Every once in a great while, somebody comes up with a simple idea and then does the hard work to build an elegantly simple design. The Bentwood Gunsmithing Universal Manual Operating System is a pump action AR upper which uses a standard AR upper and bolt carrier. This product is made in California, by Americans.

Why build a pump AR? Their big ideas are:

  1. Build an AR which can be used where hunting laws prohibit semi-autos.
  2. Eliminate the criteria for semi-auto function when developing hand loads for hunting, suppressed applications or subsonic reduced power loads (keep the gun as silent as possible by stopping automatic ejection that makes noise when the shell casing bounces around and the pressure wave escapes the breach).
  3. Create an option to build a legal AR rifle in Constitution-deprived areas of America where semi-automatics are called ugly names and are banned or restricted.
  4. Build a rifle faster than a side charger for manual division 3-gun

They accomplished all this by making a conversion kit which turns your AR upper or parts kit into a pump. The kit weighs only 21.5 ounces. It replaces the gas block, barrel nut, fore end assembly and gas key. There are other companies selling pump rifles, but they are not ARs – they just look like them. This kit uses your AR parts, no FFLs involved, and if you find your rifle banned, you can fix it.

Bentwood Gunsmithing UMOS

The Bentwood Gunsmithing UMOS will be unveiled in July at the Bay Area Rifle Challenge, in Richmond, CA. Because of my connections at the highest levels of the firearms industry, I got one of the first production models to test. I took it to the range and put on a Trijicom Reflex Sight. I figured the red dot would let me track the sights through pump action and get me back on target faster. I was right. The pump is connected by a rod directly to a special gas key on the bolt carrier group.

It is much like running a pump shotgun with the exception that it does not “lock.” The bolt locks, but the pump can move the bolt carrier at all times. The disconnector and normal safety features of the AR make it impossible to fire out of battery.

I was a little surprised when the bolt locked open on an empty mag. This is an advantage over the pump shotgun because you know when you are empty and won’t try to shoot on an empty chamber. When you get a new mag in, you can just move the pump forward and get back to work. The buffer and recoil spring remain in the gun and assist you by pushing the bolt forward.

It has been said that shooting an automatic is like marching the goose step, and shooting a revolver is like dancing a waltz. The UMOS is definitely a waltz. It is not as fast as a gas gun, but if you work with the gun and shoot as you pump, it is very smooth.

I took my UMOS it to a local 3 gun match and threw on a Bushnell 6500 elite 1.2-8 variable power optic. There were no surprises, everything works just as advertised. I shot steel out to 200 yards and skeet at 10 yards with no issues.

The kit consists of 6 pieces that will fit nearly any AR-15 and turn it into a manually operated pump rifle. There are no permanent modifications to the upper or barrel or bolt carrier group so they’re still functional as semi auto components once the kit is removed.

It’s about a 5 minute job to swap it in and out. Some heavy bull barrels will not work with the kit.

The basic kit costs $350. Here is what you get:

Bentwood Gunsmithing UMOS

Main tube, barrel bushing, and barrel nut are T6 anodized aluminum. The Pump is delrin with a “pump key” of stainless steel. The operating rod is O1 tool steel and is TIG welded into a GI gas key. Every part is nicely finished and well made. The kit has a solid feel to it.

Bentwood Gunsmithing UMOS

The kit is a universal fit. It will work on carbine-, mid- and rifle-length systems. The front Picatinny rail sight base is optional. A sling swivel stud is standard. The standard kit will not accept a Harris bipod without modification. Lightweight barrels with a .625 gas block require an extra piece, which Bentwood will supply at no extra charge. Just mention it when ordering.

They have two dealers in California:

Sierra Arms
1408 N Carpenter RD #1
Modesto, CA. 95351
[email protected]

J&R Sports Supply
2558 Old 1st St
Livermore, CA 94550
(925) 443-9691
[email protected]

You may also contact Bentwood Gunsmithing of Henderson, Nevada. They are happy to answer questions. They spend more time building guns than answering phones, so email is the best way to reach them.

Bentwood Gunsmithing
180 Cassia Way Suite 507
Henderson, NV 89014
[email protected]


Bentwood Gunsmithing is a full-service manufacturer and custom shop, so any special barrel or upper or other treatment or surface coating can be accommodated. They will also use your components and build a gun for you.


The gun in the picture has a 20″ Green Mountain barrel, DPMS grip, Ace stock, Doublestar lower, DPMS lowpro upper. Bushnell 6500 elite 1.2-8 on an American Defense mount. The brake is from Elite Iron. The mag is an MSAR.

VIDEO: Test firing the UMOS prototype