This is cool, especially if you're like me--and even though your prime Lego® years have passed you buy, you still can't get enough of them.  As a matter of fact, I'm working on the Lego® Death Star™ right now. 

Using only Legos and rubber bands, Jack Streat was able to build a fully-functional  replica of a Heckler & Koch UMP 45 sub machine gun.

This is far from the only time Streat has fashioned a potentially dangerous thing out of Legos;  he was able to build a working chainsaw out of the construction toys, and has a Youtube channel full of videos of other Lego weapons he has assembled.

The video for the  UMP 45 is below.  A Lego gun doesn't quite shoot with the same velocity or distance as a real gun, but if the TSA bans Legos from flights, you know who to blame.