Spherical shell and spherical case used plug or Bormann time fuzes*, but shells for rifled guns did indeed have safety mechanisms against going off if dropped. A common method was to have blocks between the firing pin and the primer that were spun out of the way when the shell took the riflilng; another was to have safety wires that were broken by the shock of firing, but wouldn't break just from being dropped. The trouble is that none of the fuzes of the day (or today for that matter) were absolutely reliable, so there still is a lot of unexploded ordnance out there, buried in the ground.
Here is an odd point about CW artillery that I hadn't thought of until I read something by Imboden. He said that rifled guns were less dangerous to the recipients of the favor than smoothbores. The reason was that explosive shells from rifled guns would bury themselves in the ground and go off with little or no damage. But round balls would hit the ground and roll or bounce before going off, so the damage was much greater. Of course, a timed fuze, if set right, would detonate the shell or case shot in the air over the target, causing serious damage.
As to telling solid round shot from shell or case, it is easy enough if the projectile is clean; shell or case will have a fuze hole, probably filled with either a fuze plug (which screws into the fuze hole and seals it for transporting) or the fuze itself. But if the ball is very dirty, rusted or seriously eroded, it may be hard to tell solid from a shell or case. The best practice is not to mess with any one of them unless someone has already made it safe.
*The shell or case shot was strapped to the sabot with the fuze facing forward; the "windage" or flash of the powder around the projectile ignited the fuze. Most folks think the fuze faced in toward the powder but if it did the explosion would just drive the fuze into the shell and set it off in the gun, blowing the gun up. Even today, I am told, a great fear of "red legs" is a "premature" in the bore.
Jim
