This is the best view of the mechanism. The crank is turned clockwise. This causes the large gear on the left to turn counterclockwise, and the small pinion engaged with the rack on the bolt to also turn counterclockwise. The pinion pushes the bolt back, stretching the rubber band.
The gun is fired by the lever on the very left 'falling' into one of the gaps between the little rivet-like legos stuck in the gear (there's two positions around the circumference without a piece in them). When the lever is able to fall back, it pulls the lever on the front back, and disengages the pinion that pulls the rack. The rack is then rapidly pulled back by the rubber band, ejecting the projectile out the back on the left.
This picture shows the gun immediately after having been fired. You can see how the lever on the left, and the lever holding the pinions are both futher back. The movement of the lever with the pinion on it is not that great, so that difference is harder to see.
Also note the red rubber band. This rubber band causes the pinion
to disengage, while the black rubber band towards the bottom left causes
it to disengage. The black rubber band needs to be much tighter than
the red one, because it needs to overpower the red one, as well as yank
the pinion, which is jammed against the rack on the bolt, out of the rack.
This picture shows the gun from the other side. Here you can see
two small gears tha couple the main drive shaft with the hand crank on
it to the pinion that pushes the bolt back. The shaft with the crank
on it is also the shaft used for the pivot for disengaging the pinion.
This way, the two gears coupling the drive shaft to the pinion always stay
the same distance apart.
This is the gun with the magazine removed. The piece with the rack on it is the bolt, in the 'fired' position. The smooth tiles just in front of it are for the projectile to slide out smoothly without jamming on anything. Also note that I used the pieces with the horizontal holes through them even where there is nothing going through them. This is because those pieces actually hold together slightly better than the regular legos.
I built this lego gun back in 1998, before people had digital cameras, and way before YouTube. I took the photos with my Scanning camera hack.
A YouTube user by the name of Billy Glen has since built his own version
of the machine gun based on the picturs and refined it further.
He has posted a
cool video of the gun in action on YouTube. Too bad about the soundtrack.
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