Re: 20" barrel blank??
a good share of the cost of a barrel is the labor and equipment costs, setup time, etc. It would most likely be a PITA for the barrel maker and would be no significant cost saveings. Ask and see what you find out.
I've got a good share of 8" stubs laying around the shop also, but they do make dandy stock for thread caps.
AR type barrels are made shorter, but many are ran at a time in the same setup.