Moderators really do work- not completely silent but not far off.
Without a moderator the action will be a significant influence - unmoderated PCP and CO2 are loud.
Power also comes into the equation so a lower power out put will mean generally a lower report.
I think you'll find that out of all unmoderated actions a ssp like a daisy 717 / 747 would be a sensible choice.
If you were going to fit a silencer then almost anything will come down to a low report, but you cant fit a silencer easily to all pistols - so choose carefully.
Out of all the moderators you wont go far wrong with a Weihrauch.
If you are practising indoors then you may find you will also be tasked in constructing a silent back stop