10.4mm versus 10.9mm - .410" v .429". That speaks more to the human proclivity to split literal HAIRS than to any real-world difference between the two cartridges.
Of course the .41 wasn't born to be a "magnum," but to be an improvement over the .38 Special in use by the POPO, but quickly found itself at odds with the established .44 magnum. That's because the ammunition companies established a "police load" and a commercial "magnum" load.
The two are so close as to be indistinguishable, and certainly nothing being shot by either will ever notice the difference!