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Education / Training Gas springs of the Theoben airguns: Part 2!

Gas springs of the Theoben airguns: Part 2!

by B.B. Pelletier

I’ll try to finish the topic and address any concerns today. Our first question from Markus concerned leaking, so let’s go right to reliability.

Markus saw the graphic and understood that the gas spring compressed its internal pressurized gas even more when the gun is cocked. But I didn’t put any numbers to the graphic, so today I will. These are not the correct numbers, but they are representative of the relationships between high and low pressure (cocked and uncocked). If the uncocked pressure is 500 psi, the cocked pressure might be 900 psi. Markus was concerned about leaking at the higher pressure (cocked), but, as you can see, the spring’s internal pressure is always high. The difference between uncocked and cocked isn’t that great. And, they do leak! Anything with pressure leaks in time, but let’s look at this realistically. The gas springs that hold open the back deck on a minivan will work reliably for about 8 to 10 years. By the eighth year, they’re showing their age. By year 10, you probably have to help them open the deck. Those gas spring units are made much cheaper than the units that go into airguns. Also, a minivan’s gas springs are under full compression 98 percent of the time, where an airgun’s spring is just the reverse.

An airgun gas spring should operate reliably longer than a decade and perhaps two full decades. However, there will be defective units that leak down right from the start, just as there are coiled steel mainsprings that fail within the first 50 shots. Those are the exceptions. My Theoben Fenman held reliably for 10 years, and my Crow Magnum held for the six years I owned it. A Vortec gas spring (no longer made) I put into an HW 80 is still going strong after 9 years.

Owner abuse
You just can’t get away from it, if one is good, two must be better! It’s the American way. When Theoben first began selling their guns in America through Air Rifle Specialists, they had a Schrader valve in the rear of the action that allowed the owner to modify the gas pressure. They also sold a “Slim Jim” pump to let owners make this modification. I bought the pump, and I also read the instructions that said the rifle (a Beeman Crow Magnum, which is essentially a Theoben Eliminator) came to me with the gas spring set at its maximum. They did that over a chronograph, increasing the pressure until the velocity stopped increasing, then releasing pressure until the gun shot its fastest with the lowest pressure possible. The owner was only supposed to RELEASE pressure to make the gun easier to cock. Things didn’t work out that way. American owners began increasing the pressure of their gas springs, convinced they were also increasing velocity. Heck, the rifle was MUCH harder to cock, so it had to be more powerful, no?

NO! At first, the power remained stable, but the increased heat from the more rapid air compression of the now faster-moving piston started vaporizing and melting the piston seal. As the seal melted away, a deep pit formed in the center and the compression it was able to develop dropped. Shooters countered this by adding more pressure, which only speeded up the destruction of the piston seal. When I got my used Fenman, it took 75 lbs. of effort to cock, yet it delivered less than 12 foot-pounds of muzzle energy. I sent it off to be repaired, and it came back cocking at 38 lbs. of effort with 12 foot-pounds of energy – exactly on spec.

To counter the large number of guns sent in for repairs, Theoben stopped selling the Slim Jim pumps. They eventually removed the easy access to the Schrader valve, as well. However, there are a large number of Theoben gas spring air rifles in this country that have been slowly beating themselves to death like captive impact hammers over the past 10 years. Be VERY careful when buying a used Theoben! The interesting note to my Fenman story is that the gas spring itself never leaked – in spite of being over-pressurized all those years.

Gas springs are hell on pellets!
This is a topic I’ve never addressed, but all spring guns are much harder on pellets than CO2 guns or pneumatics. Although a spring gun operates on very little compressed air, it generates that air blast instantly and delivers a hammer-blow to the pellet in the first few micro-seconds. Powerful spring guns are known to deform the skirts of lighter pellets by blowing them out flat into the sides of the barrel. Well, gas spring guns are the worst for this, and the Eliminator/Crow Magnum is at the top of the pile when it comes to destroying pellets! You have to use a tough pellet that has a thick skirt – enter the Crosman Premier. Remember how I said Premiers are made of a hard lead alloy? It may be bad for leading the bore, but it handles the powerful Theoben gas springs like no other pellet. Fortunately, it is also a world-class pellet, so you are in tall cotton if you shoot Premiers. Just remember to clean the bore with JB Bore Paste like I told you. Beeman Kodiaks/H&N Baracudas are another good pellet because of their thick skirt, and because their greater weight is more suited to the power of this gun. Also, they come in .25 caliber, while Premiers do not.

Gas springs are hell on scopes, too!
The Theoben Eliminator and the Webley Patriot share the unenviable reputation as world champion scope-breakers. Forget the .700 Nitro or the .50BMG! They’re wusses compared to these two! [I only mean that from the standpoint of breaking scopes. Obviously, these two calibers will knock anyone into the middle of next week!] I can live with the Patriot because Webley allows me to choose my own scope mounts, but Theoben provides their Dampa mount on the gun, and little else will fit. Unfortunately, the Dampa is not adjustable, but fortunately most Theobens don’t need much scope alignment. And, as frail as it might appear when you first see it, I have never had a bit of trouble with a Dampa mount. You are limited in scope placement and size because the Dampa doesn’t move. You wouldn’t want too heavy a scope on an Eliminator anyway – too much mass for the mount to control.

Other issues
The other gas spring issues are petty compared to these, but they are worth considering. The first is maintenance. When Air Rifle Specialists imported them, Davis Schwesinger did all the repairs for the guns he sold. When Beeman took over, they did parts replacement, but Schwesinger still repaired the actual gas spring units. However, Theoben has now changed hands and I don’t see the same relationships in place anymore. All that support is now gone, and who knows what has taken its place? So if you buy a Theoben, new or used, you might have to maintain a relationship with the company in England. If you buy a Beeman RX-2 you can expect that Beeman in California will back it up. No, it isn’t exactly the same as a Theoben, but it does have a Theoben gas spring inside.

The last word
I have avoided the history of gas spring airguns. Theoben did not invent the concept – it was pioneered by an Argentine airgun maker and Theoben improved on it. They hold patents on their designs, but there have been other gas springs contemporary with them. The Vortec gas spring was a drop-in conversion for the Beeman R1 that made it a little more powerful and a lot harder to cock. It was around for about five years, but has disappeared from the scene. Vortec actually did make a gas spring conversion for the HW77 and for the Diana RWS sidelevers, but these have gone, too. To my knowledge, there has never been a gas spring pistol – probably due to excessive cocking forces. That’s about all I can tell you.

author avatar
Tom Gaylord (B.B. Pelletier)
Tom Gaylord, also known as B.B. Pelletier, provides expert insights to airgunners all over the world on behalf of Pyramyd AIR. He has earned the title The Godfather of Airguns™ for his contributions to the industry, spending many years with AirForce Airguns and starting magazines dedicated to the sport such as Airgun Illustrated.

49 thoughts on “Gas springs of the Theoben airguns: Part 2!”

  1. B.B.
    Thanks for a very informative and interesting set of articles on Gas Springs. At the moment, I have three air guns, a Sheridan 20 cal, a RWS 52 .177 and a IZH 46M in .177. Now that you have given me a much better understanding of the metal Spring gun and the Gas Spring, I would like to better understand how a Single Stroke air gun like the IZH works. It seems it cannot be “pumped” up beyond one stroke. It there a release valve? It seems to me that the single stroke is almost an ideal mechanism for an air gun but most air guns use a different mechanism (spring, multi stroke, gas….). If I look at a diagram of the Sheridan and the IZH, the mechanisms seem very similar. The Sheridan requires multiple pumps to charge it and the IZH seems to do with one single stroke. Why are there not more single stroke air guns on the market? Also, the pumping effort per stroke on the Sheridan and IZH seem about equal. BTY, I bought my RWS from Davis Schwesinger and he has been a gold mine of good information on the gun and other things having to do with air guns. Clearly, with respect to the single vs multi stroke air guns, I am missing something.

  2. PCR,

    The pump head of a single-stroke mechanism is what holds the compressed air for the shot. It is the “inlet valve,” if you will.If you open the pump handle for a second pump, you lose the first one.

    The single stroke is limited by the power it can generate. Tom Gaylord just did a column in Shotgun News about a man who has invented a new type of single stroke mechanism that can generate double the power with LESS effort, but nobody will give this guy the time of day! You can read it by going to the Shotgun News website


    and scrolling to the bottom of the first page. The title is “What if Nobody Cared?”

    Single strokes are apparently at the bottom of most airgun manufacturers’ lists.


  3. B.B.,
    I’m apparently still in the 2nd stage toward airgun Nirvana so after reading the past two posts I am drooling after a “gas spring” gun since I do not have one. It will be a while before I can scrape together the $$ but do you think I would do well to stick with the RX2 rather than a genuine Theoben? The reviews seem to think the Beeman gun is a beauty and the only hesitation I feel regards the cocking effort. I’m in my mid 70’s and have lost some muscle power but I have noted that regular springers tend to ease up after continued use so maybe I could hang in there for a few thousand rounds. Do gas springers ease up too?

  4. I’ve noticed the skirts on my jsb’s domed,jsb predator, and my crow magnum blown open abit in my 1250 gamo, but I figured this would help the pellets ride the rifleing better and be more accurate. Will this effect the pellets even though there at least to me considered medium weight pellets?


  5. BB,

    Wich airguns have the best triggers in the worls?

    And,is a R9 gold finger more accurate than a hw77 or hw97?


    How can I get more foot pounds in a .22 cal rifle?

    Does pyramid air carry venom upgrades?


  6. CWI,

    Gas spring rifles unfortunately do not become easier with time. The only way to make the cocking effort lighter is by releasing some of the stored gas pressure.

    As for sticking with the RX-2, I’d say if it’s delivering the accuracy you want then it’s the gun to stay with. Theobens are very pretty and their stocks feel quite natural, but the RX-2 has the nicest stock of the entire Weihrauch line. Except for the beautiful wood, I’d say you are there.


  7. Unidentified reader,

    You’ve asked a lot of questions, so I have copied them into this comment. I’ll answer each in turn.

    Q-Wich airguns have the best triggers in the worls?

    A-I think the best trigger is found on the FWB P70.

    Q-And,is a R9 gold finger more accurate than a hw77 or hw97?

    A-No, the R9 has the same POTENTIAL accuracy as the HW77 and 97, but the underlevers are easier to shoot, so they will usually come out on top.

    Q-How can I get more foot pounds in a .22 cal rifle?

    A-To boost power in a spring rifle you must reduce friction and vibration space in the powerplant. To boost power in a gas or pneumatic gun the barrel must be longer and the valve has to be timed to remain open longer. Larger port sizes with very smooth entrance surfaces will also help.

    Q-Does pyramid air carry venom upgrades?

    A-No. Venom upgrades are sold by the Webley Custom Shop.


  8. BB,

    wich is the best trigger in a spring piston air rifle?

    Wich air rifles would you recommend from the gamo line?

    Can you give me how many foot pounds does the hw77k U.S version has?


  9. Venom had to close when Webley went down and the Webley brand is now owned by AGS (which also owns Logun).
    Steve Pope, formerly Venom, recently announced his new tuning company, V-Mach Custom Rifles Ltd.

    “The company is currently dedicated to air rifles only. Products and services include high standards of tuning, custom work, servicing, spares and repair work, stock refinishing, action refurbishment, reblueing and a range of accessories for most quality makes and models. Servicing for all previously tuned Venom rifles will still be undertaken.”

  10. BB,

    In this blog you mention a company named Vortec. Is this the same company that used to make the Lamprey hollow piont pellets? I wish another manufacturer could get the dies for theese upside down wonders and begin making and selling them again! Or maybe they do, and I don’t know. Do you have any information about theese pellets? Thanks.


  11. BB,

    I did some research about bipods for underleveler air rifles and only found the JSR PRO BIPOD.You can find them at

    The problem is that its in UK.I searched but didnt find any store selling this in the US.I dont know how to order from that website and I dont know how much they cost because they dont say it in $$$.But another symbol.If you can find or allready know of a store that sells these please tell me.If I can buy it I will since the cf-x isnt to hold sentitive maybe it wont affect it accuracy.Hey!!! maybe it could aid my accuracy because the cf-x likes to sit in sandbags and I shoot mine with a hard grip for better accuracy.Weel,BB or anyone that knows of these Bipods please tell me.

    CF-X guy

  12. B.B. Pelletier, I see that gamo has afew new rifles comming out and I’m thinkingf about buying one for rabbit and squirrel hunting. Would you reccomend one or a precharged? If so wich brand? And what caliber should I select, .17,.22? Any info you can give will be very helpful.

  13. magnum,

    For hunting I always recommend .22 caliber. I really like the CF-X, but it isn’t being imported in .22 caliber yet.

    There is a big price difference between a Gamo spring gun and a precharged rifle. I would always recommend a precharged rifle over a spring rifle for the reasons of accuracy and power. My choice for squirrels and rabbits would be either a Talon or a Talon SS in .22 caliber.

    I suggest you read some of the back posts we have here. There is more than one year’s worth of postings, plus a lot of reader comments.


  14. I’ll be sure to do that. Have you seen the rifles that gamo came out with? I would think that the hunter extreme would make the pellets tumble doing 1600fps. Their new shot shell for the viper extreme ought to be a new step for pellet rifles.

  15. To the person that gave me the bipod link,

    Hi,and thanks for that.I dont know if it will work because my rifle is an underlever rifle.And there are 2 problems.First is that the underlever rod is thinner and second,it has to be flat enough to go between the barrel and the underlever rod.If this universal bipod does that please tell as ill be buying it.Thanks again.

    CF-X guy

  16. Hunter Extreme,

    The Gamo Hunter Extreme is not a new rifle. it’s the Gamo Hunter 1250 re-named because of their new Raptor pellet. You are correct that the Raptor would be unstable at that speed (1,600 f.p.s.), so you would never shoot that pellet in the Hunter Extreme rifle. You would select the heaviest .177 pellet available and hope to get the velocity below 1,000 f.p.s.

    The Hunter Extreme is better in .22 caliber, where there are pellets heavy enough to keep it subsonic.


  17. BB,
    I’ll keep that in mind. I also had a question befor about the Tech Force 50 have you herd of it? All I know is that it shoots around 900fps with a 15 grain pellet.


  18. B.B.,
    Thanks for the for lookin. I’ll see if I can find out anything more about it. It doesnt look to shabby. I am wondering why its so cheap though. If you find out anything more let me know. Thanks,sav300

  19. sav300,

    The Tech Force 50 is inexpensive because it comes from China. China has a reputation for two bad things. 1. They usually don’t make accurate barrels. They can make them, but most of the time they don’t. 2. They do not use seals of the proper durometer rating. Read that as Leakers!

    I have heard mixed reports about the TF 50, so I can’t really comment on it one way or the other. But because it’s Chinese, I am suspicious until I see the gun tested.


  20. B.B.,
    Thanks for the info. Do you know of any other PCP’s in that price range? I’m still lookin and don’t got enough $$ to buy the talon or talon ss. If you know of an almost brand new or good condition rifle let me know. I prefer it to be new but I don’t think I’m going to find one other than a used. Any info is great B.B.


  21. RUBBISH,

    Crosman pellets have antimony in their alloy. Antimony hardens lead. That’s why I said they are hard, because they are. The thick skirt comment was incorrect – they do have thinner skirts, but again because they are harder, they don’t deform as easily.

    As for Premiers being world class. AAFTA and the Brits proved that at the nationals and the worlds for a decade running. With JSBs now on the scene they are no longer number one, but they are often the best pellets in individual guns.

    I am referring to real Premiers in the cardboad boxes, not the round tins that Crosman now sells as Premiers, too.


  22. yes same premiers that at one time had 4,5% antimony in and leaded so bad that ben Taylor had to use HEAT to clear bores! Prems came at a time when no one else had new dies on offer and as they wore so did the consistency! Tom the tool strikes again.

  23. I am thinking about ordering a Gamo Hunter Extreme but pelting a 5.4 gr. pellet out 1600 fps. results in 30.7 ft. lbs. If this is true then according to my calculations it would require a 11.9 grain pellet to go sub sonic with the same ft. lbs. I know that with heavier pellets the foot pounds often go up. How heavy of a pellet does it really take to make this gun fire sub sonic? Does anyone know of an accurate pellet that will leave the muzle of this gun sub sonic?

  24. Eric,

    You are starting from the premise that the Hunter Extreme can go 1600 f.p.s. I proved that it can’t, except by detonation. It really goes about 1350 f.p.s., which makes a 10.6-grain Beeman Kodiak feasible.

    Read the entire reprt on the Hunter Exztreme.



  25. “Those gas spring units are made much cheaper than the units that go into airguns.”

    I do not want to be intimidating, but is that just your opinion or a real fact from an engineering background? I’m just an amateur airgun nut, but reading some stuff on regular gas struts online makes be believe they are a very refined technology. They have more seals (not just o-rings) and guides in them than the gas rams used in airguns!

    Just because they cost less don’t mean they are “cheaper”. They are manufactured in (I’m guessing) many thousands of tons, not a few (guessing again) ten thousand units each year as gas rams are, so the economy of scale plays a big role here.

  26. OK, are we are sorta talking about different things here? You mention manufacturing quality, I talk about engineering sophistication.

    I never owned a Theoben or a HW90, but I do know what it looks like on the inside from pictures. We have a piston sliding on a shaft made of steel or aluminum. The air is held in place with two o-rings on the inside of the piston. Take a look at the inside of a gas spring here: https://en.wikipedia.org/wiki/Gas_spring , sure some are made of cheap materials, but they are designed to take more abuse that a airgun gas spring that probably has a very sedate life (axial load only) compared to the industry type.

    One thing that I’ve not noticed on the Theoben/HW90 that I expected to see is some kind of hard coating on the shaft that the piston slides on. From the pictures I’ve seen it seems to be anodized aluminum with just a fine turned finish. Not ideal on very fast moving sliding surfaces that must seal properly.

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