One Revolver

[CharlesHarris]

The increase in noise of the 18-1/2" Marlin vs. the 24" is about 10dB, depending upon the ammunition. Standard velocity 158-grain lead which gets about 800 fps in a revolver does about 950 fps with 95dB in the 18" barrel and approaches 1000 fps from the 24" barrel with no significant increase in noise. Generally a longer barrel is quieter because its increased expansion ratio reduces muzzle exit pressure.

Subsonic. 22LR ammunition in a rifle with properly designed moderator will be less than 80dB.1 You are lucky in UK that moderators are not as heavily regulated qas in the US. We can have them, but the procedure is cumbersome, similar to registering a machinegun.