Pete, I believe it's only available as loaded ammo at this time. The fired cases I have hold 43.8-44.0 of water and were fired in what appears to be a pretty 'big' chamber...at least when comparing dimensions to an unfired case. I would suspect a vigin case to hold just a skosh less, though I haven't pulled a bullet and measured one yet.
A reamer dimensioned to move the shoulder .070 forward...or a 'standard' reamer run in .070 deep... will just make the 45.5 minimum needed for an IBS/NBRSA 'crossover' HBR case.
Sure would be nice to have only one 6 Power class in the IBS...with no case capacity minimum, wouldn't it? And if the NBRSA would follow suit and drop the case capacity rule for it's HBR guns...now that would be some real, common sense progress that would benefit both competitors, the IBS and the NBRSA. -Al