There are various ways to make a rubber band gun. They range
from simple one-shots to the complicated automatic, 1000 rpm guns
seen on some internet videos.
The simplest rubber band gun needs no more than ammunition and
your other hand: simply make the gun of your fingers, stretch the
band from the fingertips around your thumb and back to your pinky.
To fire, simply flex it and send the band flying.
Other pretty simple designs need only a piece of wood, and
sometimes a catch for the band. You can hold a piece of wood in one
hand, stretch the band along it and use your thumb to hold it in
place until you want to shoot it. Others involve something like a
small clip or a clothespeg, secured to the wood involved; all you
need to do is open the peg to release the band being held, and thus
fire it.
Semiautomatic rubber band guns are a little more involved, with
several moving parts. Making one is a bit of a challenge, but well
worth the effort. Some can figure out the parts and how they should
shape up by just thinking for a while, others will need to observe
a premade one or perhaps dismantle and copy it. Basically, you need
a toothed wheel to hold the bands, and a trigger assembly (usually
of one piece) that can release one tooth per pull, yet prevent the
next ones going at the same time. For some inspiration, have a look
at how pendulum clocks work, specifically the escapement
mechanism.
The bigger ones, often powered by motors or cranks and shooting
sometimes hundreds per minute, are just a lot of these semiautos
attached to the same weapon, and controlled by a catch to pull the
trigger when it should. For these, you realy need experience at
developing the smaller versions, as well as some know-how in the
fields of either simple electronics or of simple mechanics - both
is pretty good.
How big you make any of these depends only on the size of the
bands being fired; if you can choose that size, then it depends
only on how big you want to make it.
