A few words about gas rams, gas springs, and Nitro Piston

Monday, April 1, 2013

nitropistoncomparison 11-09 (Medium)

A while back, Freddy King responded to one of my blogs and posted a question: “Could you give some info on Nitro Piston mechanism? How does this system work?”

Okay, Freddy, here goes. If you have ever seen the gas strut that lifts the back window of an SUV or the rear door of a hatchback automobile, you have seen the basic working mechanism of a Nitro Piston – also known as a gas ram or gas spring – airgun powerplant. At the heart of the gas spring is a cylinder with compressed gas. At the far end of the cylinder is a piston. The mechanism works the same way a conventional spring-piston airgun powerplant: when you cock a breakbarrel rifle equipped with a gas ram or Nitro Piston, you drive the piston down the cylinder (further compressing the gas inside the cylinder, which is comparable to compressing the spring in a conventional spring powerplant) until it latches. The powerplant sits there, under tension until the shooter pulls the trigger. When that happens, the gas expands, driving the piston down the compression tube, compressing the air in front of it until the compressed air squirts through the transfer port into the breech, propelling the pellet down the barrel.

To find out more about this technology, I spoke interviewed Ben Taylor, who invented gas ram technology, by telephone in England. In 1976, he and his business partner Dave Theobald were unhappy with the state of the art in spring-piston powerplants. They had made and sold eight spring rifles, and they all suffered from not keeping the energy that Taylor wanted. So he had a thought: what if you used compressed gas in a cylinder with a piston instead of a spring?

So he built one, using a brake seal for cars from Lockheed, and it worked! At first, he pressurized the gas ram with 150 psi air from the shop and got only about eight or nine foot-pounds of energy. “Then we tried 300 psi nitrogen from a bottle and got 1,000 fps in .177. I shot that same gun for five years with the same charge in it,” Taylor says.

In 1981, Taylor and his partner applied for a patent and tried to interest various airgun manufacturers in licensing the technology. A couple of times they came close, but ultimately no deals were consummated. “So we decided to manufacture it ourselves. It took 10,000 pounds to get set up. We sold 490 the first year, 1,000 by the third year,” he says.

Gas rams offer a number of advantages, Taylor says. “They are totally tireless. You can leave them cocked for as long as you like. Nothing wears out. The seals don’t wear. Recently I serviced gun number 25 from 1982, and it was the first time it had been serviced since it had been manufactured. You have to remember to shoot, or cock-and-decock, a gas ram every few months, otherwise the seal can get bonded to the bore, and that will cause failure.”

He adds, “We found that if a gas ram is going to fail, it will do so within the first week. Otherwise, it will last for years. Right now, there are more of our guns out there that have never been serviced than those that have been serviced.”

There are a few disadvantages to gas rams. Unlike a spring powerplant, which often will keep operating at reduced velocity even if the spring gets broken or bent, if a gas ram fails, it won’t work at all.

The biggest problem, Taylor says, is that, because Theoben gas ram powerplants had a valve where people could increase or reduce the pressure of the gas inside the powerplant, people, in a quest for more power, tend to overfill them.

“There is a sweet spot on the pressure vs. velocity curve,” Taylor says. “If you go beyond that, you increase the pressure, but you don’t get any benefit. The gun becomes hard to control and won’t shoot straight. In addition, there is the danger of burning the piston seal. We actually had to design our high powered guns so that over-pressurizing them wouldn’t create reliability problems.”

Taylor told of an interesting experience at the test range one day. “We had two 30 foot-pound guns of the same caliber shooting the same pellets. One was a gas ram and the other was a precharged pneumatic. We had chronographs set up at the muzzle, halfway down the range, and at the target. We found the gas ram was retaining energy much better at the target. When we recovered the pellets, they looked like they had come from two different manufactures. The pressure from the gas ram had flared the skirt of the pellet flat to the bore, so that it looked like a cylindrical pellet, and the gun was shooting flat like a laser!”

The Weihrauch HW90 incorporates the Theoben gas ram system.

The Weihrauch HW90 incorporates the Theoben gas ram system.

Although Theoben Airguns has gone out of business, you can still buy a breakbarrel rifle with a gas ram powerplant based on Ben Taylor’s design: the HW90.

I also interviewed Ed Schultz, Director of Engineering for Crosman Corporation, to get his views of Crosman’s Nitro Piston Technology.

“Nitro Piston offers an advantage in longevity in modern spring guns that operate at the velocities that people want,” Schultz said.

“When you are using a mechanical spring in an airgun, you are just doing bad things to the spring,” Schultz adds. “A rule of thumb in engineering is that you don’t want to stress a spring past 50% compression to maintain reliability, but that doesn’t work in a spring gun. Instead, you compress the spring almost 100%. You take up almost all the gap between the spring coils to get ultimate performance, and that tends to weaken the spring.  And if you leave it cocked, you’re taking some life out of the spring. So you use special materials and do special heat treatments to deal with that, but you’re basically fighting a losing battle.”

“But a gas ram, Nitro Piston, powerplant eliminates the weak link in the system. The gas doesn’t care if it is compressed, it’s not going to degrade the life of the powerplant,” he says. “A life of 5,000 shots is probably a good rule for estimating spring life in an average spring-piston powerplant. The life of a Nitro Piston powerplant is easily twice that, and at the end of that time, it will shoot close to the original numbers. It’s either working completely fine, or it’s not working at all.”

Schultz adds that a Nitro Piston powerplant has few moving parts, there is no spring torque, no vibration, no need for spring guides. “To make a spring powerplant really quiet and vibration free, you have to custom fit inner and outer spring guides because every spring is slightly different,” he says. “You don’t have to do that with a Nitro piston powerplant. There are billions of gas springs in use throughout the world. Automobile manufactures have adopted them because of their reliability, and we know how to make them with high precision. With a Nitro Piston gas spring powerplant in your airgun, you get a lot of the advantages of an expensive, custom-tuned powerplant at a more affordable price.”

Til next time, aim true and shoot straight.

–          Jock Elliott

65 Comments

  1. Ravindrane says:

    After reading this articles I like to own one Nitro-piston rifle.

  2. A.R says:

    Thank you for an informative article. Could you comment on the recoil characteristics and the “lock time”– if that’s the right expression– of a gas rammed rifle versus a conventional spring powered rifle?

    Thanks

    1. Jock Elliott says:

      A R,

      My impression of shooting a gas ram is that the lock time is very, very quick, and it is often accompanied by hard “snap” recoil (for an airgun). In the future, I hope to do some experiments involving detuning a Theoban gas-ram to see it I can identify a sweet spot where recoil is low, accuracy is high, and power is decent. The Crosman NitroPiston gas spring guns cannot be adjusted by the user, but the gas-ram guns from Weihrauch can be adjusted. Some folks I have spoken to seem to be of the opinion that the Weihrauch gas rams come from the factory slightly overfilled. As you can see from the interview, overpressurizing a gas ram is counterproductive.

  3. Brian says:

    Interesting article. Some good information in there. I didn’t think about the fact that the air ram could be left cocked without any harm. I’m used to waste pellets with the old springer if a target of opportunity was no longer available and am still in that habit now. Guess I can stop doing that and leave it cocked.

    1. Jock Elliott says:

      Brian,

      Thanks for your comments and the kind words.

      Cheers, Jock

  4. Bala Iyer says:

    A very informative and educative explanation on the difference between a spring & a gas ram in an air rifle.

    I recently replaced the spring in my 22 cal air rifle with a gas ram – It certainly made a huge difference!

    Accuracy improved, vibration reduced & velocity too gained a bit.

    A great experience!!!

  5. Zonunthara says:

    After finish the nitro gas, will we refill or replace?

    1. Jock Elliott says:

      Zonunthara,

      If the gas leaks out, a Theoben gas ram can be repaired. The Crosman Nitro Piston, I suspect, would have to be replaced.

  6. Zonunthara says:

    Thanks. i wanna know is that how many shots should be available and is it modified to spring piston?

  7. Pete says:

    Very informative and helpful Thank you

  8. Stu. says:

    hi All -Ive had a compressed air underwater speargun FROM 1969– Called a Corsario-and it works the same way–with compressed air—I WISH I STILL HAD THE CORSARIO–ANY ONE OUT THERE GOT ONE ?? THEY WERE BLUE IN COLOUR AND THE HANDLE WAS IN THE MIDDLE OF THE GUN–PERFECTLY BALANCED & THE UNDERWATER RANGE WAS 30 to 40 FEET IF YOU WERE GOOD ENOUGH ? AND I WAS -CHEERS ALL-Stu xxx.

    1. Jock Elliott says:

      Stu,

      I suggest you use a search engine to find a spearfishing forum. Perhaps they could help.

  9. Dave says:

    Hi I have a Beeman Crow Magnum with a leaking gas ram which has recently lost much of its power
    it is a series 111 serial no TB13991
    can I get a new or replacement gas ram for this rifle as it is in perfect condition otherwise
    thank you

  10. Dr. Hage Bamin says:

    Which one is more accurate ? Pcp or gas spring air gun

    1. Jock Elliott says:

      Dr. Bamin,

      PCP is much easier to shoot accurately.

      Check out this information on airgun powerplants: http://www.airgunsofarizona-temp.com/blog/2013/12/airguns-101-the-basics-powerplants.html

  11. Rijo Joseph says:

    Very informative and this clarifies my thoughts..and one remain, do what Nitro means, is it nitrogen ?

    1. Jock Elliott says:

      Rijo,

      Yes, it means nitrogen. The air we breathe is 80 percent nitrogen.

  12. PJ says:

    I have 2 crosman nitro piston air rifles. I loved my first one so much i bought another, but the new one ive shot about 2000 shots through the velocity of the pellet is HIGHLY inconsistant im going tomorrow to get my money back and get a cheap springer. My first gun has lost a lot of power aswell, its only about 3 years young any ideas?

    1. Jock Elliott says:

      PJ,

      I suggest contacting Crosman customer service.

  13. Y.Sailendra says:

    Thank you for a lot of useful information.I am a sixty one year old air gunner from India, and have been in love with air guns for half a century!

    1. Jock Elliott says:

      Y. Sailendra,

      Thank you for the kind words!

  14. eddie lapid says:

    Is it ok to use barska plinker scope for nitro piston rifle?

    1. Jock Elliott says:

      Eddie,

      If it is “airgun rated.” Ask your dealer or check the website.

  15. C. Robert. says:

    A very informative and straight forward article of which made me definite decide to buy the rifle I was recommended. (Just waiting for the wooden stock)
    Thank you.

    1. Jock Elliott says:

      C. Robert,

      Thanks for the kind words and enjoy your new rifle!

  16. john finley says:

    I have a spring power plant rifle it is a gamo and works well. It is a 177 and is a little light for my needs. I was look at the pcp power plant and did not know how the nitro plant worked. Now I know thanks! from what you say my best option is a 22cal. with the nitro plant. Am I right? Thanks!

    1. Jock Elliott says:

      John,

      You haven’t told me what you want to do with your air rifle, so it is hard to say, but since the .177 is a little light for your needs, it sounds like the .22 might be your best bet.

      1. john finley says:

        I guess I was asking if you recommend a nitro piston over the pcp system. I am mostly shooting ringneck doves in the back yard. I just need the added nock-down of the 22cal. Is the nitro piston quitter? Thanks!.

  17. Clay Cook says:

    With the Theoben gas ram charged to say 377psi , I think that is 26 bar, what does the pressure in the ram increase to when the gun is cocked?

    1. Jock Elliott says:

      Clay,

      Sorry, I have no idea.

    2. Mike Thalman says:

      The ram pressure should not increase, since you are not adding air to the ram. When you cock the gun it compresses the ram until the 26 bar wont compress any further. When you pull the trigger that 26bar pushes the piston down the compression chamber (where it compresses the air, then through the transfer port then the air expands down the barrel.

      1. Andy says:

        I disagree … the term “bar” is used to indicate the pressure of the gas, NOT the quantity of gas.

        When you take a specific amount of gas in a confined space and then compress it into a smaller space – of course the pressure has to go up from what it was before you compressed it.

      2. Albert Boerstoel says:

        Of course the pressure will increase. Basic laws of physics apply here, specifically Boyle’s law that states that, with a constant temperature, PxV=c, or Pressure times Volume is constant. So, if cocking reduces the volume in the chamber to 1/5th the original volume, the pressure will be 5 times as high as the original, in this case 130 bar. The difference, 104 bar, is what drives the pellet.

  18. john finley` says:

    I seem to be having trouble replying to you. So here is my comment . I am shooting mostly in town. IS THE nitro piston a lot quiter than the pcp system.?

    1. Jock Elliott says:

      John,

      A fully shrouded PCP system, like a Benjamin Marauder, will be quieter than a nitro piston.

      1. john finley says:

        thanks!

  19. Jiggi says:

    Hi , i m using spring piston rifle, Compare to spring, nitro piston is best. But some questions here,
    1. How many years Nitro piston will work?
    2. if seal leak then it will be repairable ?
    3. Spring piston does need any special service, does nitro piston required any special service after year…?
    Thanks
    -Jiggi

    1. Jock Elliott says:

      Jiggi,

      1. I don’t know, but it will depend in part on how many shots you put through it.
      2. The Theoben-type gas ram found in the HW90 should be repairable. I suspect that the less expensive gas-rams will be replaceable rather than repairable.
      3. A gas-ram doesn’t require any special service that I am aware of. A spring piston will eventually require replacement of the spring.

  20. Michael loar says:

    Hi
    I have a crosman venom nitro spring gun in 177 caliber that I am using for field target matches and it is shooting JSB 8,44 pellets at 925 fps. I would like to change the gas spring to one that would drop the fps to around 800 fps with the JSB 8.44s and have searched the crosman sites schematic’s for a gas spring that is lower rated fps power but have been unable to determine what gas spring I should get to fit in the nitro venom to give me a fps of around 800 with the JSB 8.44 pellets. Can you recommend the correct gas spring that I would need to get to achieve this if they do make one that fits this fps range.

    Thank Mike

    1. Jock Elliott says:

      Michael,

      I don’t know of any Crosman NitroPiston powerplants that would meet your power requirements (which would be right around 12 foot pounds, I think). I know that Crosman did experiment with 12 foot-pound gas ram powerplants, and they were very nice to shoot, but I don’t know of any that are available for purchase.

      If you want an adjustable gas spring powerplant, you can a Weihrauch air rifle that is so equipped from http://www.airgunsofarizona.com, but it is significantly more expensive than the Crosman version.

  21. ERB says:

    I am looking into the Hatsan 125th models of air rifle.
    1. Any pros or cons you would care to share on this?
    2. How much effect does air temperature fluctuation have on the air piston power?
    Can I have comparable performance in the cold winters and hot summers?

    1. Jock Elliott says:

      ERB,

      I don’t have any experience with the Hatsan 125th model, so I can’t comment.

  22. Tom day says:

    I’m new to the sight but have read every comment. I have a older Beeman Falcon I . I replaced the whole inside 6 years ago and refinished it to look brand new. I even sanded the cheek rest off because I’m left handed. The only mistake I made, was puting a quality simmons scope on it. Too much recoil and one crosshair broke. Other than that, the gun shoots so accurate at 70′ feet, it really surprises me. All within a 3″ diameter . Just pleased with this rifle.

  23. Johan says:

    Exactly the article I was looking for!!! THANKS!!!!

  24. Bill Cunningham says:

    will reducing the hw 90 Theoben gas ram effect the accuracy? If not what would that level be? Thank you

    1. Bill Cunningham says:

      will reducing the Theoben gas ram to a more manageable level effect the accuracy? If not, what level would that be? THANK YOU!

      1. Jock Elliott says:

        Bill,

        I have heard (from those who have tried it) that reducing the power of a Theoben gas ram to, say, 12-foot-pounds, makes them a lot more fun to shoot and does not reduce accuracy. (Although, obviously, reducing power also reduces velocity with the same weight pellet and therefore you have to compensate more for wind drift if there is any.

  25. Donna B says:

    Glad to Know the difference . Been wanting to get a good varmint gun for snakes and the like. How hard are they to cock as Im not as young as I used to be?

    1. Jock Elliott says:

      Donna,

      You haven’t said how far you need to shoot or how large a varmint you need to deal with, but for snakes and smaller pests up to about 50 feet. I would recommend with Weihrauch Hw-30. It’s easy to cock and easy to shoot well, but not powerful enough for larger pests like raccoons.

  26. Donna B says:

    As large as a bobcat or fox sometimes. Would want to be able to take care of turtles in the pond too. Thanks.

  27. Andy B says:

    I ve just got a Remington Thunder Jet. Gas ram is a smooth as you like hitting 15mm targes at 50 ft plus. A joy you must try a ram and feel the difference.

  28. tom bookwalter says:

    Thanks…great article. I have been looking at the Hatsan 125 in the .25. I didn’t know the difference between a “springer” and a nitro piston. I think I will go with the gas piston after reading your article.

  29. Jeff rice says:

    Always wanted a Crow Magnum. Looking now but Eliminators in U.K. easier 2 find. I have a RX-2. The Theoben ram was the point of buying the rifle. I have others(all springs) My R11 mk11’s as smooth as the ram, n I like the recoilless ness of the 54. I have the 2 monster Gamo’s but U know what? I wish they all had the Theoben gas ram n em.

  30. Kobus says:

    Hi,

    Very nice article and reply’s. I also have a Crosman NP that I would like to de-tune a litle. Are there no other options for de-tuning the nitro piston on the Crosman?

    1. Jock Elliott says:

      Contact Crosman regarding this.

  31. jesse rapp says:

    i accidentally fired my beeman dual cal QT-GP while still in cocked position and now it wont lock on to th piston any sugestions

    1. Jock Elliott says:

      Jesse,

      Send it in for repair.

  32. brian collis says:

    hi will a gas ram from a hw90 fit a hw 80

    1. Jock Elliott says:

      I don’t think so.

  33. VICENTE CARDONE says:

    Fine article, very informative. Please, let me ask: I have a Hatsan Model 85 Sniper Vortex (air piston) that since new give only 700 FPS (chrony measured) instead the 780 to 800 FPS advertised, however since in my country there are not good service I accepted that issue. Today it lost most power and give only 620 FPS with the same pellet: Crosman HP 14.3 gr. Please can you GUESS what is the most common cause for this issue?

    1. VICENTE CARDONE says:

      Vicente again…I forget to say that the cooking effort do not change, I add this since I do not know if help. I fired some 5.000 pellets and age 10 months. Thanks again.

  34. robert says:

    I’ve head that the gas ram is hard to cock.

    .

    If I could, I’d have a spring gun which I could cock………then release the spring just before I shoot, it would compress a ball of air, ………w/o releasing it !

    Then when I wanted to shoot, I could pull the trigger release the pre compressed ball of air, push the pellet, and get an instant shot.

    No wiggles, no jiggles….

    That’s the best way to do it by my mind, but nobody builds a gun like that.

  35. Thomas Hull says:

    Is it possible to buy a adjustable gas piston and get dimensions of different pistons?

    1. Airguns of Arizona says:

      We do not offer such a part at this time.

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