This is a full-sized, dummy 20mm round.

Here’s how 20mm rounds are loaded into the six-barreled M61A1/A2 miniguns found on fighter planes for close-quarter air-to-air combat or on ships for anti-air/anti-missle duty.

The 20mm round used on the hard wax body probably never distorted, and therefore the exit wound you see is just the energy passing through the sculpture. One 20mm round that actually expanded would probably turn a human body into a bloody mist, with just a few messy chunks left behind. The M61A1 minigun can shoot up to 6000 of these rounds in a minute.