I have replied in the BIS forum, but if you don't see :
I had the same problem when testing some ideas with a vehicle.
You have this problem because the mp ammo trucks are inheriting their values from the BIS ammo truck, and BIS ammo truck is getting this
transportMaxMagazines=200;
And , if you count the JAM ammo you added in the truck class definition, it goes over this maximmum of 200
So in the class definition of your ow ammo truck, add
transportMaxMagazines=300;
This will solve your problem.