You want a quick lesson in sound control, stop by the scrap yard, pick up some junk mufflers off anything and cut them in half. The energy of the gases is soaked up and re directed by baffles. Defect here is that a muffler is in use while the vehicle is running. Guns work one shot at a time.
Second consideration is how much gas is produced (directly related to the powder charge weight) and how much barrel does it pass thru. A short barreled rifle (or handgun) that lets the gas out before it is all burned will be a lot louder (and brighter after dark) than a long barrel that burns up the powder and has much reduced pressure before the bullet exits the barrel.
I was shocked at the low noise of a standard velocity .22 long rifle leaving the barrel of a target rifle. 22 or 24 inch barrel. Not as loud as my air gun. Didn't need hearing protection indoors. !!
Then there is relativity. How close are you to the gas ball? Again, handguns with normal barrels, around 6 inches, are worse than rifles and revolvers worse than autos or single shots.
As suggested the smallest is quieter. .22 Hornet is very good. .218 Bee little more pep and noise. Or you can load down a .223 to these levels.
You don't indicate why you ask. Neighbors. Ironically anyone in other countries can buy a silencer/suppressor/ gun muffler. Getting the gun is the problem. Here the devices are controlled. Guns are widely traded. For the cost plus the $200 transfer tax, last I knew, you can have a silencer on your gun that does something legally. Or you chamber a .22 Hornet in a 26 inch barrel, or load down a .223 (or
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) and get all the "quiet" you want. As a rule this comes with lower velocities and performance of the old black powder days. Long range shooting becomes more of a challenge... oh welllll luck.