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.