyrojet. It was never an issue weapon, but a couple troops carried them in Viet Nam. It didn't fire explosive rounds, they just used rocket fuel to accelerate, much like a bottle rocket. The corkscrewing was only a problem during development, as the initial model was a smoothbore, the thinking being that angling the rocket venturis would provide enough spin to stabilize the round. The production weapons were slightly rifled, just enough to impart an initial spin.
Muzzle velocity was around 500fps, so I wouldn't recommend putting your thumb over the muzzle.
There was a carbine version that was HIGHLY thought of by testers, but the pistols draw-backs had doomed the company. The carbine was light, accurate, and a HARD hitter at longer ranges.
My vote would go to the 80cm howitzer fielded by the Germans in WW2. An AWESOME weapon, firing a 10,000lbs GP shell to a range of 35 miles, and there was a 7,700lbs "bunker buster" that penetrated 50 FEET of reinforced concrete, AFTER going through 20 feet of rocky dirt.
The problem was that it was INCREDIBLY wasteful of men and material. The gun crew was 1,500 men and officers, plus a large anti-aircraft and guard section, all commanded by a major general. It took about a week to bring the gun into action, which required the use of a construction battalion. The rate of fire was slow, the best they ever managed was a 15 minute reload time, but 45 minutes to an hour was more usual.
The guns were only used a few times, the one that destroyed the bunker did so during the siege of Sebastopol, and another fired about 75 of the GP rounds into Warsaw during the ghetto uprising.
Throw in the huge logistical train required to support all of this, and the Germans would have been much better off using the money, material, and men to build and crew around 200 examples of their 120mm howitzer.