To that, I said that 100 rounds of 50cal missing the gun, would or should have the same effect as the srapnel.
Why should the .50s get such 'collateral' effect as a bonus when no other weapon in the game has such?
Your effectively asking for the ability to kill something without being able to hit it, on an assumption that is based on random factors - bullet bounce, shrapnel, etc.
A cannon is fundamentally different as a weapon. If the cannon shell itself does not hit, but the target is still within its impact blast radius, then effectively, by all means, the cannons have not missed the target at all. It hit the target.
The cannons do not rely on a secondary, collateral effect to kill an ack gun. The explosion is a primary effect.
To the AP guns such as .50s or .30s, the primary effect relies on kinetic impact of shells only.
You are asking to model an arbitrary "shrapnel/bullet bounce" perimeter to the AP guns so it can kill something that didn't hit the mark at all, so in effect, the AP guns will become like cannons in its efficiency against small ground targets.