The morning of D-Day saw American Rangers heading for Pointe du Hoc to silence the big 155mm guns believed to be installed at the top of the cliff. To assault the cliffs, bombardment from British Destroyers were to keep the German heads down along with air assault. The rangers had rocket fired grappling hooks to scale the cliff by rope and DUKWs equipped with ladders from London fire brigades. These ladders had Vickers K guns installed at their tops, to help keep the German at bay while men climbed up.
About half of the landing force made it to the cliffs, but an hour late and lost the element of surprise. The Rangers that landed were on their own, as the reinforcements were redirected towards Omaha Beach due to the delay.... Which was a good thing for Omaho Beach, as the Rangers were partially responsible for getting the stalled landing off the beach. This reduced force still made it to the top of the cliffs with few casualties and did secure the gun position. The guns weren't installed, however, but Rangers used thermite grenades to disable them anyway. Alas, the British ladders and guns couldn't be brought to bear due to the heavy surf. The real threat was the Maisy gun position a few miles inland, which the Rangers did take out 3 days later.
In a story from the Independent, test guns fired by UK forensics experts have been shown to explode on firing, even leaving a piece of the barrel in the ceiling.
The guns were made from Liberator plans, developed by Defense Distributed, and were shown to be more of a danger to the shooter than the intended victim, if it was to be used in a crime. Not many details were forthcoming and a spokesman from DD suggested that there was an ulterior motive in the authorities denouncing the gun as dangerous. They simply don't want them on the street.
The reporter said the test used 9mm handgun ammunition and failed fairly rapidly, within a few shots. What kind of 9mm ammunition that was used hasn't been identified. If they had used 9mm machinegun ammunition, which is readily available at gunshows, the bullet could blow up a real gun without any problem.
3D printed guns are interesting as a project, but it is up to you to learn what the risks are, something that's all to familiar to someone who reloads ammunition. But if criminals want to use these guns to commit crimes, and it really does wipe them out, maybe we shouldn't advertise the problem.