The .577 Nitro Express is a large caliber bullet used primarily by professional ivory hunters from the late 19th century into the 20th century.

In The Man of Bronze, Doc removes one of these bullets that was deeply lodged into the back of his safe with his bare hands. He then mentally computed the weight and determined it to be the .577 Nitro Express based on the bullet's weight of 750 grains, and fired from a double-barrel rifle.

Renny and Doc, using some string and a few other things determined that the shots were fired from the top of a nearby skyscraper that was under construction.