If you fire a PKM at the top front of the hull of the M113A3 and M113A1, the engine will be disabled and if angled right ricochet rounds can kill the driver in the M113.
Playing around with this, I noticed that 5.56 NATO does not penetrate the front hull (as would be expected).
I reproduced this issue by spawning the M113 on stratis airport and shooting the upper front hull with a PKM (SLA Machinegunner). This can be replicated with both the M113A1 and A3 variants.