Breech loading
Many types of breech loaders are employed. The most common methods are to use male and female adapters to connect the barrel to the chamber, as seen here: http://www.spudfiles.com/forums/viewtopic.php?t=183
A method favored due to speed is to use a type of cam lock fitting to connect the barrel and chamber. one such example is here: http://www.spudfiles.com/forums/viewtopic.php?t=311
One method that is one of the more challenging to make is a bolt action, which can be seen here: http://pneumaticsniper.mypicgallery.com/