Airgun Moderator Design, Performance, and Development - THE BIG TEST; MODERATE FLOW

A goal of the “big test” series is to provide an ongoing reference to the performance of different moderators at different flow levels. Moderators will be added and charts will be updated periodically however the majority of the text will not be. If you have a moderator you’re curious about, please feel free to get in touch, we’re happy to test it and add an entry below so everyone can benefit. The test series are linked below:

Standard flow - FX Crown .22 shrouded producing 32 foot pounds. Flow Factor 460

Moderate flow - FX Crown .30 shrouded producing 80 foot pounds. Flow Factor 1,111

High flow - FX Dreamline .30 unshrouded producing 87 foot pounds. Flow Factor 6,500

An immense thank you to everyone who has loaned a moderator for testing purposes here. This project would not be possible without you, and with your help will keep growing. So, again, thank you!

The test protocol

The muzzle of the moderator is exactly parallel to the pickup, and separated by exactly one meter. The milspec protocol for firearm suppressors is 1.6 meters off the ground, and that is replicated here except when there is snow on the ground, in which case the pickups are placed 1.6m above the surface of the snow. Two pickups are used, next to each other, and equidistant from the moderator. Each test was five shots. The results are an average of that, so ten datapoints. The numbers are raw output, and have not been converted to decibels. I hand select each sound profile (image) which I feel is representative of the moderator's overall performance. The host gun is as stated above. All tests were done with the shroud fully retracted, excluding the shroud-only test.

FX Crown .30 shrouded Factory configuration w/ extended
Sound - 637.6

Its loud, albeit the peak is over quickly and remaining sound substantially lower although still surprisingly loud. At one point in time anyway FX considered this quiet enough to stand on its own. This puts me in mind of RMAC 2018 incident where Giles was woken up by a similar setup through a wall/window. I would rate this as not backyard friendly. As an aside, since it has come up before, all the moderator tests were performed with the shroud fully retracted, this is the only shroud-extended instance.

Clague
Sound – 136.3
Mass (grams) – 325
Volume (mm^3) – 493,214
Length (mm) – 276
Diameter (mm) – 47.7
Exit OD (mm) – 9.6

You’ll note that, over a year ago, I tested this same Clague on this same gun for an average of 115.6. How did it gain 15 points? I do a quick cross-check on the rifle’s sound signature before running these tests to ensure reasonably consistent performance, and while a little variation from day to day is expected, this is outside that typical range. I do have an explanation though: designs like this which set up oscillations are quite inconsistent, which isn’t obvious from the average. Run a test where several of the shots peak high against one where several of the shots peak low and your averages will come out a bit different. What remains is the 80’s laser-blast sound that accompanies this moderator/host combination.

Donny FX
Sound – 207.3
Mass (grams) – 135.5
Volume (mm^3) – 118,941
Length (mm) – 126.5
Diameter (mm) – 34.6
Exit OD (mm) – 8.04

To the subjective ear, there is definitely a bit of a puff/whoosh going on here. Again an internal oscillation is set up such that the peak is not from uncorking, and this contributes to the moderator sounding louder than it actually is.

Donny Emperor
Sound – 100.0
Mass (grams) – 438.2
Volume (mm^3) – 536,804
Length (mm) – 266
Diameter (mm) – 50.69
Exit OD (mm) – 9.5

There is a real perception of weight on the end of the gun with this moderator, and the barrel droop (thanks in part to FX’s less-than-ideally-rigid barrel system) is noticeable even off-handing the rifle into a large backstop while doing sound tests. Aside from that though, it sounds relatively quiet. Some combination of pop and hiss. In general moderators tend to sound louder to someone parallel to the muzzle than to the shooter, however for whatever reason the Emperor sticks in my mind as a design which enhances this effect. (note all tests are taken 1m away from the muzzle and parallel to it, 1.6m off the ground)

---

I want to put a break here between the commercial moderators tested and my experimental moderator designs. The obvious “conflict of interest” aside, I can’t overstate the value of having both the host and test equipment on hand when working on designs. For every success or noteworthy design that ends up in the blog series, I have many MANY failures. Posting only the culmination of my work as a direct comparison is somewhat unfair, hence the break. So please keep that context in mind.

Falx Moderate Flow
Sound – 89.4
Mass (grams) – 180
Volume (mm^3) – 213,628
Length (mm) – 170
Diameter (mm) – 40
Exit OD (mm) – 8.4

This is about the right size for this flow factor with this gas diode design in my opinion, if size is a concern and ultimate sound atenuation is not the goal. More length would obviously offer space for further improvement, but after experiments with the Mus, Gladius, and now Sarissa I believe the lion’s share of the peak-reduction has been had in the Falx. The sound is more of a quick puff, it doesn’t have much “pop” to it.

Sarissa Moderate Flow
Sound – 73.3
Mass (grams) – 260
Volume (mm^3) – 320,442
Length (mm) – 255
Diameter (mm) – 40
Exit OD (mm) – 10*

*Note this moderator runs a tapered bore, which starts at 8.4mm at the rear and opens up to 10mm at the exit to reduce the chances of clipping.

But for the hiss of the pellet flying through the air, you could confuse this with a 30FPE shrouded .22. As self-aggrandizing as it sounds, that is the best description of this moderator’s sound profile I can offer. And the data supports this. About a year ago, in what is now called the big test for standard flow, this is what a good performance looked like from a conventional moderator design mounted to the FX Crown .22 both in peak and the trace.

Brevitas *tested 5.7.20*
Sound – 609.3
Mass (grams) – 30
Volume (mm^3) – 45,565
Length (mm) – 74
Diameter (mm) – 28
Exit OD (mm) – 9.0

This is the loudest of my moderator designs. It is not quiet at this flow factor, no two ways about it. The peak is comparatively high, and the absence of a devoted sound damping section results in sustained sound which also contributes to the perception of loudness. So whats the point?

All moderator designs though are necessarily a balancing act between size/weight and sound attenuation. This design was an experiment to try and find the smallest practical and effective moderator possible. The diameter is limited by the shroud, smaller than the Crown’s 28mm shroud would look goofy, so from there it was a question of how short it could be made. While shorter versions were tested, the ultimate conclusion was that 74mm was necessary without unreasonable sacrifices to acoustic performance. To put it another way, while not quiet, this design is quieter than the naked host with its shroud extended (factory “quiet” configuration) while adding about half the length.

That isn’t the only thing experimental about this design though. While all my moderators are experimental, this one takes it to a new extreme by using experimental materials and manufacturing technologies, as well as experimental designs. This is what, ultimately, allows its unusual combination of performance attributes.