This is a paint drawing of the whatever you want to call it.
Basically its the barrel, with a larger size pipe sleeved over it loose enough to slide up and down the barrel. Somewhere under the sleeve is a hole in the barrel big enough for whatever you are firing. This whole is also on the sleeve so when they line up its an open hole into the barrel from the outside. From there its just the matter of attaching a hopper of any sort onto it.

This is "it" on a gun. Its in front of the black valve on the model.







