Danny:-
it could have slowed to the point where it had just sufficient energy to penetrate the sheet metal of the spinner and then be stopped by the hydraulic "dome" of the CSU
Very true of course, but to me that still seems incredible. The spinner has a momentum of its own, propelled as it is by the aircraft. The shrapnel may have lost much of its initial velocity when the shell of which it was part had exploded, but would still have been subject to the laws of physics and retain some percentage of its original "oomph".
Stafford's spinner must have collided with this jagged hot mass of metal in that brief window of opportunity before the mass started accelerating again under the effect of gravity. I seem to recall that the danger in the London Blitz from being struck by falling debris from our own Anti Aircraft barrage was no inconsiderable part of the total hazard, as a life underwriter might have calculated. Hence the dubious value of our Prime Minister viewing proceedings from the Air Ministry roof...