(set in 1922?, written in 1922, published in 1923)
Then, in spite of my daze of fright and disgust, my morbid curiosity triumphed; and as the last of the monstrosities oozed up alone from that nether world of unknown nightmare, I drew my automatic pistol and shot it under cover of the thunder.
The only thing we immediately learn is that the weapon was an “automatic pistol.” To an American in 1922, the eminent automatic pistol was probably the Colt Government in .45 ACP, adopted by the US military as the M1911 (Investigator Weapons 1: The 1920s and 1930s, pp. 37-38). We do know that the anonymous narrator was a seasoned investigator of “strange horrors” and a veteran of many “ghastly explorations.” This suggests he probably carried a serviceable weapon rather than one of the many .25-calibre vest pocket pistols that were so popular with contemporary civilians, but offer so little real power.
However, we also know that, like his electric pocket-lamp, he carried the pistol all the time; it only came out at the end of his investigation, suddenly and surprisingly. This suggests an easily concealable weapon. The big Colt is rather on the large and heavy side, as are other, less likely contenders like the DWM-Luger American Eagle (Investigator Weapons 1, pp. 45-46) in .30 Luger (7.65×21mm Parabellum) or a pistol brought back from the battlefields of the Great War, such as a DWM-Luger P.08 (Investigator Weapons 1, pp. 45-46) in 9×19mm Parabellum. The most likely options are, in fact, a Colt Pocket Hammerless (Investigator Weapons 1, pp. 40-41) or a Savage Model 1907 (Investigator Weapons 1, pp. 55-56), both of them available in either .32 ACP (7.65×17mmSR) or .380 ACP (9×17mm). A similar foreign make is also possible, as huge numbers of automatic pistols were imported from Europe during the 1920s.