Springtime! - Part 2
Introduction by B.B. Pelletier
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Vince Brandolini's first installment about calculating spring-gun energy [insert link after blog part 1 is published] has a few more steps to go, and today he finishes it up for us.
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Springtime! - Part 2
by Vince Brandolini
After you've followed all the steps in my previous blog, there's one last step...measuring the piston stroke. You have to select an accessible datum point, such as the rear of the cocking shoe or a recognizable spot on the piston. Measure from that point to some other reference point (it doesn't really matter where) with the gun cocked and uncocked. Subtract the lesser measurement from the greater and you'll have the cocking stroke. On some rifles (like some from Gamo), you might have to cock the gun with the action in the stock, then separate it while it remains cocked. Take the upmost care while doing so!
At this point, a little number crunching and we're done. First, calculate the preload in pounds. Multiply the preload distance measured above by the spring rate. This tells you how much pressure is on the spring when the gun is at rest. Multiply the stroke (in inches) by the spring rate and add it to the preload figure. This is your cocked pressure. Take the average of the cocked and preload pressures, multiply by the piston stroke, divide by 12, and you're done. Here's the formula:
- PL=preload measurement
- SR=spring rate
What about efficiency? For that you need to know pellet weight and actual muzzle velocity. Use the following formula:
Divide the powerplant energy into muzzle energy, and you've got efficiency. Generally speaking, I've observed efficiencies in the range of 30% to 44% in .177 and 40% to 46% in .22.
There's one big unknown I've left out of this entire discussion...the effect of dieseling (burning of lubricant) on velocity. Obviously, this would drive the efficiency artificially high. I've heard of the Cardew experiments that purported to show that a lot of a springer's power comes from this effect, but personally I'm not convinced that this is always the case. I haven't seen the details of that experiment, but I tried my own test some time back. I compared the velocity figures of a dry rifle (thoroughly degreased and relubed with only powdered moly) and wet (lubed sparingly with moly/oil) and found little difference.
I've seen some springers shoot with a very tight velocity spread of around 10 fps over 10 shots. If the rifle was dieseling, I'd think that the effect would not be that consistent, since there's no mechanism to tightly regulate the amount of fuel introduced into the chamber. Lastly, there's the matter of my most efficient .177 caliber springer; the one that showed up as over 44% efficient when every other .177 was between 30% and 39%. That gun happens to be a Crosman 795, an under-5-ft-lb gun that probably doesn't have enough compression to really initiate any dieseling. And, my least efficient .177, a Walther Force 1000, has a loose seal, a large powerplant and a good-sized spring that should make it more inclined to diesel.
At this point, I'm just gonna stay out of THAT discussion. As I said before, this information can be very useful. I was always a little perplexed by the fact that my RWS Panther did about 875 fps with Crosman Premier 7.9-grain pellets, despite having a long and reasonably stout cocking stroke. When I calculated the efficiency, I found that it was well under 30% when every other springer I had was over. This told me that something was probably wrong, and I eventually discovered that the breech seal was leaking. It was recessed too far into the breech face. I shimmed it out, and now the rifle delivers over 940 fps with the same pellet.