Hi I am still learning about the whole cannon science, and I do not have anything bigger than a pop can size mortar. My understanding of hern tubes is that if you ask for a seamless tube and a properly shaped breach plug its a valid option. Also you will have to make a proper vent liner. What you need to avoid is having microscopic black power particles to go between the steel liner and the casting. With times the particles will attrac moisture, then corosion will form and since its most likely in an area where visual inspection may or may not be possible at one point your tube may be more like a pipe bomb. So based on my understanding it seem that a proper steel liner can (or almost) withstand the presure by itself, so as long a you make a proper vent liner you should be good. Also Joel Brown at Hern seem to really stand behind his products and always answered all my questions. Safety is 49% about the tube and 51% the user, because Im sure not mater what tubes you get , there is at leas one moron that can somehow blow it apart.
Now like I said my experience is pretty limited so hopefully somebody with more experience than me will chime in