Why Cap's shield is indestructible

I never consistently collected The Official Handbook of the Marvel Universe. Only issues that tickled my fancy. One of these was #15 -- the "Book of Weapons, Hardware, and Paraphernalia." I mean, who wouldn't want to get the scoop on all those cool gadgets utilized by Marvel's greatest?

The section on Iron Man is awesome enough (and why wouldn't it be -- the book is largely done by Eliot Brown, the same dude who did the terrific Iron Manual), but there's also things you'd [possibly] never even consider: Stilt Man's systems, Deathlok's cybernetics, Dr. Spectrum's Power Prism, and Mockingbird's battle staves. And at issue's end, there's a cool page devoted to two of Marvel's unique inventions: Adamantium and Vibranium. Any true Marvelite knows that Adamantium is what Wolverine's claws (and bones) are composed of, and that Vibranium is found only in Wakanda, home of the Black Panther.

What many Marvelites may not know is that Captain America's vaunted shield is ... composed of both these substances!

[Dr. Myron] MacLain began experimenting with the process that creates Adamantium as a young scientist in the employ of the United States government in the 1940s. By accident he created a special adamantium compound which is more nearly indestructible than any form of adamantium created ever since, and then through another accident, fused this adamantium sample he had created with a sample of an unknown metal which is now known to have been Wakandan Vibranium. The result was the large rounded specimen of alloy which is the most virtually indestructible substance known to man. This alloy specimen was given to Captain America to serve as his shield. The exact chemical composition of Captain
America's shield and the exact degree to which it is indestructible have never
been determined, nor has MacLain or anyone else ever learned how to duplicate
either the special Adamantium compound used in the creation of the shield, which
is known as Proto-Adamantium, or the process which fused the Proto-Adamantium to the Wakandan Vibranium.

All emphases mine.

And there you have it. Cap's shield is made of an Adamantium even stronger than that usually used in the Marvel Universe! It's stronger than Wolverine's claws and bones, tougher than Ultron's body, and harder than the Constrictor's coils. (Read the whole Handbook entry on Adamantium here.)

Take that, Count Nefaria! "Mere scrap of metal," old man?? HA!