Airgun Moderator Design, Performance, and Development - Moderator Velocity Loss (part 18)

Catch the previous episode HERE.

So this is a weird subject which seems to float around the forums and other social media and whatnot, the claim that airgun moderators reduce pellet velocity. It is particularly strange to me, given my firearm suppressor background, the expectation is typically a velocity increase not loss. In the firearms community the phenomenon is called “freebore boost.” It is usually fairly minimal, but readily measurable. So why would this phenomenon be reversed in airguns? And does it even exist at all?

While having heard about airgun moderators reducing velocity over and over again, I confess I’ve never actually seen a shred of data on the subject. I’d always written the topic off without directly testing it, because having now mounted many experimental super-light moderators to my rifles, I’d never seen any POI shift up or down to suggest a velocity delta. Clearly though heavier moderators can and do create downward POI shifts by flexing barrels ever so slightly. I’m also reluctant to totally discount “common knowledge,” because it tends to at least be rooted in something.

Curiosity therefore, finally, got the better of me and I decided to do a quick test. This is using a prochrono with lightbars producing good consistent readings. The rifle is my FX Crown .22. I want to preface this with a disclaimer though: this is not a test of all rifles, moderators, possibilities, or mechanisms. On the chopping block are a handful of moderators I had kicking around. The DonnyFX in .22 is meant to represent the “typical” architecture found in many moderators of two effectively “flat” baffles surrounded by sound damping material. While the Donny’s monocore design means it isn’t identical, I figured if it turned up anything interesting I could start building test models and revisit the subject. The Ramus Trident is a very unusual, perhaps unique, design what with its venturi and spiral “baffle” architecture. As I had them laying around I also thought I’d run a Falx, Mus, and Brevitas to try and cover the range of small-to-large with gas diode design. I took five shots with every moderator. Knowing that some guns need to “warm up” or have an errant first shot if left sitting for some time, which my Crown had done, I took 5 shots and recorded them. Incidentally a Pilum had been sitting on my muzzle, so remained there for the warmup. Pellets were unsorted, and went into the magazine dinged skirts and all. And one final thing to note. Regulators are imperfect, and will start to drift as the regulated pressure approaches the tank pressure. Not wanting this to affect the data, the reg on this gun being set for approximately 130PSI, I refilled the rifle at ~150BAR shooting before-and-after baselines with bare muzzle. There was no significant change, which is good.

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Well first off, based on the data, I would speculate this Crown shoots low for the single first shot, possibly the first two although the second shot is within the range of “normal” especially accounting for a potential dinged skirt or what have you. But the real question of course is: what is the credibility of claims that moderators affect muzzle velocity? Well, on the face of it, all of the averages are within the standard deviation of the overall average, so any change would be quite small. From a shooter’s perspective, at least on this rifle, you’re very unlikely to notice it. It is possible that, with a large enough sample size, a statistically significant difference would be found. Technically speaking the lowest average was with a moderator (the DonnyFX) and highest with a bare muzzle, but that smacks more of anecdote than evidence. First off, it isn’t what you’d call improbable it’d have happened at random (just over a 1 in 5 chance unless I’m mistaken). Secondly, you’re still talking about a mere 5.2FPS gap.

Obviously this test isn’t comprehensive. Different rifles may perform entirely differently, and so too may different moderators. So I’m not trying to stamp “myth busted” on this just yet. What I aim to do however, based on the above data, is cast a somewhat more critical light on the claim that airgun moderators reduce velocity. If they do, clearly the effect isn’t universally significant. Presented with the claim that moderators reduce muzzle velocity, I would request a data set or a proposed mechanism of function. I also want to add that, if anyone reading this is sitting on said dataset or known mechanism, please get in touch. I’d love to update this blog with that new information, and generally further knowledge on the subject.

Anyway thanks everyone for reading. I hope this quick bite of info was informative and interesting. :)