It may have been the ALR thread [or somewhere else], that said that TC used 4150 steel that is actually much stronger.
Possibly more brittle but it was properly treated by TC.
It's mostly the specialty long rifle barrel makers that use the 12L14 in order to keep costs lower for individual builders.
It's not that they are unsafe, although the thread mentions that they are easier to blow out when loading mistakes are made.
But rather that there are better steels more suitable for gun barrels.
My only purpose for posting this at all is because it shows that there are different quality barrels that are currently being produced.
The different steels used by the different makers means that it's more difficult to generalize about all modern barrels.
It's possible that the imported Spanish sidelock guns and the old TC sidelocks have stronger barrels than made by some of these "premium" long barrel makers today.
From what I've read, TC had more than one company making barrels for them, and then they may have also started making their own barrels after a period or time.
It's helpful to read the original thread as it provides some info. from both sides on the issue about 12L14 compared to other barrel steels.