GASPING FOR BREATH
Written by: Der Luftmeister
My wife and I spend a considerable amount of time in areas at high elevation. During one summer a few years ago, we had some airguns along for recreation and testing. The battery included a couple of multi-pumps, a break action, and a rifle powered by carbon dioxide. Along the way, I added a fine Crosman 140 that had languished on the shelf of a pawn shop for over a year. In one shooting session, I set up the chronograph and started shooting across it with one of the multi-pump rifles. Knowing that particular rifle well, I was surprised to see that the velocity was considerably lower than it was the last time I tested it. While walking around the area, I became aware that I was breathing differently than back at home. That should have been no surprise at an altitude of 8,200 feet. Then it dawned on me that while I was having trouble getting enough air, so was the airgun!
At high altitude, the atmosphere is less dense so each breath brings in less air. In order to get enough oxygen, it is necessary to breathe more deeply and more often. When the airgun is pumped, each stroke brings in less air. The reason that the velocity is lower at high altitude is that the pumping is less efficient in the thinner atmosphere. I decided that this phenomenon needed to be explored in some detail. The airguns I had available were all rifles that I had tested extensively at home where the altitude is only about 800 feet.
Working at an altitude of 8,200 feet, I selected a dozen types of pellets and measured the velocity produced when the rifle was given ten pump strokes. What I found was that the average velocity at that altitude was 91% of what it was at 800 feet. With a second multi-pump of different caliber the results were almost exactly the same. Although I should have known better, I thought that a break action rifle might not be affected as much as the multi-pumps. What I found was that the break action gave a velocity at 8,200 feet that was just 87.4% of what it gave at 800 feet. The rifle tested is advertised to give 1,000 feet per second and low altitude it actually gives about 900 feet/second with pellets of normal weight. The effect of altitude at 8,200 feet reduces this value by approximately 100 feet/sec on the average. Keep in mind that a 10% reduction in velocity means almost a 20% loss in kinetic energy.
Having seen the loss in velocity at high altitude, it seemed that it might be useful to determine the effect at some intermediate altitude. Fortunately, one of the ranges we use during our travels is at an elevation of 5,500 feet so velocity testing was carried out there. The results were about as expected in some ways and surprising in others. With a dozen types of pellets, the multi-pump rifles gave average velocities that were about 94% of what they were at an elevation of 800 feet. The break action rifle gave almost exactly the same result.
Here we have something of a paradox. The break action rifle lost about the same percentage as the multi-pumps on going from 800 feet to 5,500 feet but lost about 4% more velocity on going from 800 feet to 8,200 feet than did the multi-pumps. Since the results are based on velocities with over a dozen types of pellets in each case, the effects seem to be real.
When the rifle powered by carbon dioxide was tested, it was found that the altitude produced no effect on velocity. That was expected because the pressure inside the gas cylinder is determined by the temperature but it is not affected by external atmospheric pressure. Your Tech Force 78 will give the same velocity regardless of the altitude as long as the temperature is the same. So will the precharged Tech Force TF50. Although I had no precharged pneumatic rifle for testing at that time, the altitude would have no effect on velocity. That is because the rifle is pumped to reach a specific pressure of compressed air inside the reservoir regardless of the outside pressure. What would be different is the number of pump strokes needed to reach that pressure!
There is another factor to consider. Keep in mind that at high altitude a multi-pump rifle can be given an extra stroke or two to compensate for the thin atmosphere. It is not possible to do this with a break action because cocking the rifle moves the piston into firing position. The amount of air that enters the compression chamber is fixed by the design of the rifle, and at high altitude that amount is smaller because the atmosphere is less dense.
The lower muzzle velocity and resulting lower kinetic energy have serious implications for the air gunner who heads to the mountains to hunt vermin. Depending on the type of airgun, it may be advisable to select a model with a little extra power. Although it is a topic for another time, the thinner air at high altitude retards the flight of the pellet less. This means that the velocity loss is not as rapid as it is at lower altitude. So, even though the muzzle velocity is lower, it is partially offset by the velocity being retained better. We will have more on this topic in a later blog.
Until next time, stay pumped.
My wife and I spend a considerable amount of time in areas at high elevation. During one summer a few years ago, we had some airguns along for recreation and testing. The battery included a couple of multi-pumps, a break action, and a rifle powered by carbon dioxide. Along the way, I added a fine Crosman 140 that had languished on the shelf of a pawn shop for over a year. In one shooting session, I set up the chronograph and started shooting across it with one of the multi-pump rifles. Knowing that particular rifle well, I was surprised to see that the velocity was considerably lower than it was the last time I tested it. While walking around the area, I became aware that I was breathing differently than back at home. That should have been no surprise at an altitude of 8,200 feet. Then it dawned on me that while I was having trouble getting enough air, so was the airgun!
At high altitude, the atmosphere is less dense so each breath brings in less air. In order to get enough oxygen, it is necessary to breathe more deeply and more often. When the airgun is pumped, each stroke brings in less air. The reason that the velocity is lower at high altitude is that the pumping is less efficient in the thinner atmosphere. I decided that this phenomenon needed to be explored in some detail. The airguns I had available were all rifles that I had tested extensively at home where the altitude is only about 800 feet.
Working at an altitude of 8,200 feet, I selected a dozen types of pellets and measured the velocity produced when the rifle was given ten pump strokes. What I found was that the average velocity at that altitude was 91% of what it was at 800 feet. With a second multi-pump of different caliber the results were almost exactly the same. Although I should have known better, I thought that a break action rifle might not be affected as much as the multi-pumps. What I found was that the break action gave a velocity at 8,200 feet that was just 87.4% of what it gave at 800 feet. The rifle tested is advertised to give 1,000 feet per second and low altitude it actually gives about 900 feet/second with pellets of normal weight. The effect of altitude at 8,200 feet reduces this value by approximately 100 feet/sec on the average. Keep in mind that a 10% reduction in velocity means almost a 20% loss in kinetic energy.
Having seen the loss in velocity at high altitude, it seemed that it might be useful to determine the effect at some intermediate altitude. Fortunately, one of the ranges we use during our travels is at an elevation of 5,500 feet so velocity testing was carried out there. The results were about as expected in some ways and surprising in others. With a dozen types of pellets, the multi-pump rifles gave average velocities that were about 94% of what they were at an elevation of 800 feet. The break action rifle gave almost exactly the same result.
Here we have something of a paradox. The break action rifle lost about the same percentage as the multi-pumps on going from 800 feet to 5,500 feet but lost about 4% more velocity on going from 800 feet to 8,200 feet than did the multi-pumps. Since the results are based on velocities with over a dozen types of pellets in each case, the effects seem to be real.
When the rifle powered by carbon dioxide was tested, it was found that the altitude produced no effect on velocity. That was expected because the pressure inside the gas cylinder is determined by the temperature but it is not affected by external atmospheric pressure. Your Tech Force 78 will give the same velocity regardless of the altitude as long as the temperature is the same. So will the precharged Tech Force TF50. Although I had no precharged pneumatic rifle for testing at that time, the altitude would have no effect on velocity. That is because the rifle is pumped to reach a specific pressure of compressed air inside the reservoir regardless of the outside pressure. What would be different is the number of pump strokes needed to reach that pressure!
There is another factor to consider. Keep in mind that at high altitude a multi-pump rifle can be given an extra stroke or two to compensate for the thin atmosphere. It is not possible to do this with a break action because cocking the rifle moves the piston into firing position. The amount of air that enters the compression chamber is fixed by the design of the rifle, and at high altitude that amount is smaller because the atmosphere is less dense.
The lower muzzle velocity and resulting lower kinetic energy have serious implications for the air gunner who heads to the mountains to hunt vermin. Depending on the type of airgun, it may be advisable to select a model with a little extra power. Although it is a topic for another time, the thinner air at high altitude retards the flight of the pellet less. This means that the velocity loss is not as rapid as it is at lower altitude. So, even though the muzzle velocity is lower, it is partially offset by the velocity being retained better. We will have more on this topic in a later blog.
Until next time, stay pumped.
posted by Randy Mitchell at
1:52 PM
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