My experience is that a minimal amount of the fastest practical powder and a LIGHT bullet is quietest.
It takes more energy to get more mass moving, and even if the powder all burns up in the first inch behind a heavy bullet it takes more of it, thus more pressure, more heat, and ultimately more noise.
Most of the time when engineering a load to be quiet there are one or two other critical considerations to go along with it, principally those will be lethality and/or functioning of a semi-auto. When reducing the velocity below the speed of sound, shock damage is also reduced drastically, as is ability of the bullet to penetrate a target straight through without deflecting off of hide or bone and the reliability of expandable bullet noses, so we compensate the only way we can by adding mass to ensure that at the least we get two bleeding holes. Many cartridges will not cycle their intended host weapon with subsonic ammunition unless bullet weight is increased significantly to put reactive energy into the action and/or allow enough mass resistance to build sufficient gas pressure to work a gas action.
The 300 AAC Blackout in a AR-15 is a perfect example of a system which will work with both subsonic and supersonic loads with no changes to the gun, but it only works within a narrow load window and range of bullet weights. Below 200 grains, most AR-15s won't cycle with a load that is still subsonic. Even at 220 grains or more, there is a narrow window of powders that will make the action cycle reliably yet not make so much pressure as to push the bullet over the speed of sound. Recoil-operated .22 LR isn't reliable with standard-velocity (target subsonic) 36 grain bullets, so we have a specialty market for 42-grain and even 60-grain bullets which will cycle un-modified semi-autos without breaking the sound barrier. It all goes back to Newton's Third Law of Motion, and if velocity must be reduced, mass must be increased to equal the given force necessary to work an automatic.
If a quiet load is the principal need, functioning an action isn't required by the load, and you're not shooting to kill something, use the lightest bullet and smallest amount of the fastest-burning powder that is practical to get the bullet reliably and accurately out of the barrel. YMMV.
Camera and card was right at a hundred bucks and does NOT make phone calls or send emails, thank you very much. The microphone and auto-focus actually exceeded my expectations but the lens is pure garbage, which kinda was expected. At least it sorta captures what cannot be related in print.
