Airgun Moderator Design, Performance, and Development - The Target (part 17)

Catch the previous episode HERE.

What with everyone cooped up deaing with the Kung Flu, and more than a few wives nagging about the shooting sounds, I got to thinking about the other end of the range: the target. It is all well and good if you’re running a state-of-the-art moderator to muzzle that beast, but if the target impact is louder it isn’t doing you much good now is it?

For a test protocol, I went with the same setup as is used for moderators, 1.6 meters above the ground 1 meter from the pickup with the target perpendicular, outdoors with no nearby sound reflective surfaces. What had to change, obviously, is the shooting position. I ran my FX Crown .22 shooting JSB 18.13 grain pellets at 880fps, approximately 35 meters away from the target and partially occluding the pickup by a slight rise. The rifle with moderator that day averaged approximately 50, which is quite good. Suffice to say I have a high degree of confidence with these that the target impact is relatively clean here, near zero muzzle noise. Also please note these traces are not all on the same scale. The peaks of some are so high, and others so low, I adjusted them to better fit as I commonly do.

First up is steel. Average peak was 690. It was not quiet. The target itself was a 5/16” thick by 9” circle of AR500 hung on a shepherd’s hook. You can clearly see the impact and subsequent ringing. But most people who are trying to be quiet aren’t shooting steel. So how about a more common target, paper?

So what you’re seeing here is the impact of the pellet on a paper (8.5x11 printer paper with stick-on targets) with duct-seal backer in a wood box. The paper was retained only by slots into which it can freely slide in and out from the top, a typical convenient target design. Notice the large negative pressure spike, presumably caused by the momentum of the paper drawing it inward, toward the duct seal. Average peak was about 175. It was also surprisingly not quiet. It sounded much like a large piece of paper being shaken violently. Not surprising really, as that paper surface somewhat like a drum face, so can actually create an impressive amount of sound. Something which occurs to me, that may be completely idiotic or may be a not-horrible-idea, is that smaller pieces of paper would necessarily have less surface area. Might this reduce the sound signature of a paper impact? Just something to consider. In a way I was surprised by how loud this was, and in a way I wasn’t. Shooting paper indoors, it is unpleasantly loud if you’re close to it. The brevity of the sound impulse though makes it sound quieter than it actually is.

So I got curious, removed the paper, and shot just the duct seal. This one was a weird kind of mess. This is a rare case where I basically sampled until I got something I liked the look of. Why? Well, for whatever reason, the results were coming out all over the board. The “average” peak was 132, but I saw one spike as high as 336, and another where the peak wasn’t initial impact. WTF? My guess is two things were happening here. First is, without paper, the sound from the impact with the duct seal can more easily escape. That makes sense. The other though is that I may be impacting pellets in the duct seal, and that may be causing very brief high sound signatures. It is weird for sure. To the subjective ear though, these impacts were very quiet. If you were looking for a way to shoot without the wife or neighbors noticing, this might be the way to go. Definitely sounds a lot better than the paper. Interestingly though, still technically louder than most moderators. This gets into why sustained sounds are perceived as louder than impulses. (moderators produce a peak, but also more sustained noise than this)

While I was at it though, I was wondering about the “sound floor,” that is to say just how quiet you could possibly make an airgun, hypothetically speaking. This is defined by the sound the projectile makes as it tears through the air not too far off from the speed of sound. The answer, as it turns out, is about 7.6. A chrono would have been a sound reflective surface that would have invalidated the test, however it would have been nice to know the pellet velocity. Approximating using a ballistic calculator, this pellet was likely traveling between 750 and 800FPS when it passed through the air, 1 meter away from the pickup.

I should add that I don’t have the world’s highest confidence in this, it is just a guess really. Air movement can be a major factor at this low sound level. Additionally, it is hard to know for sure what you’re measuring. Just because the timing kinda works and the results are consistent doesn’t mean they’re consistently what you think. Laying safely below the rise, listening to pellets zip over your head, you can clearly hear the difference between the muzzle puff and pellet zip, but what exactly is it you’re capturing? No way to know with certainty unless I were to engage in a LOT more testing. And as the breeze was picking up, I decided simply to pack it in.

So I hope all this was interesting, if not useful. It is an important lesson on the relative silence of targets though. It makes me think about ways to make quieter targets though. The thing about a target, unlike a rifle, is that it doesn’t need to be particularly compact, light weight, or aesthetically pleasing. Nobody huffs a target around hunting or posts beauty shots of their carefully tuned box. There are simply fewer constraints. This means a wide variety of sound containing and deadening devices could easily be applied. So too could devices to create destructive interference. If this is an area of inquiry which interests people, feel free to reach out. If enough people are looking for such a thing, I’ll happily chug away at methods to limit a target’s sound signature. Developing a “silent target” could be an interesting challenge in its own right, and, of course, the whole project would be posted here. The real question is: does anyone care?