Whiscombe rifles and barrel harmonics - Part 1
by B.B. Pelletier
Whiscombe JW75 breakbarrel. I know this is a tiny picture; but in the coming reports, we'll see all the details of this exemplary air rifle.
This report has been a long time coming, but now its time has arrived. It will be a general exposure to Whiscombe air rifles with a discussion of barrel harmonics, because Whiscombe was the only airgun manufacturer who ever sold a system to tune them. I'm doing this report now because of the RWS 54 report. There are several similarities between the two rifles that I think are best discussed when both are in front of us.
Dual opposed pistons
John Whiscombe's chief claim to lasting fame in the airgun world was the invention of dual opposed pistons that increase power and eliminate recoil in a spring-piston air rifle. The pistons are opposed, which means they're facing each other. When the gun is cocked, each piston is pulled back against its own separate mainspring. When the gun is fired, the pistons come together...like the clapping of hands. Instead of a closed compression chamber, each piston acts as the compression chamber for the other piston. They come together at the air transfer port, so all the compressed air is squeezed out the port and behind the pellet.
The challenges were the timing of the pistons, getting both to release at the same time, and building a cocking mechanism that applied equal force to two pistons going in opposite directions. Whiscombe solved all these problems and wound up with exactly what he set out to create, a spring-powered rifle that shoots like a PCP.
Every Whiscombe model has a designator that tells you exactly what the rifle is. The initials JW stand for John Whiscombe and the number that follows is the separation of the pistons in millimeters when cocked. So my JW75 has a 75mm piston separation. Greater separation means more swept volume, which means more air to compress, resulting in more power. Other models were designated as JW50, JW65, JW70, JW75 and JW80. There may have been others, but these are the ones I'm sure of.
The Whiscombe is an underlever. The smaller models are cocked by pulling twice on the lever, while the 75 and 80 require three pulls. A toothed gear ensures both pistons are withdrawn exactly the same. The mechanism is well-designed and robust, but I get the impression that it's rather like a bumblebee that, according to aerodynamic engineers, should not be able to fly. I try not to think about it too much.
You are now seeing where few airgunners have ever looked. This is the cocking mechanism of a Whiscombe in action. A geared rod moves each piston in the appropriate direction. The JW75 requires three pulls on the underlever to cock.
Actually, Whiscomes are very robust, but they will not tolerate idiots. If an owner tries to disassemble his rifle without knowing exactly what he's doing, he'll break it straightaway. And, the rifle simply cannot stand being dry fired even once. The first one will damage the gun, necessitating repairs. Think about it - two pistons come together and each serves as the end of the compression chamber for the other. The only thing protecting them is the thin cushion of compressed air that stops them from colliding at speed. A dry fire is a head-on collision of the pistons that destroys their seals. A Whiscombe owner must act like the owner of a Ferrari. You operate the equipment within the guidelines set forth by the manufacturer and forget going to Jiffy Lube or Midas Muffler (or Smiling Jack's airgun shop, in this case)!
To avoid confusion
No, the rifle does not develop less power if you only pull the underlever twice. The trigger does not engage the sear until the end of the third pull. In fact, until you develop the knack, it's possible to think you have fully cocked the rifle when you haven't. Yes, it is possible for an underlever gun to also be a breakbarrel. The barrel breaks for loading only, not for cocking.
I'll cover these points in greater depth in future reports.