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The "Reactor" On/Off Valve Replacement for the Automag

İRavi Chopra, 1997

One of the oldest complaints about the 68AUTOMAG (and other paintguns of that design from Airgun Designs) is drop-off. Drop-off (also known as shoot-down and walk-back) is the condition where, under rapid fire, the 'gun starts to lose velocity after a number of shots. This occurs when the air chamber behind the bolt is not completely filled to regulated pressure between shots so subsequent shots have low velocity. A variety of modifications to the Automag AIR valve and assembly have been attempted to try to eliminate drop-off, including drilling extra holes in the regulator and porting out the channels of the air path from the regulator to the air chamber.

The most obvious gas-path bottle-neck in the 68AUTOMAG's design though, is the on/off assembly. You can drill out the rest of the 'gun as much as you like, the air still must squeeze through the on/off on the way to the air chamber. The on/off valve consists of three parts. The valve bottom goes in last and has seals to close the valve off from the outside. The valve top is a brass piece that fits between the valve top and the channel bringing gas from the regulator. Finally, there is a valve pin which opens and closes the valve, and is operated by the back of the sear. This whole assembly fills most of the space and leaves very little room for air to go through the valve top (between the hole in the brass top and the pin which goes through it) to the air chamber at the front of the 'gun.

Many people have tried to open up the gas-path through the on/off valve by cutting grooves in the stock brass top to provide extra channels for gas flow. Others have built complete valve-top replacements with little more metal than the minimum required to hold the o-rings in place. These valve-top replacements do work to a certain degree, but none of them are perfect. The reason that these can not be perfect is that the valve bottom partially occludes the hole leading to the air chamber! No matter how you modify the valve top, you do not provide completely free flow to the air chamber.

The Reactor on/off valve from HSI takes a new approach to opening up the on/off bottle-neck. The Reactor does not replace just the top. It is a complete stainless steel valve replacement of significantly changed design, only carrying over the o-ring and valve pin from the original valve. The Reactor top (the Reactor also has "top" and "bottom" parts) is significantly longer than in the stock valve and does not occlude the hole to the air chamber. The Reactor top is drilled out to a large diameter inside, has a wide channel cut around its outside, and has large holes drilled straight through to provide a free, low-resistance path for air through the valve. The Reactor bottom is simply a thin disc to hold the valve in at an appropriate depth. A pair of o-rings are sandwiched between the Reactor top and bottom to seal the valve in. In concept the Reactor is a very good design.

The Reactor packaging makes some very big performance claims. HSI claims that installation of a Reactor in your 68AUTOMAG allows its use in sub-zero temperatures, improves recharge rate with both CO2 and nitrogen/HPA, improved velocity consistency, and lightened trigger-pull.

I spoke to Dave Massey, president and owner of HSI about how they tested the Reactor. Their first test was to freeze two stock 68AUTOMAGs, one with the stock valve, one with the Reactor, down to 20°F with 12 oz CO2 tanks. They then took the two 'guns out into 20°F winter conditions and test-fired them. The stock 'gun froze on the first shot and vented down the barrel. The Reactor-equipped 'gun fired 290 fps consistently, the same velocity it had been chronoed at before freezing. Their second test involved taking quick 75-shot strings and measuring velocity. The stock 'gun dropped 15 fps shooting nitrogen. The Reactor-equipped 'gun varied only 8 fps shooting CO2.

While the results of HSI's tests are interesting, they are somewhat puzzling. Venting down an Automag's barrel indicates a problem with the power-tube o-ring (obviously frozen in this case). I don't see how a new on/off would protect the power-tube o-ring from freezing. Second, comparing a nitrogen-equipped 'gun to a CO2-equipped 'gun is comparing apples to oranges. While using compressed nitrogen as a power source is typically considered to provide more stable velocity, that is not necessarily always the case, particularly with a regulated 'gun like the 68AUTOMAG.

The really interesting test was done by Airgun Designs. After HSI designed and built the Reactor, they sent one to Airgun Designs. There, they installed the Reactor into an otherwise stock 68AUTOMAG and put it on the torture machine which cycles the 'gun 9.5 times per second using CO2 as a power source. It took 3200 shots at that rate to freeze the 'gun up. How impressed was Airgun Designs with the Reactor? Well, they won't say anything publicly, but it only takes a momentary look at the new Automag RT and 68AUTOMAG to tell. The on/off valve of the RT looks suspiciously similar to the Reactor. Furthermore, all new 68AUTOMAGs come with a new brass on/off top that looks very much like a compressed Reactor top. HSI likes to look at this as the sincerest form of flattery.

When I first received my Reactor and tried to install it, I found some aspects of its design to be something of a pain. First, anyone who's had to replace an o-ring at the top of the on/off valve knows how difficult it can sometimes be to get the small Teflon o-ring to stay inside the larger o-ring. With the Reactor you have to do it twice since a second pair is sandwiched between the valve top and bottom. Additionally, the small steel disk that comprises the valve bottom has a tendency to drop out every time you field strip the 'gun. Until you get used holding the 'gun upside-down when you field-strip the regulator, you'll frequently find yourself searching the ground beneath you for this small part.

When I tested the Reactor, I wanted to address the two biggest reasons that people experience drop-off. The first, most obvious reason is due to rapid fire and restriction of flow through the on/off valve. The second, and probably most common reason for drop-off is short-stroking. Short-stroking results from a person "riding" the trigger. That is to say, the person does not completely release the trigger between shots, preventing complete opening of the on/off valve, and preventing complete filling of the air chamber between shots.

To test the flow-rate characteristics of the Reactor, I used an Auto-Response trigger. The Auto-Response is a complete grip-frame and trigger mechanism replacement which allows the 'gun to shoot once on the trigger pull, and once on the release. This has two advantages. First, it allows you to shoot very very fast. Second, it forces you to cycle the trigger completely so any velocity drop is not caused by short-stroking. I tested my 68 Micromag equipped with an Auto-Response trigger and the stock on/off, several modified brass on/off tops, and the Reactor. Compressed air was used exclusively throughout this test. My first goal was to measure velocity drop-off over a string of shots. In the end, I didn't have to. Firing as quickly as I could, every on/off valve except the Reactor resulted in the 'gun sputtering down and hanging-up after about 20 shots. The Reactor not only didn't allow the 'gun to hang, it kept a constant and consistent velocity throughout regardless of how long I kept firing.

I then tested it under short-stroking conditions. With the various valves installed I fired the 'gun over the chrono without completely releasing the trigger between shots. I took several velocity measurements with each valve and found very consistent results with each. The end result is that the Reactor is no better than the stock valve when the operator is short stroking the 'gun. The stock and Reactor valves suffered velocity drops valve of as much as 50 fps when short-stroked. Cut-up valve tops that only replaced the stock on/off top fared better than the Reactor under short-stroking, losing 25-40 fps (depending on the valve top used). Why this is, I do not know.

My final test was my own version of the freeze test. I wanted to see just how well this valve could protect the 'gun from liquid CO2 freeze-up. After installing each valve, I screwed in a bottom-line 20 oz tank (not anti-siphon), and started firing. With most of the valves, the 'gun froze up and vented down the barrel within 10 shots. The Reactor allowed 15 cycles before freezing and venting. Ultimately, the Reactor may give some slight protection against freeze-up, but it can not be used as the only line of defense.

I also spent some time getting the opinions of players who do use, or have used the Reactor. The results are a curious mix. About half say that the valve did wonders and they don't suffer drop-off any longer. The other half said that the Reactor did nothing for them and that they couldn't get it out of their 'guns fast enough. Some people claim that the Reactor makes their trigger softer (as HSI claims). Others (myself included) found that the Reactor made the trigger feel stiffer and snappier. It also seems that dropping the Reactor into 'guns that have had custom trigger-work done can cause some problems, including making short-stroking a near-inevitability. If the trigger-job is done with the Reactor already installed, though, it seems that the trigger-work can be effectively designed around the valve to work well.

In conclusion, the Reactor is a well-designed and manufactured piece. It definitely represents less of an impediment to flow than the stock on/off valve and most of the on/off top replacements currently on the market. It will not correct a short-stroking problem, though in all fairness, short-stroking represents operator error more than it does a flaw in the 'gun or valve design. If you don't short-stroke your 'mag and you still suffer drop-off, the Reactor may help eliminate some of that problem. All material at this site is © Ravi Chopra, 1999