That is actually a really good idea. I remember reading an article in my local paper years ago where a man had shot and killed his upstairs neigbour when he got drunk and was shooting his gun randomly. His neighbour was sleeping in his bed and the shot went right through his matress and killed him.

I wouldn't doubt that, I've seen firsthand what even relatively weak firearms can do to these apartments. That's not to say I wouldn't have a firearm for defense, I'd just be careful about what I chose (a shotgun would likely be my first choice for that, in all honesty)

Use Silly putty. Cheap Frangible shotgun loads.

That's neat but how the hell do you store such rounds?

Fridge?