Corrected entry: When "Sam Spade" is shown the revolver that was used to shoot his partner he correctly identified it as being a British made "Webley". However, he also said that it is a "45-automatic they don't make them any more". The revolver is most probably .445 caliber, 45 automatic cartridges would not be compatible. The movie was made in 1941 and I believe the Webley remained in production at least to the end of WW2.
Correction: Spade also says it's an "8-shot." The original .455 model was a six-shot, but the .38 cal version was an 8-shot. That model is very rare - less than 400 made between 1903 and 1904. See full explanation by Ian McCollum at Forgottenweapons.com (https://www.youtube.com/watch?v=XzOm66SNmvo).
Corrected entry: Sam's coat is unbuttoned (with no tobacco pouch string showing) when he enters his office foyer the day after Archer's murder, but in the next shot (when Archer's widow calls to him) it becomes buttoned in order to hide the string to his tobacco pouch, which he unbuttons and reaches for in a later shot when reviewing the case with Effie.
Corrected entry: Bogart identifies the gun that killed Archer as a Webly-Fosbery .45 8 shot automatic. It is a 6 shot revolver.
Correction: The Webley-Fosbery was an unusual design, an automatic revolver, produced 1901-1915. Less than 5000 were made. However, there is an error, since it was chambered for eight shot .38 or six shot .455; there was no eight shot .45.
Correction: Actually, Sam Spade says the revolver is a Webley-Fosbery .45. The "Webley-Fosbery Automatic Revolver" was made from 1901-1918, but was carried in the Webley catalog until 1939, so the comment "they don't make them anymore" is correct. Additionally, the revolver was available in two calibers - .45 and .38. The .45s were chambered in .455 British Service caliber. While it is correct to say that .45 automatic cartridges would not be compatible, Sam Spade never says the revolver is .45 Automatic caliber -he just says it's ".45 caliber." There are more .45s than just the .45 automatic, after all.