Notes:

What makes this even worse is that the vast majority of the killings were done by hand, in order to save ammo.

The below extract comes from the Wikipedia entry on The Killing Fields and it paints a pretty harrowing picture.

The executed were buried in mass graves. In order to save ammunition, the executions were often carried out using poison, spades or sharpened bamboo sticks. Inside the Buddhist Memorial Stupa at Choeung Ek, there is evidence of bayonets, knives, wooden clubs, hoes for farming and curved scythes being used to kill victims, with images of skulls, damaged by these implements, as evidence. In some cases the children and infants of adult victims were killed by having their heads bashed against the trunks of Chankiri trees, and then were thrown into the pits alongside their parents. The rationale was "to stop them growing up and taking revenge for their parents' deaths."