Air
Home to the world's quietest airgun moderators, plus precision pellet caddies, self-healing targets, chassis parts, and shooting accessories, all designed and manufactured by Silent Thunder Ordnance. From the Accipiter and Sarissa lines to the Pocket Benchrest and Eyeshine Targets, this is the full catalog of STO air products built for performance and tested before they ship.
Mus II
Mus II
The Mus II is our large diameter moderator, carrying a dimensional envelope of 50x170mm and a weight of approximately 226 grams. It is the successor to one of our most popular early moderators, the original Mus, which was among our quietest designs at the time but used a carbon tube construction that couldn't be upgraded or serviced. When we phased out the carbon line in favor of our modular aluminum architecture, people wanted the form factor back. The Mus II is the answer to that, brought back with substantially upgraded internals and all the benefits of our current platform.
The larger 50mm diameter isn't just about aesthetics. At the extreme end of the airflow spectrum, cross-sectional area becomes a meaningful constraint. The additional volume gives us more space to work with at power levels where the Falx begins to run into its limits, and that space has been put to use with entirely new diode and damping geometry developed and validated specifically for these conditions.
The design uses Nikola Tesla's Gas Diode to reflex propellant air back on itself, resisting forward flow through geometry alone with no moving parts. This three-stage approach first strips air off the pellet to reduce turbulence and improve accuracy, then delays it through compound gas diode stages, and finally passes it through a high-performance damping section to absorb residual sound energy. The design adapts along its length rather than repeating a single approach end to end, applying the right technique at the right point in the air's journey through the moderator.
The anodized aluminum tube is serviceable and modular. Cores can be swapped as the technology advances, meaning a Mus II purchased today can be upgraded rather than replaced as new revisions become available.
Key Features:
Tesla Gas Diode design: Compound diode stages reflex and delay propellant air through geometry alone, no moving parts
Adaptive staged architecture: Design transitions along its length to maximize attenuation at each point in the air's travel
Tapered bore: Improves sound attenuation while reducing the likelihood of pellet clipping
50mm diameter envelope: Additional cross-sectional volume enables diode and damping geometry not possible at 40mm for extreme airflow applications
Modular serviceable construction: Anodized aluminum tube allows core replacement as technology advances
Big bore cores available: Expanded bore options accommodate larger caliber rifles
Built and tested in the USA
Select the core and bore configuration using the dropdowns below. Information on calculating your rifle's flow factor can be found in our Moderator Configuration & Optimization guide.
This product is intended for airgun use only. Use of this product in any manner inconsistent with its design intent is a violation of federal law. More information about the federal court ruling which clarified the legality of airgun moderators in the United States can be found here. Local restrictions may still apply. It is the responsibility of the buyer to ensure compliance with all applicable laws.
This item ships within the United States only.
Selecting the Right Variant
Matching your moderator to your rifle's flow factor is the single most important step in getting the best performance out of a Mus II. Our Moderator Configuration & Optimization guide walks through how to calculate your flow factor and what to do with that number. The Mus II is designed for rifles at the extreme end of the airflow spectrum, specifically 160 FPE or greater with a shroud, or 120 FPE or greater without. This correlates to flow factors in the 5,000+ range. The reference hosts for our highest flow tuning points are:
Standard Flow: Tuned on an FX Crown .22, 32 ft-lbs. Factory config w/ extended shroud, acoustic peak 288.0. Flow factor 460.
Moderate Flow: Tuned on an FX Crown .30, 80 ft-lbs. Factory config w/ extended shroud, acoustic peak 637.6. Flow factor 1,111.
High Flow: Tuned on an FX Dreamline .30, 87 ft-lbs. Bare muzzle, acoustic peak 3,668.4. Flow factor 6,500.
Extra High Flow: Tuned on an AEA Challenger .35, 150 ft-lbs. Bare muzzle, acoustic peak 5,255. Flow factor 16,333.
If your rifle falls significantly outside this range, get in touch before ordering.
Sound Profile Data
A note on reading these traces: peak alone doesn't tell the full story. Post-peak sound character has a substantial effect on perceived loudness, often more than the peak value itself. Each trace is provided so you can evaluate the full sound profile rather than a single number. Note that traces are not shown to the same scale.
Mus II Extra High Flow Rev.3, as tested on an unshrouded AEA Challenger Pro LB .35. Average peak 366.5.
Rev.3 represents a complete re-engineering of our damping system, combining the diode advances of previous versions into a more integrated structure while dramatically increasing closed damping material surface area and more precisely tuning open damping material flow resistivity and impedance. At extra high flow conditions the Mus II's larger diameter pays clear dividends: at the same overall length as the Falx, it delivers performance at power levels the Falx cannot adequately address, bringing a genuinely demanding test host down to a manageable sound signature.
