During WWII the British created an almost perfect silent assassination weapon and that was the De Lisle carbine chambered in 45 ACP. The beauty of the 45 ACP with military ball ammo is it is always subsonic which is necessary for suppressed weapons.
The original weapons used a British SMLE action, a Thompson submachinegun barrel and a 1911 magazine with an integral suppressor. This was a WWII firearm made for British commandos but to this day remains among the quietest firearm ever made.
While obtaining an original De Lisle carbine would probably not be possible, modern reproductions are produced. It had a sound signature of 85 decibels. As a comparison, a suppressed .22 pistol could have around a 95 dB noise level, while the unsuppressed one could go up to 135 decibels. Decibel rating are measured on a logarithmic scale.
The difference between 85 decibels and 95 decibels is a reduction in sound of ten times. The De Lisle carbine is 10 times quieter than a suppressed .22 rimfire pistol. It even had a rubber piece where the bolt contacted the stock to kill the sound of the bolt closing.