Italy unloaded most of the 7.5 Carcanos before they were issued to the troops. Nevertheless, in addition to the scenaro that you describe, the 6.8 round would chamber in a 7.7 rifle (with massive excess headspace). That situation could result in the action bursting on firing. The Carcano got a bad rep in the States because importers sold huge amounts of poor-quality 7.7 and 6.8 surplus ammunition. Much of it was simply dud ammo, but certain wartime manaufactureres had made substitutions in the powder formula that led to rapid deterioration in storage. By the mid-fifties those rounds were hand grenades waiting to go off in your face. It is actually a testament to he strength of the Carcano that few people were seriously injured, but the guns got a rotten reputation anyway.
The takeaway is that Carcanos are good rifles, but surplus ammo with headstamp dates from the '40s or no date should be discarded.
***By the mid-fifties those rounds were hand grenades waiting to go off in your face.***
Too bad a certain commie did not use military issue junk on Nov 22, 1963. Think of how different history would be if it had blown up in his face.