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General | Running Suppressed

JP Variable Mass Operating System

There's no such thing as "one size fits all" in AR operating systems. Tuning your rifle is the only way to realize its potential and provide the performance you want. In some rifles, a little tuning is the difference between reliable function and reliable frustration. JP offers more options in this department than any other manufacturer, because we have really thought it through and done the research.

Using the same weights as our Silent Captured Springs, the Variable Mass Operating System (VMOS™) enables mass tunability and improved buffering of your operating system for a much wider range of applications such as suppressed rifles or exotic caliber chamberings. Excess gas and higher fouling can easily push these rifles outside their operational window. The VMOS™ compensates for all factors, providing additional, reciprocating mass that ensures reliable function. Particularly in systems without an adjustable gas block, this level of control feels like liberation.

Like our other carriers, the VMOS™ models feature a 100% increase in bearing surface for smoother operation, improved alignment and lower wear. The longer cocking pad of our designs also contributes to optimal reliability. Exterior bearing surfaces are polished before the carrier then undergoes our 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™ for suppressed rifles by clicking here.

Variable Mass Operating System
JPBC-7JP VMOS™ Bolt Carrier
   Large-Frame (.308)
   Stainless Steel
   QPQ Black Finish
   Standard Version
JPBC-7H2JP VMOS™ Bolt Carrier
   Large-Frame (.308)
   Stainless Steel
   QPQ Black Finish
   Heavy Version
JPSCS-SMSteel Weight for Silent Captured Spring/VMOS™ Carrier$7.95
JPSCS-HMTungsten Weight for Silent Captured Spring/VMOS™ Carrier$24.95

Running Suppressed with the VMOS™

The VMOS™ variable mass, integrally buffered carrier is a game-changer for suppressed semi-autos.

The popularity of suppressors is red hot. Still, there is a lot of frustration about running detachable suppressors versus running a full-time suppressed platform. Which is easier to manage? Running detachable suppressors on gas guns generates a laundry list of difficult-to-remedy problems. The VMOS™ is the solution. For AR owners using a detachable suppressor, this carrier sooths the two main pain points.

1. Constant Adjustment

Controlling bolt velocity through mass regulation is possible thanks to the VMOS™'s dramatically superior buffering capacity. The VMOS™ carrier in its heavy configuration in combination with the heavy Silent Captured Spring (JPSCS-10H2) allows the rifle to run reliably—can on or can off. It also puts much less stress on the extractor, bolt and cam pin, all without having to adjust the gas system to regulate bolt velocity. And that's the bottom line here: no putzing with your rifle when running the two extremes.

Suppressors generate much higher port pressure just like a muffler on a car generates backpressure in the exhaust system. The engine must be tuned to run with or without the muffler in place to perform properly. The same scenario applies to the gas-operated rifles. From the factory, most rifles are set up to run without a suppressor. On these rifles, the suppressor will raise the port pressure and thereby the bolt velocity typically beyond the operational range. This knocks the rifle out of its "sweet spot." Keeping the bolt velocity in its sweet spot is the key to reliability, durability and improved sight recovery.

The VMOS™ dramatically increasing the overall mass of the operating system. In suppressed applications, this enables it to store a far greater range of kinetic energy imparted to the operating system by variable port pressures. The result is more constant bolt velocity and more reliable function.

2. Excessive Fouling

Suppressor use on gas guns creates a major fouling problem. On a bolt gun, the suppressor is able to completely vent and depressurize before the action is opened. A semi-auto platform, on the other hand, is already extracting the case while the suppressor still retains a high level of residual pressure. This gas vents forward but is also back down the barrel. This fouls the chamber around the extracting cartridge and the inside of the receiver itself. Thus, the suppressor drastically shortens the cleaning cycle on a gas gun.

Depending on the fouling characteristics of the ammunition, the rifle can quickly become inoperable. This manifests in failure to fire due to the inherent bolt bounce effect on all AR-type rifles. When the receiver and action become fouled to a certain point, the operating system can no longer overcome bolt bounce and press the system into battery. The doubling of buffer capacity offered by the VMOS™ adds twice the dead blow effect to the operating system. This will overcome significantly more fouling and bring cleaning intervals back to near unsuppressed levels. In fact, high-speed video shows that the BC-7 eliminates bolt bounce all together.

Every advantage comes at a price, and suppressors are no different. Your can will cost you functional duration (though the price tag and tax stamp are nothing to scoff at either, but that's neither here nor there). This VMOS™ carrier and Silent Captured Spring is your hefty discount to that price.


What parts do I need for the Low Mass Operating System?

Our low mass bolt carriers do not require that you also use our LMOS buffer (JPLMB) to function, and a low mass buffer can be used with a full mass carrier. Both reduce the mass of the reciprocating parts in the rifle, and the effects are essentially accumulative. Each one has some effect by itself while using both has more effect.

Both a low mass carrier and buffer are drop-in parts for a standard AR, i.e., neither part typically requires an adjustable gas system or any other special parts to function. The JPSCS is also a low mass part and is actually lighter than the LMOS buffer.

The gas system setting adjustments required for changing between LMOS and FMOS are so small that many people never bother to make any adjustments when switching carriers between rifles. If you are running the gas system at the absolute minimum setting for a particular load with no margin for error, then you may have to readjust your gas system. Otherwise, you should be able to just swap the carrier.

Why is my aluminum carrier (JPBC-1) showing wear?

The JPBC-1 is aluminum with hardcoat anodizing, which wears faster than the QPQ coating of our steel carriers. Because of this, it will not take long for you to notice cosmetic impact wear in the anodizing on both the front corners and cocking pad of the carrier. This is normal break-in wear. It does not indicate any current or future functional damage and is not a cause for concern.

The cocking pad wear can be minimized by using a JP Speed Hammer or other hammer with a radiused leading edge. Hammers with a sharp corner, such as a Mil-spec hammer, will create the most wear on the cocking pad of the carrier.