View Full Version : Adams Arms Piston Review

14 May 2008, 18:56
Look for this review to be up shortly.


20 July 2008, 09:19
Adams Arms Piston Kit


Adams Arms
Jason Adams started his efforts at creating a piston kit as a result of using others on the market. As a recreational shooter with an engineering background Jason figured that he could make a kit that would be more simple in design, made of quality materials, and still be installed by the end-user without modifying the rifle. The result of this is the Adams Arms Piston Kit.

Piston Kit Components
As supplied by Adams Arms, the Retrofit Piston System consists of the installation instructions, gas block, plug, drive rod, drive rod bushing with attached spring, modified handguard cap, bolt carrier key, upper receiver bushing, steel dowel rod, and a pair of modified M4 handguards.

The major components of the Adams Arms Piston Kit.

For installation, the following tools are needed: 1/8 punch, 9/64 Allen key, 5/32 Allen key, wrench, and a metal hammer. This is assuming that the kit is being installed on a barreled upper. For some applications, a Dremel with cutoff disc and a bench vise may be needed.

The piston kit obviously replaces the DI system found in conventional AR systems. In operation, the Adams Arms kit routes the propellant gas that is bled through the gas port into the gas block. The hollow gas plug channels the bleed gas into the cup of the drive rod that fits over the tail of the gas plug. As this gas expands the drive rod is pushed to the rear. The receiver end of the drive rod moves between the barrel nut teeth and through the receiver bushing which helps to steady it as it travels back and forth. The rear face of the drive rod impacts the carrier key, which acts as an anvil. This impact is translated to the carrier body, unlocking the bolt and moving the carrier to the rear and into the extension. Once the drive rod has reached its full rearward travel the machined step and spring-loaded bushing come in contact with the barrel nut teeth. The spring loaded bushing and the return motion of the carrier key push the drive rod back forward as the bolt returns into battery.

For the sake of brevity, I will not repeat the entire installation process. I will note that it is important to read the instructions carefully before starting, and to be sure of the configuration that you will be using. For example, if you do not intend for the handguard cap to be used (as would be the case with some rail systems) it is necessary to cut the cap in half prior to installation, and use the bottom half of the cap as an indexing shim to make sure that the gas block is properly positioned on the barrel. Also, at present there is only a one-piece clamping gas block, so if the barrel that the kit will be used on is going to require a permanent muzzle device to meet NFA regs, this will have to be planned and accounted for prior to installation beginning.

The actual installation is fairly easy for anyone who is used to working on ARs. It takes very little time, so long as you read them ahead of time and are familiar with the installation process. Compared to having to have the upper sent off for work as with some other pistons, the Adams Arms kit shines in this regard. Providing you have your tools together and have read the instructions, the kit should be able to go on your upper in under 30 minutes, and probably a lot less than that.

The modified carrier key installed on the carrier.

While there is a torque setting listed for the carrier key (which is nothing new to AR users) there is no mention of staking the key against the screws. In talking with Jason, he related that the metal of the carrier is hard enough that the screws cannot be staked. Coming from a long history of torquing, LocTite, and staking carrier keys and screws this flies in the face of my experience with the DI system. So I marked the carrier key screw heads in the key to see if there was any walkout.

The adjustable gas block with clamping screws and flip up BUIS installed. The gas plug is shown in the full power setting. The screws had not been marked at the time of this photo.

The installation of the gas block requires special attention. If the block is not properly seated hard up against the cap, the cap will rotate under firing. On the first installation of mine, the cap would not rotate by hand, but would during firing. Care should also be taken in tightening the clamping screws. Adams Arms does not specify a torque setting for the gas block screws. Tighten the screws evenly back and forth, being careful not to over-tighten them. Again going on my experience with the AR, I put a witness mark on the gas block screws to see if there was any walkout.

The drive rod with the spring and bushing.

The upper receiver bushing can be difficult to identify (there is no picture of it in the parts diagram) and install. The receiver bushing is the white metal bushing that does not have the spring attached. I found it easier to hold the bushing in the upper receiver with a pair of needlenose pliers while pressing downwards with the steel rod that is included with the kit.

The piston kit installed. The drive rod, spring and bushing, and lower handguard are visible, with the gas block on the right. The belled end of the drive rod fits over the adjustable gas plug.

For the initial review, I used the included modified M4 handguards. In following reviews, I will be using the DD rails.

On The Firing Line
Once the kit was installed, I put the test upper on a carbine lower and went to the range. I used M855, M193, and WWB. I set the gas plug for the full power setting and started blasting.

There was more felt recoil from what I felt on the standard DI upper. The piston upper ejected the rounds at the 2:30 to 3:30 position very neatly. Extraction and ejection was positive and I had a nice impact area of brass. I dialed the gas plug down to the reduced setting. As the instructions implied, the ejection changed to a soft lob, barely clearing my right arm and plopping the brass down just outside my right foot due to the slower carrier movement. At the off position, the carrier did not cycle at all. Through the first range session of around 360 rounds I did not have any stoppages. The piston ran without any problem. The carrier key screws and gas block clamping screws have not moved.

At my first firing session, I had lubed the upper just as I would the standard carbine. After talking to Jason, he urged me to fire the carbine without lube. So following the first range session I simply punched the bore with an Otis pull through and did not lube the upper, bolt, or carrier. In the following sessions with a current total round count of around 1100 rounds, the piston kit is still going strong with no stoppages even though the rifle is running dry. My PMCS is basically limited to punching the bore after each session.

The piston system with the upper handguard removed. The excess gas is vented and leaves the fouling visible on the back of the gas block.

One of the features of the piston kit is the way in which the drive rod travels. Other kits use a spring to stop the operating rod, and will not function if the spring breaks. The Adams Arms design uses a machined step in the drive rod to stop its movement, and the spring simply slows down and cushions the stop. Jason has said that their piston will run even if the spring breaks. I decided to test this claim.

The drive rod spring and bushing removed.

Going even beyond the possibility of a broken spring, I completely removed the spring from the drive rod and fired off a magazine. Just as advertised, the drive rod continued to function. Again, this was without the spring. While even if the spring breaks, there will likely be at least some part of the spring remaining on the drive rod. In my test I completely removed the spring. While the system will run like this, I urge you not to do so. In talking to Jason before doing the test, he predicted that there could be some damage to the barrel nut. Without the spring to cushion the drive rod, the barrel nut did show some slight damage.

Slight damage to the barrel nut teeth did appear as predicted. Firing the system without the spring and bushing is not recommended.

One of the selling points of the piston system is the reduction of heat and fouling in the chamber and upper. Even when using lower quality ammunition, the bolt was easily cleaned and the fouling in the upper was lower than with a DI system. In handling the drive rod and gas block in between firing strings, the carbon fouling came off without effort on my gloves simply from handling them. The bolt was cool enough after firing that the bolt could be held in my bare hands. At the same time, using the included handguards the barrel was hot enough that the heat could be felt through the handguards and my gloves, and the gas block could not be touched even with gloves.

The gas plug is shown here. The fouling seen around the hole is from the gas port. The gas plug and block are the areas that build up heat as opposed to the bolt.

The face of the bolt and carrier key.

The debate over whether a piston is needed will rage on probably for some time. The simple fact is, there is a developing trend towards piston kits, with some well known weapons manufacturers using them in modern designs, and the HK piston M4 upper adding to this. There are several reasons the reader may decide to use a piston. First, the ease in cleaning. Without a doubt, the Adams Arms kit is low maintenance cleans up very easily. Next reduced heat to the bolt, resulting in potentially increased bolt life and lower likelihood of extractor and locking lug failure. Again, the design keeps the heat away from the chamber and upper. For shooters that own suppressors, the adjustable gas block allows the gas to be regulated and the system to be tuned. While these largely are present in the very design of the piston system, the benefits of the Adams Arms Retrofit Kit should put it foremost in the eyes of anyone looking for a piston system.

The Adams Arms kit shows several benefits over other systems.
1) It is an end-user installed kit. It does not require the user to ship an upper off.
2) It requires no permanent modifications to the upper assembly. The gas port will not have to be opened up, the system does not rely on cut down conventional parts and is quite sturdy. A standard AR upper is used.
3) It works with a variety of rails without modification to the rails. The user will not be limited to proprietary or a narrow range of rail systems. Some rails will require modification to be used.
4) It can be removed from one upper and installed on another. If a barrel needs to be replaced, it is a simple matter of moving the piston kit.
5) Non-pinned gas block. While this is a negative to some, it also means that again no specialized tools or smithing is needed to install the gas block. It also does not tie one gas block to one barrel.
6) The gas block is adjustable. For using suppressors, the user is able to adjust the amount of gas being returned to the carrier.
7) Front access to the system. The gas plug is adjusted from the front, and is also removed through the front, which allows the drive rod to be removed as well. There is no need to remove rails or handguards to access the system.
8) There are a minimum number of parts, and minimum number of moving parts. Because the drive rod is the piston there is no separate piston, operating rod, piston housing, etc.
9) Quality parts and machining. The Adams Arms retrofit piston components are well-machined and made of quality materials.
10) The kit includes a railed block. Adams Arms provides an adjustable gas block that includes a Picatinny rail interface. This is a great deal in regard to total value of the kit.
11) Lifetime warranty.
12) 30 day money back trial period.