That's back when firearms were overbuilt and used low chamber pressures because they didn't have finite element analysis simulation software and their materials weren't consistent enough to not blow up, or have to weigh 200 lbs. Dumping great energy into a mass is what matters.

You can heave a cannonball at an elephant with a potato gun, but it won't do much. .45-70 Government isn't a Big 5 cartridge despite being almost 12 mm across because of its antique origins and low chamber pressures.

A tiny FMJ high velocity round like a 5.7 wouldn't do much despite having the penetrating power of being essentially a supersonic "needle".

.338 Lapua requires a 5400+ J load to legally hunt B5 in parts of Africa. Otherwise, a 12.7x108 or .50 BMG would do the trick. Anti-matériel rifles are a lot easier to acquire than useless, antique elephant guns. Barrett.net

KE = 0.5 x m x v^2.