Hey Frog , I bought a similar piece of tubing from them for my spanish mortar and have turned and threaded it for a registered US base cup I picked up on sturm.
The original spanish 60 base cup has had the threads machined away, fine for a BP but a real bummer for rewatt as a DD
If interested I have an original 60mm spanish tube perm welded to a base cup thats had the firing pin and threads machined away making it perfect for BP use . $200 if interested
I have fired at least 100 shots from it with BP and Pyrodex with projectiles as heavy as 6 lbs so I am pretty sure its not weak . I found some nice little juice bottles that hold aprox 1.5 lbs of sand that I can throw about 800yds with no problem, it is also the perfect size for tennis balls which can be tossed 3-400 yds with just a tablespoon of black powder.
here is a quick vid clip of it throwing a tennis ball in the back yard , you can see the wad fall to the ground right after it fires http://youtube.com/watch?v=dfTPpBDJFY0
If the base cup is perm demilled (threads and firing pin machined away) and perm welded in place on the tube I dont think its neccesary to make it smaller than 60mm . I have never seen any reg stating the tube has to be undersized, its generaly only a problem if you have a functional base cup and a functional bbl in possesion at the same time, which would then be considered a DD. A functional base cup alone is a just like any other title one firearm , transfers on a 4473 and the bbl is just a bbl . The two together are a DD.
I have corresponded with tech branch about making original 60mm and two inch brit mortar bbls and have been told that the bbls are unrestricted unless in the presence of a functional base cup .
Short story is if a non functioning(no firing pin) base cup is perm welded in place the bbl can be original bore size.