A friend has one. Hard to say really. Perhaps like a loud hand clap. Certainly not silent. But noise level wise I’d be happy to use it in a resident area and confident neighbours wouldn’t be bothered/concerned etc
I have a Spanish moderated .410 with full length mod ie all the way to the action. To me, firing it, it sounds no louder than a springer air rifle. However, I’m told that fro 100 yards away the noise is more of a quiet crack. What I can say is, noise wise, there’s no comparison between it and an in moderated .410 and it’s great fun to shoot.
i have a mossberg pump moderated 410. Ive found gamebore subsonics are very quiet doing about 850 feet per second. eley subsonics are faster at about 950 - 1000 feet per second but a little louder. magnasonics are similar to these. by far the quietest where nsi subsonics but really slow and not suitable for game or pests. With the nsi you can hear the shot hit a cardboard pattern plate at 20 yards over the sound of the gun being fired.
you can happily use any of these cartridges around the beef cattle in buildings on the farm where i shoot. they dont even flinch.
idid find the magnasonics didnt pattern very well but worth trying in yours if you get one
I had a Hushpower .410 years ago, yes it's less noisy than a normal .410, but also due to the ported barrel far less powerful, and lets face it a normal .410 is pretty limited in that respect.
For a while I thought I was just missing, but after shooting a few stationary things, like a can or two, and then one of those chuck away gas cartridges, the old ones with no valve I knew I was hitting it because of the marks in the paint, and that is all it did at 20 yards, at 10 yards small dents, at 5 feet a bigger dent, but not one single pellet pierced it, but by contrast a 25 yard hit with a normal 12 bore ripped it to bits, Hushpower sold on, replaced by a normal .410 and no more problems.
These days I would not look at any shotgun that is 'muffled' by means of barrel porting, same for integral silenced .22LR's.barrel porting make normal hi velocity ammo subsonic, fine it that is what you need.