Blackpowder muzzleloading firearms must only ever be loaded with black powder. Modern cartridge firearms use a powder that is black in color, but chemically is quite different and will far exceed the pressures a muzzle loading firearm can withstand leading to a ruined weapon and likely death or injury to user or bystanders.
I’ve seen it happen in person, at an informal shoot by some moron. The rifle barrel didn’t burst, but it was ruined. I had noticed a peculiar and very loud, sharp report from the weapon and stayed well clear. Technically speaking a blackpowder firearm could be completely filled with blackpowder and the barrel would hold. Even a small amount of modern smokeless powder will detonate a barrel.
It is apparently not unusual for people to believe they can use small amounts of smokeless powder in a muzzleloader. For this reason, anytime muzzleloading weapons are used in your presence by anyone for your safety and others it is imperative that you are certain your partners are not using ANY smokeless powder whatsoever. Lots of idiots out there when it comes to muzzleloaders and cap & ball revolvers, and home made cannon.
For this reason I would never allow anyone near a 4th of July celebration near a home made device. Just because.
Unless you have a ML designed for smokeless powder, like the Savage 10ML II. I don’t know why this one failed. I have one and only use moderate loads.
These were designed as a high performance smokeless powder muzzle loading rifle.
As an example of extreme firearms ignorance, here is a good one. When I was in college at MU, the KA fraternity (which was a southern based sons of Lee) had a cannon in the front yard, which got fired off occasionally when they were intoxicated sufficiently. Their brothers at UMR (an engineering school no less) couldn’t find black powder, so charged theirs with two pounds of Red Dot (fast burning shotgun powder) They only killed two or three people when it exploded.