First of ALL. THERE IS NO SUCH THING AS MOUSE FART QUIET.
Now that is out of the way. You can easily compute the amount of air your rifle uses for each shot. That volume is important when building a moderator.
Let's have an example. Suppose my HW110 gives me 30 shots on a 140cc tank from 200 BAR to 100 BAR. That's all you need to compute the volume of air your rifle uses for each shot. The math looks like this: ((200-100)*140)/30=466.7 cc/Shot. We are calculating the start volume in bar minus the end volume in bar times the tank volume divided by the number of shots. That gives us the volume per shot at ONE BAR.
For that rifle it works out to about 457cc. There are different ways that a moderator works. Some simply allow enough expansion to drop the pressure before venting to the atmosphere. Some use turbulence created by baffle designs to slow down the release of pressure over time. Some use both approaches. If you reduce the volume of a moderator by half you double the working pressure of that moderator. If you double the volume of a moderator you reduce the working pressure by half.
The HW110 primarily uses the first approach (expansion chamber) with a couple of very simple restrictors thrown in to add turbulence. IN THEORY if you reduce the pressure of the shot to ONE BAR before it enters the atmosphere you could actually achieve "mouse fart" quiet. The HW110 moderator is EXCEPTIONALLY good. It will reduce the shot volume to something like about 68 dB at 15 feet from the muzzle. That is just superb. Most moderators are struggling to reduce shot volume below about 78 dB.
Remember the shot volume I calculated above? The moderator on that HW110 is 170mm x 30mm external (6.7" x 1.2") and 162.5mm x 27.5mm internal. So the volume of that moderator is 96.5cc. That means the ratio of volume of the moderator to the shot volume is 1:4.8. The volume of the moderator is only about 1/5 of the volume of the air used for each shot. A lower moderator volume is going to mean MORE noise. Cut that ratio in half and you are going to APPROXIMATELY double the noise power +3dB. Double that ratio and you are going to APPROXIMATELY halve that noise power -3dB. That theory APPROXIMATELY holds for IDENTICAL designs. As you shorten a moderator the distance from the muzzle to the exit port reduces and that adds in additional problems (usually solved by increasing turbulance with more baffles).
Now lets go back to that assertion that an exit pressure of 1 BAR means "mouse fart" quiet. If the moderator to air volume ratio is 1:5 the working pressure inside the moderator never exceeds 5 BAR. It is a "5 BAR" moderator. So a "5 BAR" moderator is capable of 68 dB performance. A "2.5 BAR" moderator should be capable of 65dB performance and a "10 BAR" moderator should be capable of 71dB performance.
What are the dimensions of those two moderators. Well the easy (and best) way to change the moderators volume is by changing length. So the "2.5 BAR" moderator would be about 13.4" x 1.2" and the "10 BAR" moderator would be about 3.35" x 1.2".
The question was essentially what is the biggest moderator I can make without wasting space. The answer IN THEORY is something like a moderator which gives you a 1:1 ratio of moderator volume to shot volume. Well we can calculate that. Lets suppose you have a Condor shooting a 9mm slug for 200 fpe. It does that on a 200 BAR charge and uses 30 BAR on a 500cc cylinder (just guessing here). The moderator that you are wanting to build will have to contain 30x500cc = 15,000cc of shot volume. Lets use a 75mm (~3") tube for our moderator. You calculate the cross section of your tube as PI * (37.5*37.5) and get 14.06 sq. cm, By dividing that into 15,000 we get a length of 1067mm or basically one meter long. That's gonna be real quiet.
OR You can do what snipers are trained to do. Shoot through a loop hole from the back of a room. In other words USE THE BUILDING as the silencer.
I buying some metal working machines , so with the proper tools I might try it again this time would be more enjoyable..
