The threads used on the Fergusons breech plug (screw?) is called a "multiple thread".
Common screws have one thread that is wrapped around the centerline so if you look at the end of a bolt or screw you will see one place where the thread starts.
Multiple threads have more than one thread starting at the end of the bolt at one time.
There are "double threads", "triple threads", "quadruple threads" and so forth.
Because more than one thread is starting at the same time, each thread must move much faster up the body of the bolt to get out of the way of the other threads.
For instance, a double thread with two threads starting must move up the body twice as fast as a single thread would.
A quadruple thread must move up the body 4 times faster than a single thread would.
I don't know what thread size was used but if it was, lets say, a 8 threads per inch single thread size if you turned the bolt 8 times it would move an inch. If the same thread was a double thread the threads size would be the same but you would have to turn the bolt only 4 times to move it an inch.
If it were a "10 start" thread of the same thread size, moving it one full turn would move the bolt 1 1/4 inchs.
By using threads such as this, just turning the trigger guard about 1/2 turn will move the breech screw down so it is totally out of the way of the barrels breech.
