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).