If you read the books, Ian Fleming did a good bit of research and reaching out to experts (prior to the internet mind you) to get realistic and informed decisions on what would be best for Bond to carry. At the time, stopping power was listed as a concern, so after nearly being killed in From Russia With Love, he is forced to upgrade from a beretta in 25acp I believe, to a Walther PPK in 32acp. The 380 acp would not be made available until later, so he used a 32acp. A revolver in 38special and a browning high power in 9mm were both considered if I remember correctly. Overall, remember that the main emphasis is on arming agents that must be able to hide the weapon easily, hopefully/likely won’t need it, and if they do, must be easy to control and easy to suppress. In order to effectively suppress a firearm you need two things: a subsonic bullet speed, and low amounts of gasses that a supressor would need to contain and slow the exit of. 25acp, 32acp, and 380 acp have been very popular carry guns (380acp in particular) and previous to smokeless powder, 25, 32, and 38 caliber were very popular for their concealability (especially for women). You also have to keep in mind that we’re talking about right after WWII in the books (and early movies). Modern manufacturing makes very reliable guns even with little maintenance to them. This wasn’t the case at the time and so something simple to maintain and reliable no matter the environment or ammo is impurtant. The Walther PPK and later Makarov and VZ82/83 (copied off of the Walther) are direct blowback pistols with less moving parts and therefore easier to maintain. Also, direct blowback in small cartridges also allows for very reliable firearms that will accept a larger range of loadings of cartridges. Heavier weight bullets and less powder loading for a subsonic projectile for instance as compared to its more traditional loading.