An Anti-Aircraft cannon similar to the Flak 88 in that it fires a single, highly explosive shell with timed fuses (in the case of CCNet, basically a proximity fuse). I'd say it would have the reload time of about 8-16 seconds so that it can't be rapid-fired. The shell would travel farther than your standard AA before exploding on its own, however if it came within 5-10 blocks of a plane, it would explode. The shell has a 20% chance of not exploding when within range of a plane and will continue to fly past the plane before exploding on its own or coming close to another aircraft, which it runs the same chance of detonating or not detonating. There are two ways the damage could work.

A. Release shrapnel (literal bullets from normal AA). This also means that a skilled pilot can still avoid the shrapnel.
B. It simply damages any plane within a 5-10 block radius, almost no damage at farthest, maximum damage at closest, and immense damage, almost one-shot for large planes, on direct hits. There would be a 20% chance the shell does not do any damage at all if it detonates, even on direct hits.

The heavy AA's shell would travel slightly slower than your standard AA shell. Heavy AA would not be allowed on warships but would be allowed on forts and trucks. I suggest the shell be made from a canister shot shell with aluminium and gunpowder surrounding it to make 6-12 heavy AA shells. The heavy AA would be excellent against planes traveling at max altitude or even heavy bombers (my previous suggestion) as they would be cruising even higher than standard aircraft and untouchable by standard AA.