Don't reference "the smell of cordite in the air" after a gun discharges, unless your novel takes place a hundred years ago.
Keep in mind that British ammo used Cordite for years beyond what one might think. It was standard until at least after WW II. British .38-200 ammo marked 2Z used flake powder.
But he wants to set stories in the 1920's. If US or German ammo is used, no Cordite. That was a UK propellent. German bullets for shooting game were also better, the Brenneke Torpedo and Ideal patterns. The American Nosler didn't appear until about 1949. But British ammo was reliable and bullets by Rigby and others were well suited for the heaviest animals.
Keep in mind that Britons called shotgun shells (American) "cartridges", just like they did metallic ammo.
And learn the difference between bullets and complete cartridges. TV newscasters frequently muff this.
BTW, Colt had an agency in the UK from about 1851 until handguns became illegal there. Their .32 automatic was quite popular in the UK and the Empire, where pistols were allowed. Churchill and other officers also liked the Model 1911 Government Model .45. Yes, it was designed by John Browning, but made by Colt. In WW I, several others were licensed to make it, as they were in WW II. Lord Lovat was a famous user of the Colt .45 auto in WW II, and Churchill tried to get enough to make it the standard pistol for his Commando regiments. However, beginning in 1920 (with passage of the new gun law), I think officers had to carry issued sidearms. Prior to that, they bought their own. Churchill got his in 1915. As Prime Minister in 1940, he bought many new ones and tried to get his police bodyguard to carry one. But that man preferred the Webley .32 auto, the std. police pistol then, and back at least as far as the 1920's. So, your stories can say that the cops had Webley .32 automatics. Of course, most carried no pistols at all. I have a character using a Webley .32 in a short story on Fan Fiction.net, but he was a villain. Still, that gun suited his nature and the times.
Keep in mind that if you use our advice, you may need a translator. As Churchill (or maybe Shaw originally) said, the English and the Americans are two peoples separated by a common language! I am of British descent, quite probably a hereditary Baronet, maybe an Earl. (Genealogy research continues.)
I read more books by British authors than do most Americans and see some UK movies. But even I don't know many British slang terms, etc. And we say chick where you say bird, for girls. Bonnet vs. hood on cars, etc. Ian Fleming messed up on some US terms in his Bond books.
In those, Lost World stories mentioned, the arrogant London editor tried to jack me up because I had Prof. Challenger eat eggs cooked "over easy." (US term for eggs turned in the skillet; not "sunny side up," with the yellows staring at you.) Said that Britons don't eat eggs cooked "over easy."
Now, those eggs were cooked by Anglo-Brazilian character Veronica Layton. Otherwise, they'd probably have been cooked by her virtual sister (after Third Season episodes), Finn, also Anglo-Brazilian. These blondes were Anglos, but born in Brazil and def. would be
brasileira. in some regards, despite speaking American English. (Marguerite Krux, later Lady Roxton, Countess of Avebury, was lazy and a poor cook.) And Veronica was involved with American reporter Ned Malone. How do you think she'd cook those eggs?
My son has eaten eggs "over easy" in Britain and had no trouble ordering them that way, nor did other diners. The order raised no eyebrows. And Ian Fleming, like me, was a Gemini who liked scrambled eggs. He even published his recipe for the best scrambled eggs! Some Britons just hate to praise American authors, picking at our spelling, too. I'm sure that member Kurusu will confirm that Brazilian Portuguese differs from "pure" Portuguese. But is Alessandra Ambrosio any less beautiful than Sara Sampaio? Both supermodels are pretty and very smart. Alessandra also designs clothes and Sara is a very shrewd poker player, professionally. Alessandra is
brasileira, and Sara is true Portuguese. Both are outstanding women. Do their spelling differences really matter, if we grasp the meaning of the words? To be sure, a
carioca in Rio de Janeiro may use a word that a friend just in from Lisbon may not recognize.
Anyway, check gun terms to be sure you use UK versions. Otherwise, it'll set off some editors who dislike Americans. Yes, I'm quite serious. Ditto for some UK readers!
And, beware of those who saw, "Downton Abbey" and other period-centered lifestyle shows. If they think you got a minute detail wrong, they'll slam you! In my case, some thought I had Finn being too familiar with her chauffeur and maids after marrying Challenger. (I killed off Jessie Challenger via flu.) So, then some, especially middle-aged ladies, objected to Challenger marrying a girl less than half his age. But I think they "click" as I wrote them.
The only thing you can be sure of as an author is that, as Lincoln said (paraphrase), you can please some of the readers all of the time and all readers some of the time. But you'll never please all readers all of the time! And that goes double for editors, many of whom are self-centered elitists. They frequently think they know more than they do.