FMJ ammo was designed to pass through-and-through human flesh and remain as intact as possible (as per the Geneva Convention).
Additionally, look at the butt-end view of CE399 and imagine your head being crushed to that extent.
Call that 'pristine'.
You assume that CE399 was at room temperature when it was deformed slightly.
But it carried 2250 Joules of kinetic energy (10 g. at 2200 fps or 670.6 m/sec muzzle speed). The specific heat of lead is .16 Joules/gram degree C. The melting point of lead is 375 degrees C and the addition of tin and antimony will lower that a bit. That is the melting point. The lead core will soften at temperatures much lower than that.
To heat a 10 gram MC bullet at 20 C to say 170 C (150 deg. C change) would require around .16 x 10 x 150 = 240 Joules of energy. If half of the bullet kinetic energy loss is absorbed as heat by the bullet, that represents a loss of 480 Joules of bullet energy, or a reduction from 2250 to about 1750 Joules (670 m/sec to 591 m/sec).