It was dimes. Nickels are too large.
Fact is, no one I've ever talked to knows whether this is valid or an "urban myth", because I've never heard verifiable reports from anyone who's ever actually tried it.
Over the years a lot of people have mentioned it.
Once, Jeff Cooper wrote that he'd load a shotgun with dimes as a "down the hallway" load.
Supposedly, Vietnam Special Operations people like LRRP's, Rangers, SEALs and Special Forces loaded shells with dimes, and called it "Keep the change, Charley".
It was supposed to "cut through" jungle brush better than buckshot.
The ONE real test I saw was a year or so ago on the Discovery channel????? in a program about the "Real Billy the Kid".
In the show, they discussed whether Billy the Kid really shot Bob Olinger with his own shotgun, which was reported to be loaded with dimes.
In the show, they loaded up some shells with real silver dimes, and shot a side of beef covered with cloth.
Most of the dimes bounced off or only penetrated slightly.
Buckshot spectacularly blasted all the way through.
The sticking point: The gun used was a 10 gauge shotgun, NOT a 12 gauge.
Dimes are a tight fit in a cylinder bore 12 gauge, but are much too small to seal a 10 gauge bore.
So, the question is, did the dimes fail to perform because they didn't properly seal the bore, and the gas escaped around the column of dimes, or do dimes really not work?
What's needed is some experimenter to try some silver dimes in a 12 gauge shotgun.
Until then, I'd have to put the dime story in the urban myth category, and say NOT to use them.
Whatever, the same statement I made above holds true: You don't want to put yourself in the position of finding yourself sitting in a courtroom full of people, explaining your having shot someone with your "trick load", and having them all looking at you like your some kind of unique insect.
NOTHING works as well a standard buckshot, or is as politically correct.