Back, oh, forty years ago I read a short story that mentioned an imaginary cartridge by name. I came across the story again in various anthologies. While browsing Gutenberg I found that it's out of copyright and up on the Project Gutenberg web site.
The story is "Police Operation" by H. Beam Piper. It debuted in the July 1948 issue of Astounding Science Fiction. It's a pretty good story, which is why it was anthologized several times. Some bad guys from an alternate timeline dump a carnivorous alien animal in the Pennsylvania woods, and an officer of the Paratime Police is dispatched to dispose of it. He has been kitted out so as not to attract the attention of the natives of the United States of America, circa 1948, but the prop department gave him a bolt action rifle that was just different enough to attract attention when he encounters the natives. As described, it's a "Sharp's Model '37 .235 Ultraspeed-Express," which turned out to have been made shortly after the Second War Between the States, somewhere else. Oops.
.235 would be 6mm or .243 as we measure bore size today. If anyone ever worked up a high velocity 6mm wildcat, that would be a nice name for it...
Here's the short story:
Police Operation by H. Beam Piper - Project Gutenberg
Yeah, it's from 1948, but if you like SF, guns, or hunting stories, it still looks pretty good. Piper eventually wrote several novels and a bunch of short stories based off this one.