Did some interweb searching and nickel has a hardnessof 4 Mohs, iron 4.5 Mohs Steel 4-4.5Mohs according to a jewelry website.
Wikipedia says iron is 4.
Another place I found them staring the Rockwell hardness of 99.5% nickel is 44 where iron is 79 and stainless is 70...that's just a couple minutes of browsing...who knows how accurate any of it is. And it's all talking about pure nickel which is highly unlikely, more likely an alloy, which in all actuallity stainless steel could perhaps be considered a nickel alloy ( I build houses, definitely not a mettalurgist) so without more information we're all just guessing unfortunately. It would be nice to believe they wouldn't be making bullets that would be known to destroy barrels....unless maybe shilen and Krieger teamed up on the project to get more sales lol.