[narwan]

The 'problem' is the ammo reload; the reload capacity is based on the warhead size, not the cost of the rounds. So the very expensive cluster rounds reload as quickly and easily as the much cheaper HE rounds.

There are a few work arounds though for PBEM games. One is to disallow onboard arty with cluster rounds, only offboard arty with clusters allowed. That will solve the 'problem' in most cases. But there are instances where that eliminates a substantial part of a nations on-map arty purchase options. For those you can use another agreement, to only use the amount of AP rounds allocated at the start of game. Reloaded AP shells aren't used (simply don't plot new cluster strikes for the unit), only the HE rounds. That represents that cluster rounds weren't usually available in large quantities and would be used for specific targets only. So no more blind cluster strikes all over the map.
I've used that agreement in PBEM games myself and it does work. You'll have to trust your opponent to play it fair off course.

Another way to solve the problem of not being able to buy an on map arty unit because it has AP rounds is to look for a similar unit in another OB without the AP rounds and then purchase that (set as captured). Now you'll have the same unit type but without cluster rounds (used that too in the past).

Narwan