Remmi-
I think that you're making this too complicated.
By the way, if you're a writer, you should know that Writer isn't normally capitalized.
If you want a Webley MK IV, go for that. You might have him order one with a honed action (for smoothness) and a commercial blued finish. They also had an Army revolver that was basically a MK. IV with a six-inch barrel and a square butt, almost identical to the later MK VI. It was not an issue item, officers buying their own sidearms then.
You could have him use a Webley-Wilkinson or WG revolver chambered for .476, which threw a 280 grain bullet at some 700 FPS. That was about the max practical in such a gun. Although the .476's original home, the Enfield revolver of the 1880's, was obsolete, the ammo was still available, and it would hit harder than standard .455.
I'd say to let the guy carry a MK. IV with nicer than standard finish in settled areas, and have him use a .45 Colt for meaner places. That's what I thought Sherlock Holmes should have taken after the Hound of the Baskervilles. But Sir Arthur Conan Doyle knew very little about guns.
Speaking of Doyle brings me to my own Fan Fiction about a TV series based on a book of his, not in the Holmes series.
Have you read, "The Lost World"? A syndicated TV series made in Australia was made about it recently (1999-2002) and several movies were based on it over the years. The TV series has some die-hard fans, like the first, "Star Trek" series. There are a number of fan sites still operating.
I noted the guns in the TV series, and stayed with those for my Fics, adding some types that they probably had, or could have had, whether the explorers were seen with them on the show or not.
Because this was very much in the time frame that you mention, 1919-1923 on the show, that data would apply.
Will Snow, as Lord John Roxton, wore (depending on the episode) a brace of nickled Webley MK. VI's in shoulder holsters, or a Colt M-1911 .45 auto . Ned Malone, the American reporter, used a Webley MK IV .38 (really not made until 1927) or another .45 auto. Prof. Challenger had a Colt .45 SAA. Dr. Summerlee (killed off at the end of the First Season) had a MK. IV Webley .38, as did several transient characters. I guess the prop house in Queensland had some on hand...I just assumed that they MEANT to have them represent MK. III .38's. (These guns are NOT numbered in the military Mark series.)
Finn (blonde chick in the Third Season) used a small crossbow, but I replaced it in my Fics with a Smith & Wesson M&P .38, taken from some dead slavers after an early story. It has a five-inch barrel.
Finn also (in my Fics) uses two rifles. One is a 6.5mm Mannlicher-Schoenauer and a .44/40 Winchester M-92 carbine. Which depends on what she expects to encounter on a given day.
Challenger uses a H&H .450 double rifle, like he had on the show. Lord Roxton has a .416 Rigby, true to the show, or a .318 Westley Richards for a lighter rifle. Margurite Krux, whose handgun was a S&W hammerless .38, (sometimes subbed for on TV by Webley WP or Iver Johnson hammerless guns) has a .303 sporter made by BSA. Actress Rachel Blaklely DID use a .303 on the show, but it had a Monte Carlo stock, and probably dated from the 1960's. I have her carry one made in the REAL style of the day. She also has (in the Fics) a Rigby .275.
Ned Malone has a .30 cal. Springfield M-1903 rifle. Also a .450 that I suspect was made for the Army & Navy Cooperative stores, given that his newspaper probably wouldn't spring for a Holland & Holland or Rigby or Purdey double for him.
In the book, which I have just bought and not yet read, I saw a .577 double rifle mentioned. This was in fact a popular elephant rifle around WW I, even later.
Now, what the explorers had to fear in that book/ series was dinosaurs that had survived on a remote plateau in South America. They would also have to be concerned about any real Amazonian animals and hostile Indians. (In one Fic, I had Finn get up a tree with her Mannlicher and snipe a Xingu headhunter from some 300 yards.)
I mentioned all this because the time frame is the same, roughly, and because I know guns of that day very well, and already did my homework before writing those Fics, 17 of them so far.
Except that you seem to be concerned with monsters for a horror story, the need seems much the same. I don't know what monsters you have in mind, but doubt they'd be any tougher than the larger dinsoaurs that the Lost World explorers faced. So, I made my suggestions.
I didn't equip anyone with one, but S&W's .44 revolvers like the famous Triple Lock and the Second Model Hand Ejector were available then, too. In fact, Sasha Siemel, who lived in Brazil in those days, used S&W .44's quite extensively. He later had S&W .357's and one of the first .44 Magnums, but that was later than your time frame. I recommend his books, such as, "Tigrero" for your reading, also the works of the African hunters of the day. Interlibrary Loan may help you locate a reading copy of, "Tigrero", printed in the early 1950's.
Hmmm...I just recalled that I DID have one bad guy use a Smith & Wesson .44 Special. He was one of Zoth's minions, in my Fic, "The Death of Zoth". Zoth was a demon who had caused a horrible war in the 21st Century, which Finn survived, to be brought back to 1922 by Prof. Challenger, in a one-shot time machine built into a cave.
That's all that I have time for. But those are the very guns that I chose for this time period, roughly. Oh: I once had Marguerite tell her lover, Lord John Roxton, to wear a Colt .32 automatic under his tux in a scene where he dressed up for a more formal occasion. A British gentleman of that day might very well have carried such a gun for concealed needs. Colt was always well represented in Britain, and they had no bad gun laws until 1920. Even after, the peerage and other well connected persons could easily get gun licenses.
I hope that this entertained some here.
You can hear Australian actor Will Snow's voice now on the Outback Steakhouse commercials. That's Will as himself, not faking a British accent. He and co-star Jennifer O'Dell (Veronica) have also had some guest roles on American TV, at least once together. (On, "THe Closer".) Jen is American, the only actress in that series who was. Finn sounded American, but actress Lara Cox was born in Canberra, and lives now near Sydney. She has also gotten some later guest roles. Alas, the shows weren't seen here, although if anyone is interested, I can probably tell you how to see her in one series in episodes on YouTube.
Is anyone else here here a Lost World fan? If so, you may like this video of the three Treehouse ladies, made by a Dutch fan. She's pretty good. The music is, "Harem" from the CD of that name by the famous Sarah Brightman. See Finn, Veronica, and Marguerite here, and understand at least some of the appeal of the Lost World series.
http://www.youtube.com/watch?v=UAX6HSQuNAI&mode=related&search=
Lone Star