I mean, the gun thing is more on the adaptation artist than the author, perhaps. Without looking at the novel version, I can't say if the actual author ever specifically said what guns were being used when and where. Doesn't really seem like the kind of story that would've went into excruciating detail about the weapons in any scene. But even with very little direction on what to use when and where, just Googling random guns from any and all time periods and drawing those into a setting that looks as old timey as this was probably on the artist if anyone. Collective responsibility since a publisher and perhaps the original author took a look at it and okayed it?