In every Bond film except Moonraker, James Bond carries a small pistol, and it’s usually in a shoulder holster concealed under his suit jacket. The pistol is either a Walther PPK, a Walther P99 or, in Octopussy, a Walther P5. Due to the magic of filmmaking, the prop department can hold onto Bond’s gun and place it and/or the holster inside the jacket only when needed. Consequently, Bond’s suits usually do not need to tailored to fit a gun underneath. Bond only needs to wear his gun inside his suit jacket when he’s about to pull the gun out or when he removes his jacket. Many of Bond’s suit jackets could not realistically hide a gun.
Sean Connery’s, Timothy Dalton’s and Pierce Brosnan’s suits would be the best at concealing a firearm since they are cut full in the chest. Anthony Sinclair specifically tailored Connery’s suits to conceal his PPK, at least in the earlier films. The closer fits that George Lazenby, Roger Moore and Daniel Craig wear would not be so effective at concealing a firearm. A fit that hugs the body won’t leave any room for a firearm, and the tight suits in Skyfall that hardly leave enough room for Daniel Craig certainly won’t fit his PPK. An open jacket can conceal the gun well and also allow quick access. Wearing the jacket open works better with suits made in heavier cloths that won’t blow open in the wind. A three-piece suit is more presentable when the jacket is worn open
High-buttoning single-breasted suits and double-breasted suits are the worst for carrying a gun in a shoulder holster because the smaller opening makes it difficult to access the gun. Roger Moore manages to pull his PPK out from inside his buttoned double-breasted suits on numerous occasions with impossible ease. Lower-buttoning double-breasted suits like Roger Moore’s dinner suit in A View to a Kill aren’t so much of a problem.
When a suit is tailored for concealed carry, the chest has extra fullness and is shaped around the gun. A stiffer canvas like what English tailors traditionally use helps prevent “printing”, which is when the gun leaves a visible imprint in clothing. Anthony Sinclair’s suits had the characteristics that a good suit for concealed carry should have. Sinclair spoke about his suits in a GQ article in 1965:
If you use a good woollen, tailor the insides properly, you should be able to take the suit, roll it into a ball, shake it out and have it fall into perfect shape. It’s the fabric and canvas and inner work you invest in a garment that should do the work.
This kind of resilience will also help to hide a firearm. Softer Italian suits need to be cut even fuller to better conceal a firearm.
I have never carried a gun myself, so I do not write from personal experience. If you have personal experience in concealing a gun under a suit, feel free to share below.