I suspect those are assumed units rather than truly unitless...
As a writer, Hemingway was subject to Editors and even Translators as well as whatever Knowledge he may or not have had. "The unbelievable smash of the .505 with its 525-grain bullet at a muzzle velocity of 2,300 feet per second" "No, Mr Hemingway, you can't write that! The readers won't like it! Let's just say a muzzle velocity of, ooh, four tons. That gets the idea across, don't you think?" "But..." "Too much? You're right, let's make it two tons."
And where do the Translators come in? Well, they don't so far, but I'm just wondering how they've translated it. Are those long, short or metric tons? Or have they mangled it into some other, maybe more sensible, unit, because all readers in language X are familiar with muzzle velocity in terms of goose wings per fortnight? And what are grains in any case?