Have measured wall thickness? I ask as you described "massive back bore," For safety sake you should verify if you have not already.
Now to your original question, low noise level. Sound, like recoil is both measureable and relative. Most of us perceive both recoil and sound from a gun differently when hunting a live bird, than shooting at a fixed target. Perception may be part of the reduced sound or different sound signature.
Further I have never fired federal featherlights and cannot comment on them specifically; I have fired gamebore shells and they definitely produce a totally different sound that anything else I have used, especially the 20 gauge cartridges.
Ultimately it may fall into simply the cartridges you are shooting are quiet.
I find that if something is working for me, I try not to overthink it too much about it.
I once discovered after patterning a gun I had always shot very well running an average of less than two shells per bird, my shooting dropped off the cliff. I had patterned it and discovered it was very tightly choked compared to what I thought was wrong. Somethings are mental and I outsmarted myself.
