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.
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.
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.