[Marcello]

"And how many rounds is possible to carry on the APU'ed gun? Not much I suspect."

Judging from the pictures of these artillery pieces, zero is probably the answer to your question.Personally my idea was to use that mobility to move the guns between prepared positions:leave a shelter, drive to a fire position, execute the fire mission and then roll back into a shelter to avoid eventual counterbattery fire.Ammunition in this case would be prepositioned and thus not an issue.