The can be unsafe, however properly made, bulge more than blow, unlike steel barrels.
Oft times, a gun barrel maker would get bulged twist steel/iron barrels back from the Proofing House, hammer down the bulge sometimes having to do it twice,
before it passed proof, but eventually it would pass proof. This info from W.W.Greener's "The Gun & it's Developement" 9th edition.
With steel, I'm pretty sure that doesn't happen - but - I don't know that for a fact.