If you unload the ammunition of the magnum by drag and drop in the inventory window, you can only load (the option combine is not showed) the bullets to other ones in the inventory. If u do that, the former ammount of the bullets where u load in to, are reduced by half. You have to go to in the inventory, right klick on the bullets, choose "eject" and then you can combine the ammo with other bullets to a stack.