JP Enterprises Release VMOS – Variable Mass Operating System

JP Enterprises, known for the LMOS (Low Mass Operating System) and SCS (Silent Capture Spring) has announced the fusion of the two systems into the VMOS (Variable Mass Operating System). In short, the VMOS is a bolt-carrier group that combines the basic shape of the LMOS with the adjustable weight system of the LMOS on a single bolt carrier.

The VMOS is initially released for large frame (LR-308 / DPMS “G1”) pattern firearms. The small frame (AR-15) is expected to launch early 2017. Retail for the large frame is set at $319.95 with the small frame expected to be slightly less.

I, for one, welcome the large frame AR release first. Simply put, .308 weapons are far more tempermental in their function than their intermediate cartridge cousins. While adjustable gas blocks are a solid solution to the problem, often adding mass to the large calibers will help with function as it increases the dwell time.

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.”

The VMOS BCG adds in a few additional welcome features. The bolt “rails” are both polished and increased in size and the bolt carrier itself is QPQ nitrocarburized for corrosion, wear, and lubricity. Unfortunately, forward assist functionality is limited to the forward-most part of travel and looks to be compatible only with JP’s receivers.



Nathan S

One of TFB’s resident Jarheads, Nathan now works within the firearms industry. A consecutive Marine rifle and pistol expert, he enjoys local 3-gun, NFA, gunsmithing, MSR’s, & high-speed gear. Nathan has traveled to over 30 countries working with US DoD & foreign MoDs.

The above post is my opinion and does not reflect the views of any company or organization.


  • Anonymoose

    Wait, so it won’t work in a DPMS G2? G1 is the mass-market generic standard, isn’t it?

  • MIKE


    • VanDiemensLand


      • MIKE


        • VanDiemensLand

          Hahaha, touche

          • valorius

            LMAO, he owned ya there Van. 😉

          • VanDiemensLand

            True dat 🙂

    • Ron

      Dwell time is the time between firing and unlocking of the bolt, the projectile is not effected.

    • mig1nc

      Reciprocating mass can affect lock time, but not dwell time.

      Dwell time is usually referred to as the time the operating system mechanism is under maximum pressure, meaning in an AR15 the time between when the bullet passes the gas port and the time the bullet exits (or “uncorks”) the muzzle. That’s why you see people say a 10.5″ SBR has less dwell time than an 11.5″ SBR both with carbine length gas systems. The bullet exits sooner on the 10.5 thus having less dwell time.

      Lock time in this context has to do with when the operating mechanism is acted on and when the system unlocks, in an AR15 when the bolt has been rotated by the carrier and unlocked from the locking lugs.

      The whole thing has to be timed so that there is enough pressuring during dwell time to unlock the bolt but not before the pressure in the barrel has dropped to safe levels.

      • Sledgecrowbar

        Dwell time is between the bullet passing the gas block and exiting the muzzle, it’s the total time that the gas is acting on the bolt carrier group, and it’s normally referred to when talking about a rifle being undergassed or overgassed. Weight of the BCG has an effect on dwell time because it means the same amount of gas has to accelerate a greater mass for the action to cycle.

        Reducing the gas to the BCG with an adjustable gas block also changes bolt speed, but it does this without changing dwell time, which results in a smoother cycling gun, provided it’s not reduced so far that the gun doesn’t cycle reliably when dirty.

        Lock time is the time between the trigger sear releasing the hammer and the hammer striking the firing pin. A shorter lock time means less movement of the gun during that moment and helps accuracy. The main thing that affects lock time is a lightweight hammer, but this negatively affects primer ignition in military ammunition because the primers are stiffer for safety so you want the hammer to have more inertia to hit the firing pin harder and ensure the primer is strongly pushed into the anvil.

        • mig1nc

          I like your post, this is a good discussion.

          “Dwell time is between the bullet passing the gas block and exiting the muzzle” Indeed, that’s basically what I said when I said “meaning in an AR15 the time between when the bullet passes the gas port and the time the bullet exits (or “uncorks”) the muzzle”.

          I’d suggest though that I think you stated it backwards in regards to carrier mass and dwell time. There is no physical way that the BCG can have any impact on dwell time because it isn’t holding the bullet back from getting out past the muzzle after it passes the gas port.

          Operating system mass is dependent on dwell time as it relates to gas flow to function properly. Overgassed/undergassed has to do with how much gas flow you have during dwell time. A larger gas port can allow for more gas volume to flow during dwell time which might be reduced due to a shorter barrel in front of the gas port, which is why shorter barreled guns have larger gas ports. It allows them to compensate for less dwell time by increasing the volume available during the time window.

          I agree about lock time, I’m aware of that term being used in the trigger context as well. My mind is failing me, I’ll have to see if there is a better term to describe the time it takes to unlock the action in a self-loading firearm.

          • Sledgecrowbar

            I don’t know if there’s a term either but I get what you’re saying, the time between the bullet passing the gas port and the bolt completing it’s rotation to unlock is what is affected by BCG mass. I’d rather adjust other things and have a lighter BCG but a heavier BCG increases reliability in extraction and feeding so you have to find a sweet spot so to speak.

            Having more gas means the gun will operate reliably over more conditions, including using cheaper ammo that tends to be underpowered, but the less gas you can get away with, the nicer a gun is to shoot. Since you can’t adjust actual gas pressure other than moving the gas port forward where it’s slightly lower, the next best thing is to reduce the orifice it travels through with an adjustable gas block, which is also nice because you can re-adjust it easily on the fly. I think having as much dwell time as possible is good in this case because when you reduce the orifice size, you need the pressure to be acting over a longer period to ensure full cycling of the BCG.

            That said, I see that 18″ AR barrels with rifle-length gas systems are becoming popular with recreational shooting (as opposed to combat) because they cycle very softly. I’m not sure if it takes much work to get them to run well, or if they need a comparatively very large gas port like a true dissipator barrel (a rifle length gas port in a 16″ barrel, which sounds ridiculous to me). I think there is a lot of extra gas built into the tradtional AR design for reliability so experimenting with different barrel lengths using different gas systems and adjustable gas blocks (since changing gas port sizes is much more difficult) is something I want to do.

  • VanDiemensLand


  • Kyle

    I’m pretty sure the forward assist cuts on their carriers are in the same spot as the DPMS ones. I know DPMS carriers function in the JP uppers with the forward assist without any issues. Also, you should really only ever need the last little bit of forward travel, if you’re needing to use the forward assist before that then you’re likely doing something wrong.

  • Steve

    Gas guns and suppressors – it takes some work to get those two to function properly…

    …by ‘some work’ you mean an H2 buffer and having a gas port that wasn’t drilled improperly?

    • mig1nc

      I agree. IMHO there was quite a bit of FUD in that video. Bolt velocity and bore blow back are problems with all guns, not just DI ARs. Also, why would he say that you need to adjust mass instead of adjusting gas flow for cans? That’s just ridiculous. Doing both is the best answer. Usually just throwing in a heavier buffer and not being over-gassed to begin with is good enough.

  • Don Mei

    I’ve been doing essentially this for years. And for free. By removing or adding mass to the buffer you accomplish exactly the same thing.

    I have a semi-auto gun that cycles like lightning. I have removed all mass from the buffer and am using a relatively light and cheap semi auto bolt carrier. Is there bolt bounce? Probably But this is a competition gun and I’ve never had a problem.

    The bottom line is you can remove mass from the reciprocating assembly in an AR and get all the benefits of the JP low mass or variable mass operating system, for little to no money.

    Note that you do need to combine this with an adjustable gas block since the lighter reciprocating mass will be over-gassed with a “normal” amount of gas. And that’s the whole point. Put less energy into the reciprocating mass to make for faster follow up shots.


    • Jack

      Nothing’s free. You’ve put your time in to mod parts that you’ve already have on hand or money in to buy different parts. It’s easy to pull weights out of a buffer but that’s only a fix for the low end. If you’re taking about any sort machining to further reduce weight from a carrier, not everyone has the time/equipment to do that. An adjustable gas block will help with a lot of the issues with a can but it’s nice to have options and different avenues for tuning.

      I’ve got a couple of buddies that will make their own AR Doo dad in their shop to save $35 retail because they can do it for “free” themselves. BUT they’re out the material, wear and tear on their equipment and most importantly their time. The several hours on the mill/lathe aren’t worth the perceived savings… for me anyway. Ymmv

      • AD

        I fully agree that it’s often worth spending money to save time.

        As far as your buddies go, perhaps they’re enjoying themselves, and maybe they like owning/using things that they’ve made themselves. I know I do.

        • Wow!

          I build things myself to improve my skills and as you get good, doing something like turning a new buffer takes less than 15 minutes. No way you can beat that kind of speed and cost savings.

          Plus by DIY you can make new stuff not even on the market. Like I made my own AS VAL using custom reloading dies to form 7.62×39 into 9×39 running cast bullets and Custom AK parts (basically an AK blackout). I also made an IWI Negev semi auto clone using pictures of it online. None of them are “exact” to the original, but functionally and nearly in appearance, they are the same.

  • Wolfgar

    Another great innovation from JP, looks good.

  • valorius

    And i was just thinking, “Gee, i wish someone would come out with a $300 plus dollar BCG for an AR.”

    OK…i’ve never thought that.

  • noob

    oh come on, they almost had the acronym VAMOOSE. or even just VAMOS.