I think that H. Rider Haggard ought to at least receive an honorable mention. His adventure stories usually included firearms, and though, like Heinlein, he did not go into great detail, they usually were acceptable.
Nothing ruins a story for me quicker than really sloppy writing about firearms. I don't understand why writers do not get some one firearm savvy to check their work.
The author of the Parker stories usually got his firearm models right, but I feel like his knowledge of guns came from reading catalogs. This is better than the research many writers do.
Robert E. Howard's character, Francis X. Gordon carried a "fat black automatic". I have always wondered if this was a model 1905, or a model 1911. Could you even buy .45 ACP in Afghanistan at that time?