421 has it correctly. The usual calibres of ammunition are harmless without a firearm, even when incinerated, they are no danger to the aircraft and of minimal danger to crew and pax.
This is a non event.
Security have been trained to look for more if someone brings bullets, or an empty magazine, or a part of a firearm. Not so surprising really. A dozen people could bring items that aren't dangerous alone, but could be assembled into something dangerous. Hand-loading bullets on board would be feasible, as would assembling a firearm from parts. Agree that this is a non-event. But I do understand the reaction of German authorities. Then again all the tax free liquor bottles also make formidable weapons...