Since I just got mine, I've been doing some research.
I would say the rarest GP100 barrel length is a 5" inch.
And then, I would say the next rarest is a 3".
The most common are 6" and 4".
Also, the FULL lug is the most common.
I never even saw any half/short lugs in my search.
This is just to add to what Terry just said. What he said seemed to be very true during my search.