The inefficiency of a standard knock-open valve results in some PCP guns having a power curve over the duration of their air charge. However, Phil Bulmer looks at how one engineer solved it for British PCP gunmaker, Daystate.
Shooters of precharged pneumatics (PCPs) will have heard the term ‘power curve’ – the rise and fall of the pellet’s power output as the rifle cycles through its usable charge of air. These days, some PCPs are fitted with air regulators that level out any curve – and other manufacturers have perfected the design of the main valve such that they can achieve an extraordinary number of shots with minimal power variation. The diagram above explains this graphically.
The modern-day PCP became ‘mass market’ in the late 1980s. Back then, they had a simple knock-open valve – where the pulse of air from the main air reservoir came courtesy of a tap of a valve by a spring-driven hammer. A problem with this system is that the hammer hits the valve and continues to bear down on it, so more air is released than is actually needed.
Strictly speaking, we now know that a hammer bounces five or six times before coming to rest. While the first valve strike typically sends the pellet on its way, subsequent (bounced) strikes simply exhaust more air unnecessarily until eventually the pressure in the main air reservoir, behind the valve, is enough to overcome the hammer’s inertia and reseal the system.
So an unregulated, knock-open system is reliant on the internal pressure of the main air reservoir to slam it shut. But the fact is, the opposing forces of hammer and air pressure don’t always make for a happy marriage – and this can result in significant losses in efficiency.
While an internal regulator can design out this relationship, there’s no denying that a ‘reg’ adds a further complication to the system – and like any high-performance kit, having ‘one more thing to go wrong’ isn’t to everyone’s liking. They’re good when they work… but many a field target competition has been lost owing to a ‘mechanical’ on the part of a regged rifle.As the pioneer of the modern-day precharged pneumatic, Daystate has approached the problem of hammer bounce in quite an uncomplicated manner with its slingshot system . It not only leads to a flattening out of the power curve, but also increases the rifle’s shots-per-charge-count through the more efficient use of air.
Slingshot is the brainchild of airgun engineer Steve Harper. It’s actually patented and Daystate has been fitting Steve’s system into all its non-electronic models since 2008 – yet many Daystate owners aren’t aware of it. Even if they are, few admit to understanding it – which I think is a crying shame given what an amazing feat of airgun engineering it is. Buy an Air Ranger, Huntsman or Wolverine model today, and it’s part and parcel of your rifle’s action.
Put simply, the slingshot system involves a two-part hammer – an inner and an outer, if you like – where the hardened firing hammer (which strikes the main air valve) sits within a cage  in which it’s free to move. Once released from its cocked state by operation of the trigger , the whole assembly travels forward by way of the hammer spring, which drives it toward the main air release valve.
When the cage reaches the valve housing, it stops dead . However, under inertia, the hammer inside – which has a slightly recessed face – continues to travel on , until it then taps open the valve to release the pulse of air that ultimately drives the pellet out of the breech and along the barrel. The valve is then closed by a combination of the air pressure behind it, and its return spring.
Here’s where the slingshot gets clever, though. An independent anti-bounce spring then returns the hammer back into its cage; unlike a conventional hammer, it never comes to a rest on the head of the valve stem .
And neither does it get the chance to bounce back and forth on that valve, unwittingly opening and closing it to release unnecessary pulses of air long after the main pulse has driven the pellet. Ergo – air volume is controlled to maximise consistency and shot-count efficiency.
Translated into figures, Harper’s design brings practical gains of around 40 per cent improvement on air consumption – and although the flatter power curve resulting from the better air release isn’t quite as good as a regulator, it’s not far off and comes courtesy of far less complicated componentry.
Daystate’s tests have also shown that Harper’s slingshot system is good in PCPs producing muzzle energies up to 30ft/lb, too. Pernickety shooters may notice that there’s a slight increase in cocking effort (as there’s a second spring in there), but it’s nonetheless a significant gain in performance for a modest amount of cost that won’t hit the consumer’s pocket quite so hard.