Chugalug,
...If you tried to fire a chunk of red hot jagged metal at high velocity into the spinner of a fast rotating propeller driven by a Napier Sabre, without disturbing the performance of either, I strongly suggest that it would not be possible...
Very true, but if the shell casing fragment had travelled some distance 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 but be unable to damage it (I'm assuming a hydraulic prop), and remained trapped between.
When the "butterfly" tail fuse safety caps came off our wing bombs, they were quite often found jammed into the lower wing or flap, but, being so light, just stuck there and did no further damage.
(Just a guess). Cheers, Danny.