
You can buy one of these cool rubber band machine guns (for $400), but that is not geeky enough. To qualify as a GeekDad you need the get the $30 plans and build it yourself.
The for-sale assembled gun -- almost an art piece -- has twelve barrels and loads 12 rubber bands for each barrel. That means that when you crank it you can launch an enemy-leveling barrage of 144 bands in a matter of seconds! As they say on their website:
The plans for the DYI rubber band machine gun are for a gun of a different design and lower capacity. It only holds 72 rubber bands, and looks a bit less elegant but to my eyes more military. Both contraptions are found on the same website, called Backyard Artillery. Indeed there's a bunch of other cool geeky items.

One big caveat. I had not seen, used, or built either of these guns, or any other items on the site, so I can't vouch for their performance. If you have experience with these or similar, let me know.