According to the autopsy doctors, the bullet that struck JFK's head hit near its hardest part -- the External Occipital Protuberance.
The bullet that struck JBC's wrist (CE-399) did not fragment -- probably because, having passed through JFK and JBC, it had slowed down quite a bit and because it was twirling when it struck said bone.
D'oh
Hi Tom, as the WC showed, a full speed bullet striking a human wrist would literally smash the wrist bone therefore for CE399 to only fracture Connally's wrist it must have been travelling slower! 
CE399 was partially slowed after moving through the soft tissue of Kennedy's neck, it then went sideways through Connally and after exiting it struck Connally's wrist at an angle hence why it left lead fragments and subsequently CE399 buried itself shallowly in Connally's thigh.
In fact the main provenance for the authenticity of CE399 is the fact that it was discovered before anyone knew the true extent of the injuries, so in other words who could possibly know what condition of bullet to plant?
CE399 was intact when found by Tomlinson.
CE399 shed a small amount of lead and a small amount of lead was discovered in Connally.
CE399 was flattened on one side consistent with it's passage sideways through Connally's ribs and even though a Carcano bullet fired through water will deform it won't exhibit the same flattening on only one side.
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Also worth noting is that John Lattimer removed a similar amount of lead from another Carcano bullet and made 41 fragments which easily accounted for the missing lead from CE399.
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JohnM