***The Arisaka and the Carcano have the strongest actions ever fielded in bolt action battle rifles.****
I have read that the reason for the bad rep is that the governments changed calibers during the war. As a result, people would often get a smaller caliber rifle, then try and force the larger caliber cartridge into the chamber, driving the bullet back into the case. When fired, the rifle would self destruct.
It is like some idiot who forced an 8MM Mauser cartridge into a 30-06 rifle, then wonders why the rifle exploded.
Yes, there some idiots that will do that. I had a working companion who blew up a muzzle loading rifle, split the stock, blackened his eyes from the recoil. He had no powder measure and just put several TABLESPOONS of powder in his rifle.
Then there was another idiot who was determined to fire a odd caliber military rifle and wanted a caliber cartridge to fit it. It did not matter if the case did not fit, HE WOULD MAKE IT FIT! I quietly took the clerk aside and told him not to sell this man anything as he was obviously a danger to himself and others.
Then another idiot who wanted to shoot a black powder revolver so he grabbed a can of smokeless rifle powder and wrong sized bullets. I stopped him before he blew himself up.
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.
I know a couple of guys who were trying to fire a powder charge sans ball in a musket and got no action.
After trying several times the guy holding it tells the other, “put your finger over the hole and see if you sense any pressure”. BOOM! Blew the end the finger off.