Vacuum bazooka

When you attach the vacuum cleaner to the tube it can suck up to about 10% of the air out of the tube. This may not sound like very much difference, but air pressure is huge. Air pressure pushes with a force equivalent to 10 tonnes on every square metre. This means that the air inside the tube is pushing out on the projectile with a force of about 160N , but the air outside is pushing in with a force of about 180N. This means that overall there is a force pushing it down the tube of 10-20N.

10-20N is very large force for the 10-20g mass of the projectile, so when you let go it will accelerate down the the tube very rapidly. When it hits the card it is going fast enough to knock it off and fly through the air.