Nick Stanning shows how a spring-powered rifle fires its pellets – and how to take care of the components that make it possible
Before we can really look at ways to improve a spring-powered air rifle, we must first understand the basics of how it works. Here we will go through the major components, and what happens between the moment you squeeze the trigger and the pellet exiting the barrel.
To demonstrate that the principle is the same across the board, rather than use some exotic, expensive top-of-the-range hunting or target rifle, I will be using a cheap garden plinker that I bought for under £100! It’s the AirForceOne AFO-16, which I got from The Shooting Party in Staffordshire.
The AFO-16 appealed to me as it is quite unique. It’s a sidelever bullpup design, which ever so slightly resembles an M-16 if you squint. It’s great fun for tin cans, but it is a bit smoky…
How it works
There are two main types of spring-powered air rifles on the market today. The first is the break-barrel: this is cocked and loaded by pulling the barrel down and compressing the spring until it latches onto the trigger sear, where it is held until the gun is fired.
In contrast, lever-actuated air rifles use a lever on the side, or underneath the gun, to compress the spring. Many of these have a sliding compression tube, like those in the AFO-16 and the HW 97.
Once the gun is cocked, you have a set amount of air ahead of the piston in the compression tube. The power potential of any air rifle is largely limited by the swept volume of this chamber. It doesn’t matter how big a spring you cram in: if there’s not enough air to push out, the only thing that will increase is the recoil.
So now you have the potential energy of the spring and piston to the rear, a compression tube full of air in front of that, and a pellet sat in the barrel waiting for the air to come through the transfer port to push it out. On firing, the first thing that happens is the energy from the spring pushes the piston forward, and the rest of the gun backward. This is the initial phase of recoil.
The piston travels forwards, compressing the air ahead of it – which cannot escape through the narrow transfer port, as the pellet is blocking its path. During this compression phase the air temperature can reach in excess of 400°C, and the pressure gets so high that any oil or grease ahead of the piston seal will detonate in the same manner as a diesel engine. Unsurprisingly, this phenomenon is called ‘dieseling’. This is bad for the gun, its components and, most importantly, your accuracy. To avoid this, always degrease the compression chamber and use lubricants sparingly.
So the piston has travelled forward, the air in the chamber is under extremes of temperature and pressure, and eventually the pressure overcomes the pellet’s friction and the pellet starts to move. The size of the transfer port is crucial to attaining the correct pressure: too big or too small, and power will drop dramatically.
The pellet starts its forward movement, which relieves some of the pressure in the compression chamber – but not enough! The increasing pressure coincides with the lessening force in the expanding spring until the point where there is more pressure in front of the piston than behind, the piston comes to a sudden stop and ‘bounces’ off the cushion of air and heads back up the tube toward the trigger. This pulls the rifle forward, and is known as ‘surge’.
If you’ve been following carefully, you will know what happens next: the spring pressure overcomes the air pressure and the piston goes forward again. Another rearward recoil. This may happen more than once in a firing cycle.
The other recoil happening is called ‘torsional’ recoil. A spring will wind and unwind as it is compressed and expanded. This can cause the gun to twist as it fires, and is most obvious when using a higher-magnification scope at distance. The simplest cure for this is a good-fitting spring guide and a top hat with a slip washer. This gives a low-friction bearing for the spring to rotate on.
All of this happens very quickly – in fact, it happens before the pellet has exited the barrel. The time taken from squeezing the trigger to the pellet exiting is called the ‘lock time’: the longer the lock time, the worse the hold sensitivity, which is what makes the rifle shoot different points of impact if you change your hold or your grip.
All of these shenanigans going on inside explain why it is so important to maintain your gun; making sure all the parts fit together well, all surfaces are smooth and correctly lubricated.