Here is Galena/AMT's response to my email inquiry of selling an extra barrel blank, from which to make a 5.7mm Johnson barrel (in case you cared):
In our manufacturing process we drill and rifle the barrel blank before we
machine the the locking lug, etc. This means that we don't have any barrels
with the outside machined but no hole drilled. Hopefully the first of next
year we will set up a custom shop where we could do this but for the time
being it is not possible - Sorry
David Small
Galena Industries