Guys,
Thanks for the interest in researching barrel forging processes. Maybe someday we will be able to recover some of the lost skills and figure out HDTDT (How Did They Do That). The following is taken from patent application 27,539 describing the "old method" of forging gun barrels without the weld. The improvement in the patent application does include the use of a mandrel. Apparently, before this patent idea, barrels were forged to length without a mandrel. I envision that this resulted in the reduction in size of the central hole, but not to the extent that the boring reamer could not follow the still existing, but smaller hole.
"Drill a hole through the whole length of a short cylinder or bar, which was then forced through plates with graduated holes or passed through rollers as used in the old process of rolling iron, but without the use of mandrels until the required length was attained"
This is from patent application 27,539 Manufacture of Gun Barrels, J. H. Burton, Mar 20, 1860
What would be interesting to do is to try the method of barrel forging that Mr. Burton describes as the "old method".
Jim