That's probably most powerfull lego gun, in terms of projectile velocity. Sharp tipped bullet archieves 50m/s and goes through:
- 32 sheets of paper
- 5mm of oak wood
- 0.5 mm of steel (hard to measure, but very visible dent)
- 20-25 mm of meat

Lone minifig head reaches astonishing speed of 80m/s and despite being blunt goes through:
- 5 sheets of paper
- 1 sheet of cardboard

All velocities calculated by measuring bullet drop at 10m distance multiple times, with error of about 10%. 

Due to super high speeds and the fact that bullet is not accelerated at the barrel end (so it can stabilise itself after hundreds G's of acceleration), rifle is very accurate at up to 20m. Shooting at longer ranges is less practical since air resistance on the bullet is pretty huge.

The mechanism is very efficient and almost frictionless (indicated by the fact that weapon is very silent when shooting). The pusher brick is stopped by rubberbands at the end of the barrel to prevent it from smashing into bricks and shattering.