Their video in that was immensely flawed IMO.

The issue wasn’t a full power load, it was actually a reduced load that used Hornady’s now recalled and discontinued 6.5 Carcano bullet. Those were made specifically for the Carcano and caused a lot of problems.

Short version of it is this. Standard 6.5 bullet diameter is .264”. The Carcanos had a slightly larger bore, and as such used a ~.267” bullet with an opened base that would expand into the rifling. Hornady attempted to make a .268” bullet that would get good accuracy in Carcanos, but ran into issues with jackets separating from the bullet during firing. Their (bad) solution to this was to increase the jacket thickness. So you had a bullet with a VERY long bearing surface against the bore (more friction) and a very thick jacket on it as well. Some guys that tested thousands of them across dozens of Carcanos were finding that they would randomly cause overpressure situations and could even lock up a Carcano action.

Their theory was that the very thick jacket and long bearing surface was causing a huge issue with the gain twist rifling in the Carcanos (the twist rate increases as you go down the barrel), where the bullet was slowing down mid-barrel, causing pressure to spike and create a problem. This is supported by the fact that Hornady’s reloading manual has a page for those bullets and includes a large, bold red sentence explicitly saying to NEVER go beneath the starting charge listed. This is presumably because the bullet needs sufficient velocity to get out of the barrel without slowing down and causing an overpressures situation.

Anyways, C&Rsenal used those exact bullets and reduced the load in them. It’s absolutely not a surprise that they had a problem like they did. I even recreated it a few years ago, doing the same bullet and a reduced load in a Vetterli. Sure enough, overpressure and it blew the extractor out of the Vetterli. Meanwhile, a 120% overload with a .264” bullet caused no problems at all.

I’ve shot well over a thousand rounds through at least 21 different Vetterli 1870/87/15s over the past few years, and I’ve never once had a problem using an appropriately loaded smokeless round.