Probably one of those things folks argue about.
Myself,I hate to mess up the fine work of a barrelmaker.
You can take a piece of threaded rod that will be about 1 1/4 or 1 1/2 in longer than the bbl.It can be brass with threaded ends,or you can cover it with plastic or brass tubing,soda straws,or something to protect the bore from damage from the rod.
Make some handles to screw on the rods,like tool handles.Blind tap them to accept the rod ends.
You can make o-ring grooves concentric with the bore,or gaskets,or turn teflon washers.for seal on each end of the barrel
If you have been practicing with junk,practice again to make sure the end seals don't leak ,and the whole thing works for you.Beware what materials you use,some plastics relax and leak,maybe aluminum spoils salts.
I use this method to protect bore when browning or rust bluing in a boiling water tank.
I prefer not to recomend trapping the ends of any vessel that will go into 300something degree bluing salts .The danger of a high pressure steam explosion in the salts is obvious.
My suggestion is for hot water processes only.