The UMOS – Universal Manual Operating System Pump Action AR-15 Kit

Updated. Your eyes do not deceive you. That is not a free-float fore-end. It is indeed a pump-action AR-15, named the UMOS by Bentwood Gunsmithing of Nevada.



The UMOS, or Universal Manual Operating System is a novel retrofit kit for AR-15s that utilizes existing barrels, bolt carriers, and receivers, to convert a semi-automatic rifle into a pump-action centerfire wunderkind. As an added bonus, converting the rifle to manual operation has quite a few benefits.

  • Manually operated rifles are not “banned” or “controlled” in most states, allowing you to have detachable magazines and other “naughty” bits
  • You can create a ridiculously quite subsonic build
  • Hunt with an “AR” in states that ban semi-autos.
  • Have a familiar stroke for manual 3-gun divisions.


Compatible with carbine, mid-length, and full rifle barrels, the kit itself weighs in at 21.5 ounces (which is not bad, considering it replaces the gas block, handguards, etc.) and consists of six parts, with six screws holding it together tightly:

  1. Barrel nut,
  2. Barrel bushing,
  3. Main tube,
  4. Operating rod,
  5. Pump
  6. Pump key


There are a few limitations, namely the barrel is not free-floated as the tube affixed to the gas block journal, completely plugging covering the gas port. Bentwood says this may affect accuracy but match grade barrels will maintain their groups when used appropriately. Bentwood tested splits and the best were coming in at .39 seconds between shots.


The kit retails for $350 with the sight base an extra $20. It is 100% made in the USA of T6 aluminum and machined Acetal Nylon. It is currently available direct from Brentwood or currently through two California dealers:

  1. Sierra Arms in Modesto, CA
  2. J&R Sports in Livermore, CA.

Gallery Below:

The brown rifle is a Brentwood custom gun with an Aimpoint PRO and Samson 3x magnifier in a RAM mount. Brentwood did their own Cerakote. The black rifle photos are courtesy of TheLoadoutRoom

Update: Corrected from “Brentwood” to “Bentwood” and the company is from Nevada. 

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.


  • Renegade

    But…. but…. where is the bayonet lug?

    But really, this looks nifty. I may have to run by J&R and take a looksee.

  • Eluoci

    Oh! If only those guys start produce magazines for that guns it could be a game changer in Canada, due to retarded gunlaws that we have

    • Sulaco

      Eluoci what are the magazine laws in the far north?

      • dan citizen

        I believe in canada possession of gun magazines like “Shooting Times” and “Field and Stream” are among the most severe of felonies, earning you a stern talking to for a single offense and a 1 week maple syrup ban for repeat offenders.

        • Blake

          Plus they take away your Kraft Dinner privileges if you’re caught with an “American Rifleman”.

          • dan citizen

            the horror!

        • itsmefool

          Say you possess a “clip,” say one written by Zumbo or Metcalf…does that get you the syrup ban, plus no back bacon or hockey privileges?

          • dan citizen

            My daughter has a hair clip called a “princess barrette”, which is both a clip and a barrettte and I heard a barrette can shoot down a commercial airliner from 2 time zones away. So in canada she could get an entire winter sports ban,

          • itsmefool

            Which would be a shame, considering how long Canadian winters are! Oh, well…guess you could spend all that time reloading, eh?

      • David Sharpe

        To answer your question….semi auto centrefires are a max of 5 but there are a few loopholes. I can use a 10 round AR pistol mag in a rifle. I can use a mag designed for a manually operated rifle (Rem 7615) or one designed for another calibre (25 round Aussie Lee-Enfield mags fit in the Norinco M305/M1A, 5 round .50 Beowulf mags hold 15 .223 rounds)

        But now, 10 round .50 Beowulf mags are legal!!

        • Wetcoaster

          I’d hold off onto the celebrating until there’s an explicit RCMP ruling (per the 10 round AR pistol mags). There’s too much room for a stealth-ban like the Type-97 or BX-25 mags.

          Schroedinger’s legality. Until there’s a ruling, it’s neither explicitly banned nor explicitly permitted.

      • Wetcoaster

        10 round for handguns
        5 rounds for semi-auto centerfire long guns (rifles and shotguns)
        unlimited for rimfire rifles and manually operated guns

        It’s determined by the gun the magazine is designed for and the verdict is delivered via arcane RCMP tech lab divination (not really, but they don’t release their methodology, so who knows).

        There’s a few catches of course. Rock River Arms’ 10 round AR magazines designed specifically for AR pistols for compliance with restrictions in some US states are allowed and are popular with owners of rifles that accept AR mags (like AR-15s, for example).

        On the flip-side, several large .22 magazines have been declared prohibited devices unless pinned because they are also used in pistols – specifically 10/22 mags advertised as working in the 10/22 Charger (no more BX-25s, for example) and .22 ARs that also have pistol counterparts (the Chiappa Colt/Umarex .22 ARs, HK416 .22 in particular have or had pistol variants that the RCMP used as justification that those magazines needed to be pinned at 10 rounds.)

        Glock .40 mags pinned to 10 rounds will happily accept more than 10 rounds of 9mm and will fit and cycle in 9mm Glocks.

        Because the dividing line for limits wasn’t set at something far more technical and clear-cut (for example, 15 rounds for any centerfire, unlimited rounds for rimfire), but on more layman’s terms (semi-auto, long gun, pistol) the overlaps cause a number of legal gray areas.

        PS. A local AR-maker designed a pump-upper years ago, but since the RCMP declined to allow its existence to justify unlimited AR-magazine capacity, the project never came to market.

    • David Sharpe

      10 round .50 Beowulf mags are available.

      • 1leggeddog

        Wait what? They made a 10 round “pistol” mag for .50 Beowulf?!?

        • David Sharpe

          They’re starting to.

          • 1leggeddog

            i think my brain just exploded in extasy

  • Zachary marrs

    Looks neat, if I lived in a restricted state, id own one, hell, if it was 100-150 bucks cheaper, id own one. Id bet it’d work well with a can and some hand loads

    • dan citizen

      Years back at a business I worked at we had an AR with a screwed up gas system (gas port misalignment IIRC) that would only work as a straight pull rifle (manually cycling every round). It was fun to shoot and got enough use we never bothered fixing it, just used it as is.

      Somebody finally bought the thing for their 13 year old son.

  • jamezb

    Its sleek and sexy, I love pump rifles and I want one yesterdsy!

    • jamezb

      I aldo really like the look of the OD recover and smber magazine against the blavk metal…they want me to buy this from them pretttttttttty baaaaaaad….and it’s WORKING!!!

  • cabbak

    think they have these in Britain if I’m correct. would like to see video.

    • Dave L

      Looked into it… the UK does not allow pump rifles. Straight pull ARs are common though.

      • Mr Mxyzptlk

        What he said, lever action rifles are allowed but due to a reason I have never been able to fully fathom pumps aren’t (in anything above .22 anyway). A bizarre kind of weapon which is allowed in the UK is a gas operated AR-15 were the bolt locks back after each shot, requiring you to release the working parts forward with a special bolt release on the left side above the pistol grip which is pushed by your thumb. Anybody know if this kind of thing would fly in America to allowed otherwise banned weapons in certain states, or would it be classed as a semi-auto?

      • Wetcoaster

        I had no idea. I thought the pump ban was just an Aussie thing

  • Ben M

    Very cool concept for those in banned states. On the suppressor side though, I see this as being less than ideal. If I were aiming to build a manual action rifle for use with a suppressor, I would go with a bolt-action for a variety of different benefits. That said, does anyone make a contraption that is essentially a bolt lock of some kind for the AR-15? I’m talking something that allows you to flip a switch to go from manual action to semi, mounted on the lower somewhere or even an integral part of a lower? Since I live in the beautiful state of Missouri where such bans do not exist, I would find a product like that to be verrrry interesting.

    • Ben M

      Or maybe a gas system disconnect switch of some kind… did a quick google and don’t see anything like what I am talking about. Anyone know of a product like this?

      • Dave L

        Benelli M3 style? Sounds expensive. I’ll take 2.

        • Ben M

          Yes. You and I are on the same page. This is essentially the “locking the bolt” concept.

          The problem with comparing the two is the Benelli inertia system essentially takes the recoil forces to the bolt head which causes it to unlock and cycle. Locking the bolt head in place is no problem, since this is one of the strongest parts of the gun, designed to take high pressures. The AR-15 gas system on the other hand… I feel like if you just locked the bolt in place, the pressure on the gas system would eventually (if not immediately) cause a gas tube rupture, and would certainly cause some premature wear.

          Can anyone with more knowledge of AR-15 gas pressure tolerances weigh in on this?

      • jd

        There are adjustable gas blocks that can do what you’re looking for. They can be adjusted so that they just act as a gas port plug.

        • Ben M

          While better than nothing, most require tools for adjustment, which is a far cry from the convenience a switch on the lower receiver would provide.

      • Blake

        Yugo SKSs have a gas system block-off built into the grenade sight, so when you raise it up (i.e. to target a rifle grenade) it provides the full power of the combustion gas for launching the grenade (as well as preventing the chambering of a live round afterwards, which probably wouldn’t work too well anyway as the gas system certainly isn’t calibrated for rifle grenade firing).

        • the ammo addict

          Actually, the cut-off is to protect the gas system of the rifle. The high port pressures generated by the special grenade launching blank would destroy the gas system if the gas port were open.

        • Kivaari

          A Belgian firm has sold bullet trap rifle grenades for several decades. To launch one simply puts it on the muzzle, and fire a regular round. Range is reduced compared to suing a blank and closed system.

      • Blainestang

        Noveske’s “Switchblock” allows this:

        “The OFF setting of the Switchblock will operate the rifle in a single-shot mode, eliminating the sound of the action cycling and controlling the brass ejection.”

        • Ben M

          Yeah, but they only make them for 5.56 and they don’t come in pistol gas length. Which is unfortunate, since pistol gas length 300 blk sbrs are the ideal AR suppressor platform. They also apparently get too hot to operate with bare hands… because you know, its the gas block itself.

          • Mr Evilwrench

            Well, it blocks the gas port anyway, but otherwise, what (aside from possibly the barrel diameter) would be the objection to 300BLK? Nothing aside from the barrel changes between calibers.

            Pistol length is a different issue; you need to have enough room to cycle the action with a grip big enough to grab. With the pump trapped there between the upper and the can, there’s a physical limit to how short the barrel can be made.

      • Hokanut

        Like the M1A gas valve?

  • dan citizen

    Now that’s direct impingement!

  • dan citizen

    This would be great for law enforcement, helping to curb the number of incidents where an officer let’s 30 rounds fly at an assailant in an urban area and scores somewhere less than one hit on the target.

  • Nicks87

    Cool I like it but I live in a free state that doesnt prohibit evil gas operated “weapons of war”.

    • BryanS

      Yes, but here in PA< still cant use it for hunting (unless its a boar or a coyote)

  • clinton notestine

    i remember seeing pump ak’s at my local Big 5 years ago. This however is kinda interesting. Perhaps this could be a way to get a .308 or something in your ar.

  • Alex will be reviewing this in the near future

  • Drew Wood

    would this remove the need for a buffer tube?

    • gunsandrockets

      That is the question. Potential for short pump action handgun AR?

    • Mr Evilwrench

      No. It just pushes the bolt back like the gas system would if you had one. You’d still need the recoil spring to drive the bolt back into battery, plus, the bolt carrier goes back into the buffer tube during the cycle anyway. In fact, I would worry that using this system, you’d wind up riding the bolt (like riding the charging handle) and fail to go all the way into battery.

      • Dave L

        Ya… runs best with buffer and spring still in the stock. Some have used reduces power springs, and emptied the weights from the buffer, this is even better.

  • Spidouz

    I’m sure it could be a nice way to avoid some stupid laws, including in Europe.

    In France for instance, anything that is “semi-automatic” requires to be allowed by government after long background check; NFA-like, but with a maximum of 12 firearms, including anything that is a handgun, SBR or using some “assault” caliber (5.45×39, 5.56×45, 7.62×39, 12.7×99 and 14.5×114).

    But there’s some loophole, such all “hunting” and sport firearms (such a .308 rifle with manual action and 10 rounds magazine for instance).

    So having a rifle (which is not a pistol), that is a manual action (so is this pump action), with a 10 rounds magazine and with a caliber that is not on the “assault” list, such the 6.5, 6.8 SPC, .308 or even .50 Beowulf… would be allowed and could be obtained with a simple registration (still painful in comparison to some lovely US states, but still better than nothing, right?).

    So, I’ll keep on eye on this UMOS 🙂

  • DiverEngrSL17K

    Although I am not an AR enthusiast ( I personally prefer vz.58’s, assorted AK variants, Molot VEPR’s, FAL’s, M-14’s, etc., I still own an M-4 and M-16 which I greatly appreciate and enjoy, and can fully understand the enormous following that the AR generates. In that light, I really like this pump-action conversion ( although I live in a state that has no restrictions on AR’s or any other weapon type for that matter, so it is intriguing from a technical standpoint ) ; it appears to be sleek, well-balanced and user-friendly, not to mention visually attractive.

    Now for the nitty-gritty : Some readers brought up the topic of potential mechanical issues with regard to this system, so a further test and evaluation session with a mechanical tear-down would probably be a good idea. This testing would also confirm or deny the converted weapon’s ergonomics, balance and overall usability. Nathan, thank you very much for presenting us all with this most interesting article — would such an evaluation be possible in the near future as a follow-up, and would TFB need some additional support to see it happen?

  • Crocodile Tears Dundee

    Wonder if I could get an SCR with this on it into Australia? Can’t import single shot or straight pull ARs as they look like ‘machine pistols’. I laugh then I cry.


    the horror

  • gunslinger

    so what happens with the charging handle? do you use it to chamber the first round? or the pump action? what “locks the pump” in place until it fires or press the release?

    • Nathan Redbeard

      I would think it makes the charging handle about as necessary as a charging handle on a Remington 870 i.e. completely superfluous. The pump action should be able to cycle the first round same as every other round. And the bolt on a AR15 is a locking bolt, it’s not like it’s a blowback action or anything. The bolt will stay locked until you apply rearward pressure on the pump action.

  • mrsatyre

    Try convincing a dumb LEO that that’s not a conventional AR before he arrests you, takes your rifle and impounds it and you waste time and thousands of dollars in bail and legal fees to get it back.

  • RocketScientist

    Very interesting, and I think a great idea (especially with a wooden slide and furniture, would make an awesome rifle). My only concern is from a technical point of view. Based on the pictures and brief description, it appears the only thing that constrains the movement of the slide/pump is the rod connecting it to the bolt carrier, which goes through the barrel nut? If there are no rails or raceway or something on the main tube that the slide/pump rides on, which restricts its movement to a purely linear motion, I see problems. If the slide/pump is free to rotate around the barrel, there will be side loading of the action rod against the barrel nut (increasing wear) as well as increased risk of buckling (since the action rod seems pretty small diameter). I’m hoping that the pictures just don’t show some feature of the slide or main tube that keeps things linear, but if not I would be concerned about that.

    • Dave L

      The pump is keyed into the main tube. Not to worry.

  • itsmefool

    So kinda like the Remington 7615P ‘cept with real AR parts and perhaps better build quality?

  • Adam

    18 months ago, i started to build an AR15 dedicated pump action and straight pull in Australia…It looked like an AR15, but only the stocks and magazines were compatible… Sadly, my government didnt like the idea… Bummer as i had pre production orders around 3000 units…

  • Secundius

    Isn’t that an Oxymoron! A Pump-Action, Semi-Automatic Rifle.(e.i. PA/AR-15), this gives new meaning to the word “STUPID”.

  • Can we send one to Jerry Miculek?

    • Secundius

      @ lalahsghost.

      Yeah, the idea of seeing him with one in each hand, would be a interesting sight-to-see.

  • Secundius

    It sort of reminds me of a Mossberg 464 ZMB Zoombie, Combination Lever/Pump-Action Shotgun.

  • firebert

    Cool idea but I dont even like pump action shotguns

  • bmartin79

    I could see this being an accurate rifled or long range with the right set up .

  • bmartin79

    And great for kids .