Effects of Altitude on Velocity
Introduction
Have you ever wondered how the altitude affects your FPS when traveling to national airsoft events? Check out the following review from Fox Airsoft.
Fox Airsoft is based in Colorado and as a result, variations in altitude might be more important than in many other parts of the country. At least locally, there has been a lot of speculation on the exact effect that changing elevations has on the velocity, especially for those who set up their guns to chrono at or near the published velocity limits of whichever events they attend. It is the intent of this review to provide a quantitative analysis of the effect that altitude has on velocity and provide an explanation based on the results.
Before we present the results, we’ll quickly go over the basics of atmospheric pressure and how it changes with altitude. The first and most important thing to recognize is that air has weight. The normal atmospheric pressure at sea level is 14.7 psi (pounds per square inch); the easiest way to think about this is to imagine a one inch by one inch column of air reaching all the way to the upper edge of the atmosphere, roughly 100 miles. This column of air weighs 14.7 pounds on an average day at sea level. Because of the weight of air pushing down on the air below it, the air is much more compressed on the bottom of the column and there’s almost none at the top. This means that if you go up just 2000 feet above sea level, you have bypassed a full pound of air and your column now weighs about 13.7 pounds.
Thinking about this from a more practical airsoft point of view, if you are shooting 100 feet, the BB will fly horizontally through 1200 of these columns of air so the less air in that column, the farther you will shoot. On the other hand you will also have less air inside the AEG cylinder when you fire resulting in less compression and theoretically a lower velocity. The data we have gathered is intended to determine which high altitude characteristic affects the velocity more, the lower drag on the BB or the lower compression in the cylinder.
Test Procedure
This test was performed in according to our standard test model at two different altitudes. With hop up completely turned off, 10 measured shots were fired on semi-auto, followed by approximately 10-30 unmeasured shots on full auto, and finally another 10 measured shots on semi-auto. The test gun was an ICS M4 with a Madbull blue hop bucking, a Madbull tight bore barrel and M120 spring.
The same Xcortech X3200 chronograph was used at both locations and the tests were performed two days apart to minimize any other outside disturbances that could affect the test with time. Two brands of BBs were tested; Excel .25g Bio and Airsoft Elite .25g Bio. Weather conditions were measured at the time of testing and based on NOAA information at the closest available weather station.
Data/Results
The posted data show the results with both BB brands and the velocities measured after the full auto burst described in the test procedure section.
High Altitude
Altitude | Temperature | Relative Humidity | Pressure | Air Density |
6189 ft | 51°F | 48% | 29.84” | 0.0615 lb/ft3 |
Shot # | Excel Bio .25g | Airsoft Elite Bio .25g |
1 | 341.0 | 369.3 |
2 | 353.2 | 368.1 |
3 | 341.4 | 364.0 |
4 | 333.5 | 364.5 |
5 | 345.0 | 364.7 |
6 | 343.3 | 365.4 |
7 | 342.7 | 367.3 |
8 | 339.7 | 366.1 |
9 | 342.8 | 368.4 |
10 | 335.8 | 367.3 |
Average | 341.8 | 366.5 |
Low Altitude
Altitude | Temperature | Relative Humidity | Pressure | Air Density |
906 ft | 55°F | 77% | 30.07” | 0.0742 lb/ft3 |
Shot # | Excel Bio .25g | Airsoft Elite Bio .25g |
1 | 376.0 | 380.0 |
2 | 375.6 | 377.0 |
3 | 375.1 | 380.0 |
4 | 376.6 | 381.4 |
5 | 372.1 | 381.6 |
6 | 376.6 | 378.9 |
7 | 372.1 | 384.7 |
8 | 370.3 | 381.0 |
9 | 379.8 | 377.0 |
10 | 368.8 | 382.1 |
Average | 374.3 | 380.4 |
Conclusion
As you can see from the results, we saw an average 32.5 fps increase from the Excel Bio .25g BBs and an average 13.9 fps increase from the Airsoft Elite Bio .25g. The discrepancy from one brand to another could be due to a number of reasons. If these BBs are not precisely the same size, a smaller BB will escape its seat on the bucking before a larger one. This would result in less pressure being built up behind the BB and a lower velocity measured at the chrono. The same reduced velocity would be experienced by a BB with a smoother surface finish. Additionally, a slightly smaller BB allows more air to escape around it in the barrel and reduce the total velocity. One thing these results do point out is how important BB selection is at the chrono for guns close to the limit.
Regardless of BB choice though, from these results it’s clear that decreasing elevation has the effect of increasing velocity measured at the chronograph. The important thing to remember is that we are measuring velocity at the end of the barrel. At this point the most significant effect on the BB is the pulse of air delivered by the piston. The drag from the denser air makes almost no difference at the end of the barrel because of the much higher pressure of air pushing the BB out the barrel. Beyond the end of the barrel, the BB is only affected by the air it’s traveling through. In this case, the higher density of the air results in higher drag and the BB will slow down sooner. Overall, even though velocity is reduced at the muzzle at high altitude, the same gun will have a bit more total range.
Please let us know what you think of our online review. We welcome any and all comments, questions and suggestions at service@foxairsoft.com or sales@foxairsoft.com. Also, be sure to check out our other reviews at www.foxairsoft.com
