Variations in bore diameter for one thing. Then there's the issue of pressure spikes. If you reload for the .268" 160 grain bullets using other than WW 760 or H414 with magnum primers, you might experience a pressure spike.
Before the recommended reloading data became widely available, I was experimenting with IMR 3031 in moderate doses, and had a pressure spike. The case markings were completely obliterated, the primer vanished altogether, and I was barely able to open the action. Never again!
In fact, I use only cast bullets now.
Long story short, there are dimensional variations to deal with, and experimentation is sometimes needed in different Carcanos.
But accuracy is certainly possible. My best shooters are an M91TS upgraded to M91/28 and an M41 that was refurbed at Terni for export.
I strongly believe it's worth the trouble to work up accurate loads.
This link on reloading 6,5 Carcano is very much worth the time it takes to read it - reasons for only using the two recommended powders is explained. SW
http://personal.stevens.edu/~gliberat/c ... emary.html