Alternative title: Why Energy Isn't a Good Way to Measure Pressure Intensive Impacts
For this I'll be assuming the object being hit is completely rigid and does not deform at all against the bullet. A Superman kind of no-sell. Most energetic bullet in the world is the .700 Nitro Express, so I'll be using that as my reference point.
Bullet Velocity - 610 m/s
Bullet Mass - 0.065 kg
Bullet Length - 0.018 m (Subtracted the case length from the overall length)
Bullet Radius - 0.0089 m
0.018 m/610 m/s = 29.5 microseconds
610 m/s/29.5 microseconds = 2.068e7 m/s^2
F = (m)(a) = 2.078e7*0.065 = 1,350,700 N
W = (F)(d) = 1,350,700*0.018 = 24,300 J
Now if we use the pressure of the bullet instead...
Circle Area = (pi)(r^2) = (pi)(0.0089^2) = 0.'000249 m^2'
Pressure = F/a = 1,350,700/0.000249 = 5.424e9 N/m^2
See how much more that figure makes sense?
Results
Durability Needed to be Bullet-Proof (Energy) - 24.3 kJ, Wall level
Durability Needed to be Bullet-Proof (Pressure) - 5.424 GPa, 9.9x stronger than steel