Fast question about formal literature.

I was just asked how one should go about writing ”.50 caliber” as in relation to the weapon “Barrett M107 Rifle,” in formal literature, such as a fantasy novel that is involved on the lower end of the “technical” spectrum, meaning most of the readers will know what a .50 cal is, but not what a .50 Barrett M107 .50 Cal BMG.
I told the person, that since it was a name, it could be spelled out, or left abbreviated. Which is correct? I know if it were a scientific or paper of option, then ”.50” would be correct, but what about Harry Potter?