Bushing modification refers to removing the stop built into a reducing or hub bushing, so that a pipe may freely pass through. This is commonly done when fabricating coaxial cannons and piston valves.
Several methods exist that can be used to remove the lip from a bushing. One of the easiest ways is to use a Dremel or other rotary tool. Sandpaper or a file can also be used.
The best method of doing this is to remove only portions of the lip at a time. Check the pipe inside the bushing periodically until it will fit.