Sorry to be 'that guy', but ramjets don't *need* to be supersonic to work. In fact, the change in airflow / shockwave is enough different that a different kind of ram-air engine is used, a scramjet. Given a typical 155mm muzzle velocity of about 850m/sec (more than Mach 2) I would call the shells scramjets but WTH do I know.
Written by Bob Koure
Retired software architect, statistical analyst, hotel mgr, bike racer, distance swimmer. Photographer. Amateur historian. Avid reader. Home cook. Never-FBer