Indeed. I am well used to using various techniques to get round the inadequacies of OFP, the setpos being a command I use exhaustively. However, it will not work in this example as you would still have to set the amount of ammo the other weapon of the tank has (e.g. the machine gun) if the player has used this weapon - and that is not possible.
I have tried numerous things to try and get round this particular problem, but it looks as though it is not possible.
Anyway, thanks for your suggestions. However, it appears that there is no way round this problem.