Ballistic matting
Words of Gp Cpt Bennett RAAF
"The ballistic matting is intended as a protection against small arms rounds that might penetrate the aircraft, as occurred in an incident earlier this year when a US civilian died in a Hercules hit by small arms fire. The matting acts as a form of armour, similar to the underfloor and seat armour fitted to helicopters in Vietnam."
Believe me the Aussies do their homework.