Rock 22". To echo what Nerflad alluded to, the shell packs I have had over the years typically been standardised to the Rock configuration - i.e. 12, 13,16, 22. I doubt this is relevant to your survey, but I'm on the lookout for a 10, 12, 14, 20 shell pack that I would use for exactly the same type of music but I just fancy something a little more portable (I'm thinking mass rather than volume).