How spring-piston rifles behave
by B.B. Pelletier
Okay, Grasshopper, enough Wax on! Wax off! It’s time to use your skills.
If you’ve been following the discussions over the past month about accuracy, you should now have the tools to be a pretty good judge of the potential accuracy of an air rifle and the relative ease with which that accuracy comes — even before taking the first shot. We’ll confine today’s discussion to just spring-piston guns, since they’re the most difficult to shoot.
How a spring-piston airgun works
This is a review for many of you, but we have enough new readers that perhaps it’s good to go over the points of how the spring-piston gun works. What I’m about to say holds true for guns with gas springs as well as guns with coiled steel mainsprings. They all work the same when it comes to their operation.
When the sear releases the piston, the piston starts moving forward rapidly at 50-60 miles per hour or 73-88 f.p.s. Unless there’s something like an anti-recoil mechanism to prevent it, the gun starts moving in the opposite direction. Since the piston weighs but a fraction of the weight of the whole gun, the gun’s movement is very slight.
Within a few hundredths of an inch of the end of its travel, the piston has compressed the air in front of it as high as it will ever go…given the piston diameter and length of the piston stroke. Due to this compression, the temperature of the air has also increased to a very high point. The piston wants to slam into the end of the compression chamber, but the thin cushion of highly compressed air actually slows it down and can even stop it. The pellet in the breech is sealing the air in front of the piston, and it hasn’t started moving yet.
However, at some point — and that point changes with each pellet used, the pellet can no longer remain stationary. There’s too much force pushing on its tail and it begins to move down the bore. The piston can now go all the way forward and rest against the end of the compression chamber, or it may have done so already and rebounded off the air cushion and now needs to go forward again. Each different type of pellet will determine exactly how this relationship of movement plays out, which is why some pellets feel good when you shoot them and other pellets seem to make the gun buzz and vibrate and even make noises that you may never have heard before.
When the piston reaches the end of its travel, it stops suddenly. When that happens, it imparts a hammer blow to the airgun, sending it in the same direction the piston was traveling. This is the second recoil, and it’s much more noticeable. At this point in time, the pellet is probably between three and six inches down the barrel and the entire gun’s moving.
The movement is in several forms. First, there’s high-speed vibration running through all the parts of the gun. You can’t see this vibration, even on a high-speed camera, but you can feel it. This is the buzz that you feel from some guns, and it can be so sharp that it actually hurts to hold the stock against your cheek.
Next, there’s a lower-speed vibration that’s both larger and much slower. If you had a high-speed camera, you could actually see the various parts of the rifle moving. The pellet is still inside the barrel when this happens.
Finally, there’s the recoil in both directions. Both are visible on a high-speed camera; and the forward movement, assuming we’re talking about a conventional spring-piston setup, is by far the largest. The gun starts moving forward before the pellet leaves the muzzle, but completes the movement after the pellet has gone.
Which spring-piston guns will be accurate?
Simply stated, breakbarrel spring guns are the most difficult to control. They may be just as accurate as underlevers and sidelevers, but they’re almost always more sensitive to the movement of the gun when it fires. That’s not to say that sidelevers and underlevers are not sensitive; but in comparison to breakbarrels, they’re less sensitive.
Let’s stay with breakbarrels for now. The ones with the longest piston stroke have the longest period of time for movement. That includes the high-speed vibration, the low-speed vibration and the recoil in both directions. As a rule, long-stroke spring-piston guns are the most sensitive to hold, and long-stroke breakbarrels are the most sensitive of all.
Then there’s the weight of the piston to consider. A heavy piston causes more rearward recoil when it begins moving and more forward recoil when it comes to a stop. You tend to find heavier pistons in guns with more power.
Put this all together, and you know that a breakbarrel spring-piston rifle that has a long piston stroke and high power will probably be the most sensitive airgun, as far as hold goes. It may be potentially very accurate; yet also be so sensitive that unless the hold technique is perfect, it’ll spray pellets everywhere.
Listen to this!
When I was doing the testing that lead to my R1 book, I tested my .22-caliber Beeman R1 with the factory tune and then with four different custom tunes. One of the tunes — from Venom — increased the power of the 18 foot-pound rifle to 23 foot-pounds, but it also removed nearly all vibration. It was by far the smoothest tune for that rifle. As a result, the rifle became easier to hold and shoot.
I then destroyed all of the mainsprings used in the testing by leaving the rifle cocked for a month with each of them, so the Venomac Mag-80 LazaGlide tune went away. While I had it and used it, I learned that it’s the vibration and not the power of a gun that determines how difficult it is to hold.
That tells us that if the gun is powerful without vibrating, it can be easier to shoot. You might think that a gas spring would give you exactly that, but they don’t always do so. The more powerful gas springs, while smoother than most steel springs of equal power, still vibrate a lot and require compensation with the hold.
What do we know?
If you believe what I’ve said to this point, then you know what it takes for a breakbarrel spring-piston air rifle to be the least sensitive to hold. It must have the following:
- Low vibration
- Short stroke
- Low recoil
Put all of that together and you’ll have a lower-powered, spring-piston rifle. Time for a short story.
Several years ago, I tested a Mendoza breakbarrel called the Bronco, oddly enough, that was very low powered. It had a strange-looking Euro-styled stock with a too-short pull (about 10 inches) and a hideous kidney-shaped cutout in the center of the butt. The stock was firewood, but the action was good. No, it was better than good. It was great!
The gun cocked easily, had a very short piston stroke, a wonderful crisp trigger and an accurate barrel. I proposed to Pyramyd Air that we have this rifle restocked with a western-style stock, like the old Beeman C1 carbine. They agreed, so I found the stockmaker and had the job done.
We then sent the newly-stocked rifle to Mendoza and asked them to create a model that had a similar stock, though with a pull suited to older youth as well as adults and a couple other important changes. Voila! The Air Venturi Bronco that you all know was born. You can call me an airgun designer if you like, but what I really am is someone who knows what it takes to make the right kind of airgun. Mendoza was already making most of it, but they needed prompting to change those few important details that turned their oddball Bronco, which wasn’t selling, into our Bronco, which is now a best-buy. It’s the same gun, with just a few important things changed. Think of it as the Jeep with the V6 engine that everybody loves, as opposed to the same Jeep with the underpowered 4-cylinder powerplant that someone buys because, on paper, it gets two miles per gallon better mileage. In real life, the details matter.
The Bronco is very insensitive to hold for a breakbarrel and as a result, deadly accurate in the hands of almost everybody. Contrast that with the guy who has to have the absolute last foot-second of velocity, so he buys the air rifle that’s guaranteed to make his life miserable — hard to cock, violent when shot and requiring the skill of a concert airgunner to shoot well. He may have some bragging rights; but at the end of the day, the Bronco owner will shoot a lot more and have more fun doing it.
There are many more stories, but I think my point has been made. You now know how to select a spring-piston breakbarrel that will be the least hold sensitive when shot. Now you know why I went bonkers over the Crosman TitanGP (Lower Velocity) that’s a really fine shooter.
On to other springers
Let’s talk about the underlevers and sidelevers. Within these, there are the underlevers that use a sliding compression chamber, like the Beeman HW97K, and those that have a loading tap, such as the Hakim (made by Anschutz). There are sidelevers with loading taps, as well, but they’re not common. Sidelevers usually have sliding compression chambers, like the RWS Diana 48.
For whatever reason, both underlevers and sidelevers are less sensitive to hold than breakbarrels. Of these, the taploaders seem to be the least sensitive of all, though the TX200 Mark III from Air Arms has a sliding compression cylinder and is also very insensitive to hold.
The hold sensitivity for both underlevers and sidelevers does increase as the stroke length and vibration increase. Notice that I didn’t say anything about the power. The TX200 Mark III is very powerful, yet still very smooth and insensitive to hold. I would describe it as having a shorter piston stroke.
The RWS Diana 460 Magnum, in contrast, has a very long piston stroke and does need a lot of hold technique to shoot its best. The RWS Diana model 48 sidelever has a shorter stroke than the 460 Magnum and is also less sensitive to hold.
It seems that the same things that drive the hold sensitivity for breakbarrels also affect underlevers and sidelever guns. It’s just that these types of airguns start out with an advantage over breakbarrels in the sensitivity to hold.
What does that leave?
I have not discussed any of the other types of spring guns, such as the overlevers (they act just like underlevers) or those that cock via a lever that works in a different way, like the Haenel 310 and the VZ 35. All of these airguns are low-powered enough that they have good characteristics to begin with; as a result, they don’t cause any of the hold problems we’ve discussed.
To this point, I’ve said nothing about the quality of the barrel, the breech lockup, or the overall fit and finish of the working parts of the powerplant. These items do affect the performance of an airgun and will break your heart if they’re not taken into account. Some air rifle barrels, for instance, look like 40 miles of rough road and will never deliver pinpoint accuracy no matter what’s done to the rest of the gun. Some barrels are crooked from the factory and can never be fully straightened. You can put lipstick on the pig, but that won’t change its manners!
The bottom line
What all of this means is that no one has to go into the airgun selection process blind. If you can determine the three important characteristics I’ve discussed here — vibration, piston stroke and recoil — you can generally know how difficult it will be to shoot each airgun well.
If you want to hunt with your new rifle, then by all means pick one that has plenty of power. But choose it to use it! Now that you’ve been informed, don’t buy a mega-magnum spring rifle, then whine that it’s too difficult to cock or too hard to shoot accurately.
Many of the veteran readers on this blog seem to keep harping on the low-powered springers for a reason. Guys like Kevin and others keep going back to rifles like the Beeman R7 and the HW50S because they know what wonderful shooters they are. Don’t kid yourself that these guys are not experienced with the powerful springers, too. Most of them have tried the big guns and found they didn’t enjoy all that it took to make them do their jobs.
There’s a place for the RWS Diana 350 Magnum and the Walther Talon Magnum, but some thought has to be given before purchasing either of them or any other spring-piston air rifle of equivalent power. Both rifles are built for a specific purpose, which is hunting. They’re hard to cock and take a lot of technique to shoot to their potential. Neither rifle is the best choice for a first airgun for someone who is either new to airguns or new to shooting altogether.
I hope this report helps some of our newer readers narrow their selections of possible air rifles to purchase next. As always, there will be exceptions to what I have said, but they only serve to prove the general rule.