I saw a sudden fall in stock prices. But he has made money by buying stocks in two new airlines.

The development of guided round offers the potential for substantial reduction  in ammunition expenditure.

1. in stock prices 和 in ammunition expenditure，是一个类型。在表示上升、增加、下降、减少的名词之后，用 in 表示“方面”。

a rise in...

an increase in...

a growth in...

a decrease in...

a reduction in...

a drop in...

a fall in...

2. stocks in two new airlines：stocks in + 上市公司，in 说明“公司对股份的拥有关系”（Stocks are shares in the ownership of a company.）

3. potential for substantial reduction： for 表示目的，对（做）某事有“潜力，可能性”。