Mike Morton examines airgun powerplants and velocities – and reckons that if you feel the need for consistent speed, you’d better be prepared to break out a notebook and pencil when shooting a PCP
The other day I overheard a discussion where people were talking about airguns, during which someone compared them with “real” guns, meaning powder-burning firearms. They weren’t trying to be demeaning towards airguns, but clearly felt there was a significant difference between the two. They’re right – and that difference is power, just not in quite the same way they were meaning.
What’s the real difference between an airgun and a powder-burner? It’s not so much the raw power, which is what the person who made the “real” guns remark was referring to. It’s the way that this power is stored and delivered.
Let’s take a quick look at a centrefire rifle. This uses a cartridge, made up of a case, a bullet, a measure of powder and a primer. When the trigger is activated, the firing pin strikes the primer, causing a small detonation at the rear of the case which in turn creates a massive detonation of the powder, building up pressure and sending the bullet on its way. The power supply for that bullet is therefore contained within the cartridge itself, without which a centrefire rifle is just an inert lump of metal. It’s a similar idea for rimfire rifles, too.
In the case of an airgun, the power supply is contained within the gun itself, and that power is present with or without a pellet being anywhere near the breech, making an unloaded airgun potentially more dangerous than an unloaded powder-burner. It goes without saying that both types of gun need to be treated with the utmost respect. Both are very much real guns.
Airguns get their power from a variety of sources. CO2-powered guns date back to the 1870s, and nowadays usually operate with the use of 12g or 88g aluminium capsules filled with compressed carbon dioxide. CO2 definitely has an important part to play in powering many of the airguns we use today, but is subject to variations in temperature and worthy of a more in-depth look another time.
Spring-powered airguns – whether break-barrel, sidelever or underlever – work by compressing a spring during the cocking phase. When the trigger is activated the spring is released, pushing a piston forward and creating a sudden compression of air, driving the pellet out of the barrel.
Gas rams – also known as gas springs or gas struts – operate similarly to coil springs, containing a chamber built into the piston full of pressurised air or nitrogen. When the gun is cocked, the gas is pressurised still further; then when the trigger is activated the gas is allowed to expand and the piston is forced forwards. As long as the rifle works well, both springers and gas rams offer good consistency of power from shot to shot.
While all the powerplants described above are examples of pneumatic technology, using air or other gases under pressure, we now come to the pre-charged pneumatic, or PCP. This type of rifle is often associated with new technology, but is in fact the very oldest type of airgun tech, with a bellows airgun in a museum in Sweden dating back to around 1580.
Self-contained pneumatic rifles may offer an onboard charging facility for each shot, requiring one or more strokes of a built-in handle or lever, but the PCP is different because it contains a high-pressure reservoir which is filled with compressed air from an external source, such as a pump or scuba tank. This reservoir allows multiple shots to be taken from one full charge of air before the pressure has depleted too much for any further shots to be taken accurately.
It’s this stock of high-pressure air that is both the PCP’s main attribute and its biggest drawback. With the reservoir filled, the rifle is all ready to deliver multiple shots with no discernible recoil, unless you are shooting at power levels of around 80 ft lb or more. PCPs are not hold-sensitive, meaning they can be shot from a variety or stances and off bipods, bags or even hard surfaces with no, or very little, change in point of impact.
But they have a power curve.
In a perfect world, the pressure released for the first shot of the available string would be exactly the same as the last, but the world is imperfect and there will be some fluctuation within that string. PCPs may be regulated, unregulated or electronic, with an unregulated PCP showing the greatest fluctuation around the average power output per shot. As Simon Everett points out in this issue’s Airgun Answers (see page 67), you need to find the maximum and minimum fill pressures that deliver the minimum variation between shots. This is often called the ‘sweet spot’, but Simon refers to it as the ‘sweet sector’ or ‘sweet zone’ because it’s a range of pressure, not a single point, that will deliver the most consistent, and therefore the most accurate, shots.
Regardless of the type of PCP you shoot, you therefore need to work out a number of things: the maximum pressure you should fill to, the minimum pressure you should shoot to and the number of good, consistent shots you will get per fill of air.
This information is not only useful when determining your rifle’s accuracy, it has a more immediate and practical use as well. I remember when one of my shooting buddies failed to work out how many shots he’d get from one fill of air, but carried on shooting regardless, going well below the minimum level. Even though he was aware that something wasn’t quite right he continued firing – ultimately to find no fewer than nine pellets stacked up inside his barrel because the pressure had been too low to propel them out! My, how we all laughed!
So how do you go about finding the sweet sector? It all depends on whether or not you have access to a chronoscope. Depending on the size of your PCP’s air reservoir and air efficiency, you’ll also need access to plenty of pellets. If you do have use of a chronoscope, such as a Skan or Shooting Chrony, then the best way to do this is to fill your rifle to the recommended working pressure, shoot, measure each pellet’s velocity, record it and look for the variation.
For example, I filled my Daystate Saxon – an unregulated rifle – to 210 bar and fired 220 shots. Velocity was a little inconsistent from the full 210 bar, but settled down nicely after the first 60 shots, after which the rifle delivered no fewer than 160 totally consistent shots before velocity started to look a little inconsistent again. I now know I can get up to 160 solid shots from a single charge of air. More chrono experimentation would find the optimum fill pressure, but for now I’m maxing out at 180 bar.
If you don’t have a chrono then don’t worry. There’s a method that is perhaps even better in delivering real-world data – it just means you won’t be able to record any velocity figures. And what is that method? Shoot paper targets. A lot. Pick your chosen zero range and set out your choice of target. Shoot a series of five-shot groups and see whether there is any variation in point of impact at the start of the string, and when POI shifts beyond your acceptable variation towards the end. This method is more mentally demanding than using a chronoscope, because even off a bipod or other rested position you are relying on POI and group size to give you the data you’re looking for. That means you need to concentrate to ensure any change in POI is down to a change in velocity rather than user error.
I used my old Daystate MK3 RT, which is regulated, but I cut the distance to just 25 yards to account for the wind speed. After rezeroing I proceeded to plug away at a series of 10-bull targets, shooting five-shot groups. After 150 consistent shots POI shifted for the next five, and shots 160 to 165 started to open up, indicating that I’d taken more shots than the rifle was actually capable of delivering consistently. But I now know I have 150 solid shots from a full charge. Job done!