The Model 1870 Vetterli was designed.......you guessed it!.......in 1870, as a black powder single shot. The majority of these rifles were then upgraded to repeaters in 1887 with the Vitali box magazine at which point they became Model 1870/87s. However, they were still in the original chambering of 10.4X47mmR. This is a very low pressure black powder round that approximates the ballistics of the .44 mag revolver firing a heavy lead bullet (NOT the pressure of the .44 mag!).
The conversions that were performed during WWI were a response to an emergency shortage of serviceable small arms in the Italian Army. The Model 70/87/15 (or 16s according to some sources) was issued during WWI to artillery, lines of communication, train, military police and prison guards and other non-combatant units. They were issued to troops that were never expected to fire their small arms other than in a dire emergency. The various models of the Carcano that were originally issued to these same troops were then freed up for issue to front line combat units.
The Vetterlis were understood from the very beginning by the Italian Ordnance Department to be ABSOLUTELY MARGINAL WARTIME EMERGENCY CONVERSIONS. They were not expected to see regular combat duty in the trenches. The barrels are sleeved. When these rifles were originally produced, they were contemporaries of the US Trapdoor Springfield. I don't hear a lot of people advocating shooting modern smokeless loads in a Springfield? Metallurgy in the mid to late 19th Century was not what it was even in the early 20th Century. These were produced out of very soft steel. Take a close look at the small lugs on the back of the bolt. Then look at the lugs on a Carcano.
In addition, many of these rifles were converted by brazing the new bolt head on the old bolt body. I have examples of both types of bolt in my collection. The original bolt face was designed for a case that is near identical to the .348 Winchester (I am not at home or I could give you the original case rim and head diameters).
This subject has been covered countless times on the Forum since it opened in 1995. Over the years, we came up with verified examples of two barrel sleeves shooting loose from the original barrel and ruining the rifle. In addition, we had two people who had the brazed bolt heads separate from the bolt body. These rifles were never specifically designed to direct gas away from the face of the shooter, although I would imagine that a lot of the gas would channel down through the magazine in the case of a pierced primer or separated case head. My biggest concern beyond these two problems would be an eventual headspace problem due to the soft actions gradually stretching over time if fired with full house loads.
I have never heard of a Vetterli completely letting go, however, that does not mean it can't happen. Fire these rifles with full house Carcano loads at your own risk. Common sense and a full understanding of their history would in my opinion, cause an intelligent person to tread lightly. These were marginal conversions undertaken on an emergency basis during the biggest war in History up until that time. I can't see as how the past 90 years has done anything to make them stronger?
Just my $ .02 worth.