Did they ever do a forensic test on the "magic bullet" to see if its markings matched the riflings in the Carcano and or the brass from which it originated from? I would assume they likely did. It still wouldn't mean anything. That bullet was brought in via 5 different people in hand-offs after the crime scene was abandoned. This is maybe why there was a mixture of different casings and bullet types used.
Anyone could take the Carcano in the evidence locker and shoot it into a pillow and gather up the round! If you need to invent evidence, there is nothing better than slipping it in as required and giving it a nice long voyage from the stretcher to the crime lab, passing it through 5 different individuals and placing it in the evidence locker to match the outline of your story.
Its like having a December 18, 1963 letter gathered up to make sure that there was no hole in the windshield - a paper necessary to rebuttal those that actually witnessed a hole - that needed to be dismissed as well by introducing evidence gathered later to match the story narrative.
"Could you write something up for me, I need to ask you for a BIG favor......" 
