I would guess that the 20 Gage would be quieter than the 44 or 500. Looking on Hodgdon's data site, peak pressures for the 20 Gage were under 12,000 PSI, 44 Mag was 40,000 PSI, the 500 was 50,000 PSI.
The loads I saw for the 20 Gage used faster powders in smaller amounts than either of the pistol cartridges. Also, the 20 gage has a greater diameter than the pistol rounds, giving the barrel a greater volume.
All would most likely mean lower pressure when the projectile leaves the barrel of the shotgun than the rifles. The shotgun will be quieter than the others with standard loads.