From Rifle Magazine:

"From about 1899 on for the .45-70 and 1903 for the .33 WCF, barrels were nickel steel with a tensile strength of approximately 100,000 psi."

From Encyclopedia of metallurgy for 4140 steel:

"In the heat treated condition tensile strengths of 170,000 PSI. for small section and 140,000 PSI for larger section are attended all combined with good ductility and resistance to shock."

Note:

Rifle barrels are usually made from 4130 and 4140, but are not heat treated. Heat treating makes steel more brittle. The tensile strength is still greater than 100,000 psi.

In conclusion:

The discipline of making steel in the nitro era (1898-2007) has been such that a sufficient margin of headroom is afforded.