There are many Weihrauch aficionados who’ll tell you the earlier HW77s – those with a 25mm diameter piston – are better than the later, 26mm versions. That 1mm, they say, makes all the difference because a weightier piston upsets the HW’s firing cycle. The average shooter probably won’t notice it, but to those of an engineering persuasion, the change is food for thought – and one airgun technician, Tony Leach, has explored the concept more than any other. After much research and development, he’s found that the relationship between piston diameter and weight can have a huge bearing on a spring-powered air rifle’s efficiency.
In simple terms, an air rifle’s efficiency is calculated from output versus input – or, in layman’s terms: how much power you get in relation to the cocking effort required. Normally, a sub-12ft/lb .177 springer is less efficient than a .22; smaller calibre springers are around 27 to 30 per cent efficient, while bigger bore guns are in the region of 30 to 35 per cent.
However, what Tony’s discovered potentially rewrites the ‘airgun design book’ – and some of his experimental set-ups have returned efficiency figures in excess of 50 per cent! Ironically, the major air rifle manufacturers started getting close to the ideal set-up back in the 1980s and 90s, when ‘tuning’ was all the rage. But they’ve since strayed off the path, producing rifles with larger diameter pistons and longer strokes. This, says Tony, is why the early, sub-12ft/lb Weihrauch HW77s are in such demand among ‘those in the know’!
The concept behind Tony’s experiments is ‘cylinder sleeving’ – reducing the inner diameter of the air chamber to allow for a smaller diameter, lighter piston. Tony recently invited me to ‘feel’ the difference it’s made on one of his HW77 rifles, sleeved down to take a 23mm diameter piston.
It’s not something Tony’s discovered overnight, mind you – he’s been involved with airgun tuning for decades. In that time, he’s realised that a lighter piston speeds up an airgun’s firing cycle and reduces recoil into the bargain. While this has resulted in Tony tuning up some fabulously sweet HW77s, it was the oversized HW80 – with its 30mm piston – which turned his inquisitive mind to cylinder sleeving.
Tony sleeved an HW80’s cylinder down to an internal diameter of 25mm, and shortened the piston stroke in a bid to speed up its lock time – and the result was a highly accurate break-barrel with barely any recoil.
Intrigued, he undertook a comparable exercise with the Diana 52 sidelever springer, sleeving its cylinder down to 25mm from the original 28mm. Again, with a shorter stroke, it returned similarly impressive results as Tony had witnessed on the HW80.
But it was while Tony was experimenting with his own Diana 54 AirKing one day that he had one of those Eureka! moments. The AirKing is a semi-recoilless springer where the action runs on a sledge to negate the ‘felt’ recoil at your shoulder; effectively, the whole action slides back around 18mm toward the shooter on firing.
But when he was using one as a test bed for his sleeving, Tony found he could reduce the movement by over half, to just 8mm without losing any power. It was a real eye-opener, and formed the foundation of his experimentation going forward.
His next development was to incorporate a rotating piston – one that was ‘skirtless’ and so really light – with just an O-ring as the main compression seal. The concept of the O-ring seal was to keep friction to a minimum. For the same reason, Tony also machined a new piston from phosphor bronze, which has self-lubricating properties and isn’t susceptible to temperature changes.
With a 22.93mm diameter piston running in the 23mm sleeved cylinder, Tony’s HW77 test bed returned an efficiency figure the like of which he’d never imagined in his wildest dreams – 57 per cent! It was full-powered, smooth-firing and easy to cock by virtue of the fact it could run off a really light mainspring. In fact, performance was so good that the rifle even produced a muzzle energy of 9ft/lb without the sealing O-ring in place!
As the experiments unfurled, Tony found that the piston’s optimum stroke length seemed to be between 80mm and 85mm – that’s what returns the smoothest firing cycle. Although Tony reckons such a downsized piston can generate 12ft/lb of power with a shorter stroke, he’s found that the rifle then becomes very harsh to shoot.
He’s also been able to determine that the piston diameter is inextricably linked to both the diameter and length of the transfer port – the small hole through which the air is channelled from the main air chamber to the back of the pellet on firing.
What Tony’s discovered is that with smaller pistons, the transfer port can be enlarged to improve the airflow – which itself is why the mainspring pressure can be softened right down. Another benefit of using a lighter piston is that it needs less of a cushion – normally provided by the piston ‘bouncing back’ violently on a build-up of compressed air toward the end of its stroke. This, says Tony, is another reason why his sleeved springers feel so smooth to shoot – there’s basically less mass coming to a sudden, abrupt halt.
Through hours of painstaking research, Tony’s found still more efficiency gain courtesy of a shorter transfer port – and though he’s keeping his cards close to his chest on the exact dimensions, he is happy to say that he’s also shaped the inside face to aid airflow.
The result of all this is one heck of a smooth firing cycle – and although trying to get across the feel is akin to reporting darts on the radio, I can promise you that all Tony’s tuned springers shoot with hardly any movement.
I’d be lying if I called them ‘recoilless’, but their ‘felt’ recoil was so minimal that I could, through the scope, watch a full-powered, .177 pellet travel all the way to a 50-yard target! What’s more, the sight picture never blinked and the scope’s crosshairs remained on target during the follow-through as though I’d fired a PCP. Oh, and the rifle’s underlever was about as easy to cock as a PCP’s bolt, too!
Actually, through his discoveries, Tony reckons he’s now in a position to design a spring-powered air rifle that is simply not capable of ever being taken over the limit, regardless of what size spring you put in it. It would appear Tony has cracked the code for piston weight, stroke length, air chamber bore size and transfer port dimensions. In terms of airgun efficiency, it could very well be the perfect formula…