A "Geek Breech", aka restrictor tube gun is a fast gun type BB cannon made with a tubing type breech instead of the O-ring type breech. Instead of an o-ring pinched between the barrel and brass fitting, a short length (1" or so) of model fuel tubing is used. One end of the fuel line is attached to the breech and the other end is attached to the barrel. It was called a Geek Breech because it did not need tweaking and any old geek can use the cannon. An illustration and more technical explaination can be found at Visit this site. It is the second cannon down the page.
