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The JP Rifles Silent Captured Spring (SCS) system, reviewed here, is one of my favorite AR upgrades. Tuning an AR to perfection is done by adjusting the balance of reciprocating mass with the amount (and timing) of gas impinging on the action. Furthering one’s ability to do this, JP has now released the Variable Mass Operating System, designed for suppressed and un-suppressed rifles alike. JP Rifles’ press release follows…

JP Releases Variable Mass Operating System


Hugo, MN – JP Enterprises introduces their Variable Mass Operating System (VMOS™) enabling mass tunability and improved buffering of AR rifles. Operating systems can be easily tuned for a much wider range of applications such as suppressed rifles or exotic calibers.

“There’s no such thing as “one size fits all” in AR operating systems,” states JP founder John Paul. “Tuning your rifle is the only way to realize its potential and provide the performance you want. That’s why we pioneered the adjustable gas block concept more than two decades ago. Tuning can be the difference between reliable function and reliable frustration.”

Suppressed rifles or exotic calibers can experience excess gas and higher fouling that can easily push AR rifles outside their operational window. Using the same weights as JP’s Silent Captured Springs,
the Variable Mass Operating System (VMOS™) enables mass tunability and improved buffering capacity. The VMOS™ provides additional reciprocating mass that ensures reliable function in even difficult-to-run rifles.

Like JP’s other carriers, the VMOS™ models feature a 100% increase in bearing surface for smoother operation, improved alignment and lower wear. The longer cocking pad also contributes to optimal reliability. Exterior bearing surfaces are polished before the carrier then undergoes a state-of-the-art, multi-vendor QPQ (Quench-Polish-Quench) process. The final result is a rock-hard surface with remarkable natural lubricity for a ball bearing smooth function with proper lubrication.

Read more on the advantages of the VMOS™, especially for suppressed rifles by clicking *here*

The VMOS™ is currently available for large frame (.308) AR-type rifles starting at $319.95. Expect to see a small frame (.223) version early in 2017.

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    • Yes, and there are both steel and tungsten weights to adjust from fairly light to really heavy as needed.

      I’m guessing that they also slide slightly, adding a dead blow effect like the SCS does. This can eliminate bolt bounce.

  1. Yes. Looking at the product on their website, the weights can be purchased and swapped around as you see fit.

  2. Jeremy, just curious –

    Are there any AR platform bolts that allow being ‘locked’ closed to eliminate bolt noise entirely?

    (Yeah, I’m aware that means the bolt will need to be cycled manually…)

    • With an adjustable gas block, most can shut off the gas completely giving you your desired result.

      • Yeah, I only know of doing this through an adjustable gas block (not all have an “off” setting, but most do) or a malfunction 😉

  3. Silent Spring system is awesome, like the idea of the mass system.

    Why not NP3 or Nickel Boron coating?

  4. Ugh. “Tuning” Stoner’s puke-in-mouth-piece-of-scheisse-in-varmint-caliber means less that jack-effen-sh1t to anyone who actually knows guns.

    Look kids, if you ever actually need one of these turds for the ammo availability, complete with giggle switch, they’ll be someone located nearby who isn’t gonna need theirs anymore. Why would you ever own one other than because you can? (Which is a completely valid reason, but still you don’t get select fire, so you’ll be upgrading as soon as you see that first donor. So really, why bother? )

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