Why powerful mainsprings don’t always increase velocity
by Tom Gaylord, a.k.a. B.B. Pelletier
Today’s report is the one I mentioned forgetting in last Friday’s blog! Blog reader Errol reminded me about it yesterday.
I hear this so often from airgunners — how they think they’re going to add a more powerful mainspring to their airguns and increase the power. It sounds logical, but it often doesn’t work; and it nearly always doesn’t work as well as you think it should. Today, I want to discuss why that is.
The Weihrauch HW 35 was always considered to be one of the most powerful airguns in its day — which was the 1950s. They delivered over 700 f.p.s. when new in the 1950s; and over time, this rose to 750 f.p.s. Careful tuning could get close to 800 f.p.s. from certain guns. This model is still being made today, but now it sells because it’s so pleasant to shoot and doesn’t produce excessive power. How times change!
The HW 35 was so-named because the length of the piston stroke is 35mm. Piston stroke is the distance the piston travels from being cocked to being at rest at the end of the firing stroke. When Robert Beeman set out to make what eventually became the Beeman R1 rifle, he used the HW 35 as the starting point and increased the piston stroke to 80mm. And that’s where the additional power comes from — the piston stroke length and not the strength of the mainspring. Greater stroke length means greater swept volume, which means more air to compress for the shot. It doesn’t matter all that much how fast the air is compressed, which is the only thing a stronger mainspring does.
Then, there’s the story of the man who wanted to build a supersonic .22 pellet rifle. So he took the Beeman R1 as a starting point and built one that was 125 percent larger. The late Steve Vissage built a Frankenstein rifle that I documented in a report called Steel Dreams. It weighed 11 lbs., took 53 lbs. of force to cock and was larger than the R1 in every way, save one. It developed the same power! Yes, bigger mainspring and wider piston did not increase the power of the gun one iota.
Why is this true?
I know you want an explanation of why a more powerful mainspring doesn’t necessarily increase power. Here it comes:
THE MAINSPRING DOESN’T ACT DIRECTLY ON THE PELLET
The mainspring pushes the piston. The piston compresses air in front of it, and it’s that compressed air that gets behind the pellet and pushes it up to speed. The mainspring never touches the pellet. So, changing the mainspring has no direct effect on the speed of the pellet.
Here’s a good example everyone will understand. We have a house with a hollow-core door as the front door. Forget the fact that it violates all building codes — your cheap Uncle Rufus put it on when the old front door finally broke. This one was dirt cheap, which is why he got it. But your Aunt Thelma is justifiably worried about a break-in; so when Rufus is out of town, Thelma has a locksmith install a super-duper triple deadbolt lock on the door. Is that going to protect her? Of course not. Any burglar can simply break the door apart with one good kick. The lock will still be secure, but there won’t be any door attached to it.
Do any heavy mainsprings ever work?
Am I saying that heavier mainsprings never work? No, I’m not. When I was testing the Beeman R1 for my book, I installed the Mag 80 Laza Kit from Venom in England. Ivan Hancock created a drop-in kit of parts that worked well in the R1/HW 80 and increased the power. The mainspring was a very long stiff spring with thicker wire, and it was coated with a black tarry substance that I named black tar in my newsletter articles. That’s where the term black tar comes from. Black tar is also called velocity tar in some circles.
The R1 went from requiring 36 lbs. of force to cock to 50 lbs. with this kit. But the heavy mainspring was not directly responsible for the power increase. The kit also included a new piston that had 6 synthetic bearings that are now called buttons. These buttons rode against the spring cylinder walls and kept the steel piston from touching the steel spring cylinder.
Here’s the deal. The new piston was harder to slide inside the spring tube because the synthetic bearings pressed tightly against the sides of the spring tube. The powerful mainspring simply brought the piston’s speed back to parity with the factory piston. What increased the power was a combination of a better piston seal and the elimination of all piston vibration when the gun fired. The gain was just a few foot-pounds of energy, but the rifle was now getting everything the R1 design could possibly give.
A parallel in the pneumatic world
Most of you readers are aware that pneumatics work within pressure limits, and over-pressurizing them doesn’t add power — it takes it away. The reasons are different, but the end result is the same as for heavier mainsprings. The design of the gun is being overcome by one thing (the mainspring in a spring gun, or too much reservoir pressure in a pneumatic) and the performance balance is tipped toward the negative.
CAN a heavier mainspring increase the power of a springer?
Yes, it can if you also change the rest of the powerplant along with the mainspring. And no, it won’t if a heavier mainspring is all you add. The secret to more power is to balance all the components so the gun performs at its optimum. With all airguns, there’s a limit to how far you can go. Where that limit is depends on the rest of the design — the parts that are too expensive to change.
You can break your heart trying to buff up a dirt clod to a high shine. Or you can start with a gun that has some potential and make real progress by artfully changing the things that matter. The secret is to know which is which.
One last remark
Pyramyd Air is stocking the most recent issue of Airgun Hobbyist magazine. If you want to try just a single issue to see if a subscription is worthwhile, now’s your chance.