JP Rifles Variable Mass Operating System – Review

I’ve recently been playing with the JP VMOS, ordinarily for competition use I’d prefer as little reciprocating weight in my AR as needed.  However, when using the large frame AR, the AR10 or SR25 platform.  There is a point where it would be nice to be able to have less issues with flattened primers or pierced primers, simply because you cannot get your gas system to use less pressure and still get reliable functioning.  If you’re not running an adjustable gas system this is another way of solving problems as well.  Sure you can send your rifle off to a good ‘smith and have it tuned to work with your primary load.  That’s not what the majority of AR owners get into the platform for, the AR is the perfect tinkerers weapon.  Most everything can be done by the owner, it is pretty much a self sufficient platform.  You are also stuck having to rely on the load the ‘smith tuned your rifle for.  

The weights can be added or subtracted as well as vary in weight.

This is from the JP website,……..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

The VMOS allows me to have more leeway with what is or isn’t an overly pressured load.  This item seems to be perfect for the person who may not have the benefit of using the same exact load in their rifle every time.  If you’re buying your ammo and switch brands or bullet weights based on what is available, this could help keep your rifle running more efficiently.  

Cases on the left show shiny spots, flattened primers and a partially pierced primer. Cases on right, shot with VMOS show no abnormal signs of pressure, Both sets of cases shot with the same load.

I noticed the benefit when I was testing loads for a new rifle in 260rem. and was comparing the results from the 18” barreled rifle to another one I had with a 22” barrel.  The new rifle came with the VMOS and the 22” gun had JP’s Low Mass steel bolt carrier.  The 22” gun had proven itself reliable and accurate,  but when I tried the loads from the 18” gun.  The loads  that were showing zero high pressure signs, no shiny spots on case heads, no flattened primers, no issues whatsoever from it.  Suddenly, I was flattening primers and piercing primers, as well as sticky extraction.  I thought “what the h€ll”……….then it occurred to me to swap out the bolt carrier and retry.  BINGO!!!, all signs of over pressure disappeared.  The issue for the 22” gun was that the BCG was opening up and starting to unlock from the breech while there was still significant pressures working on the case and in the chamber.  

Both VMOS and LMOS are coated in a very slick coating that is also very hard.

By using the VMOS on my 22” gun I can now safely push bullets to within 50fps of my bolt gun, with an increase in velocity of about 100fps across all the bullet weights I use.  You can see from the pictures that the primers have nice round corners and case heads show no shiny spots from ejectors.

The VMOS, works by delaying the unlocking that fraction of a second so the pressure has a chance to subside.  It doesn’t sound like much, and perhaps running a full weight BC would allow the same effect, but you can’t tailor the stock carrier the way you can the VMOS.  Especially if you’re running a suppressor on your rifle, now you can tailor you’re own gun to run reliably with and without the can.  If you run a can on your rifle you really should look further into this.  

If you want to keep from taking your buffer tube off, you can simply grind the buffer detent down.

It does add weight compared to a low mass operating system, and you will need to either remove your buffer detent in your buffer tube or as I did, simply grind it down with a roto tool and grinding stone.  However it allows you to use a wider range of loads and not have over gassing issues with your gun.

Once done though, your buffer constantly removes itself from the buffer tube, unless you go with a captured system.

Overall, I think the benefits outweigh the detractors, and I plan on picking up a couple more for my large frame AR’s.  When I do a review I try and list pro’s and con’s, so here is my take on the VMOS, you can decide for yourself. If you want more info and details, visit the JP website and look up VMOS under the pull down menu.

Biggest Surprise: That it simply eliminates what was previously over pressure signs, with a simple drop in part.

Biggest Disappointment: You are required to either swap your normal buffer assembly to a captured system or do away with your buffer detent.  At which point your buffer assembly exits the buffer tube every time you open up the two receivers.

Most liked Feature: It can be tailored to work with your range of loads by you, no gunsmith required.  If you run into a problem call JP Rifles and they can talk you through any issues.

Least liked Feature: The total weight it adds if you decide to go with a captured buffer system and the VMOS together.

MSRP varies based on product but starts around $300

Visit JP Rifles for more information

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{ 2 comments… add one }
  • Link Lackluster September 28, 2020, 9:01 am

    Wow. The second sentence of this thing is only a fragment. That’s a pretty bad grade school writing oopsy.
    Did anyone proof read this article before hitting the “publish” button?

    • Trapr Swonson September 28, 2020, 7:52 pm

      However when using the AR10 or SR25 frame, weight is a necessary evil.

      Good catch!! I don’t know how that happened, I reread everything several times, before sending

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