A 6.5 Mannlicher-Carcano is the name of a particular design of cartridge. There are other 6.5 mm calibre cartridges if different designs. You can look it up online.
I don't want to. You made it sound like only a M-C could fire a 6.5 mm cartridge. Is this true or not?
That's not what he said at all. He said that there a number different cartridges designed for a 6.5mm bore, but the design of these cartridges differ.
There are a number of military 6.5mm rifle cartridges out there: 6.5 Carcano, 6.5 Swedish Mauser, 6.5 Portugese, 6.5 Romanian Mauser, 6.5 Arisaka, etc. There are also a like number of civilian cartridges. While all of the bullets might satisfactorily pass through a 6.5mm barrel, the case designs differ, and a cartridge for one design won't fit in a rifle built for another.
BTW, if you spent a little more time searching, and less time making pointless arguments, you might find that the 6.5 mm round provided by Mrs Penn was a surplus Carcano round made by SMI:
https://www.maryferrell.org/showDoc.html?docId=62272&search=pc-78441#relPageId=72&tab=pageBTW, Mrs Penn only said that "she saw a photograph of OSWALD in the newspaper and stated that a side view of OSWALD does not look like this person. The full face photograph, however, she believes does resemble him."