A couple of things... (Score 1)

After spending a little while actually building the thing, one major drawback became apparent.

Sure, it has a high firing rate (assuming the handle doesn't break), but it has no power to match!

Using 1x4 bricks yields a consistent result with a somewhat higher muzzle velocity, but there are better alternatives...

Take the classic axle 'crossbow'. It's contstructed completely out of technic lego, and fire the longest axles around at frightening speeds. Even better, simple versions can be slipped into a pocket

The most basic form is simply about 10 fairly wide beams held securely together, lined up so the axle can slip through with minimum friction. Then the rubber band wraps around the front (you need to make sure that it doesn't cover the exit hole. THen you pull the band tight, line up an axle, and release...

If the band is tight enough, the axle will pass through at least 2 sheets of corrugated cardboard before noticably slowing down

That's raw lego power power in anybody's language

Then there's the highly advanced pump action axle crossbow, but that can hardly be detailed through text...so...