Are you planning to put the beer in the barrel and then put the bladder in the barrel over the top of the beer to fill in the airspace above the beer?
That was my intent when I posted the question. yes. However, now I have found a 50 liter oak barrel
and so I no longer am persueing that. Still If I got into barrels, and had a bladder, I could double up
on oak ageing. I even thought of putting water into the bladder just anything to fill the void so I could
get beer into the oak.