18.13 22cal JSB BC
- Thread starter lhandy
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They have it listed on their website, it's also gonna be different for every one cause not everyone lives in the same area, have the same barometric pressure, altitude or run them at the same speed. You need to conduct your own tests.
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I use .28 as general reference - usually adjust using Strelok to calculate for each rifle and pellet setup
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Neither barometric pressure nor altitude will have any effect on BC if it is calculated correctly, as all atmospheric conditions should be corrected to standard atmosphere conditions. BC should not change significantly with speed either if the correct reference drag law is used, such as GA2 in the MERO Easy BC calculator.
G1 is not the correct reference drag law for pellets, particularly at high speeds. It is a pity some manufacturers continue to use it when they should know better.
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Well the reason I say altitude is because anither one of our members has a 1:18 liner fir his .177 but the projectiles he is using are shooting as if they were being shot from the correct being being a 1:16 saying that may be due to the altitude. But thank you for that useful information, I'm not looking to spread incorrect info
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That would be me. What I said is that, using slugs, as air density decreases with altitude, less spin is needed to achieve a certain stability factor. Therefore, at my elevation, calculations show that I can get the same stability factor for my 20g 177 slugs using a 1:18 liner, as I could using a 1:16 liner at sea level.
As for pellets, while BC is constant and corrected for standard conditions, any given pellet (or slug) will experience less reduction in speed (and therefore also drop) per unit distance as altitude increases. That’s because less dense air provides less resistance and drag.
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I gotcha, I wasn't gonna name drop from the get go. I was under the impression that the bc would change with different variable like slugs would, not entirely why but thank you for clearing that up!
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Here’s something that you could read when it’s too dark to see those lizards:
Effects of Altitude and Temperature on Rifling Twist - Lilja
In an earlier article in NBRSA NEWS, we looked at the effect changing atmospheric conditions had on the ballistic coefficient of a bullet. To summarize briefly we found that the ballistic coefficient of a bullet as published by bullet manufacturers and others is valid only for a standard...
riflebarrels.com Upvote 0
I can tell you from personal experience that both bullets from a firearm and pellets and slugs from an air rifle have BC values that will most definitely be affected by changes in elevation. I shoot and hunt primarily in two different areas. One is roughly 1500 feet elevation and the other varies from 7500-8500 feet. I determine my own BC values by measuring velocity drop at various distance. There is a significant increase in the measured BC value of a projectile when going up from 1500ft to 8500ft.
Kenny
Kenny
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Kenny,
You are correct, but it is anathema to ballisticians to state that a BC can change in such a way because BC values are constants derived from calculations or empirical validations that utilize standardized atmospheric conditions (sea level, 70F). The point here is that BCs need to be standardized so that they are comparative. If that notion was abandoned, manufacturers could conduct tests on the top of Mt McKinley and hoodwink the consumer. It’s certainly accurate to state that “an increase in temperature and/or elevation acts to effectively increase the BC of a projectile”
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Thank you!Here’s something that you could read when it’s too dark to see those lizards:
Effects of Altitude and Temperature on Rifling Twist - Lilja
In an earlier article in NBRSA NEWS, we looked at the effect changing atmospheric conditions had on the ballistic coefficient of a bullet. To summarize briefly we found that the ballistic coefficient of a bullet as published by bullet manufacturers and others is valid only for a standard...
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Maybe a better way to say it is that each projectile has a form factor. That form factor is indeed constant. The BC value is the relationship of that form factor to the ever changing environmental conditions. So while the projectile’s form factor doesn’t change the BC value does.
Kenny
Kenny
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It is akin to what we see with hp and torque values of engines. The manufacturers claims comport to standard atmospheric conditions to facilitate comparison. However, at 10k feet, you have <70% of that hp with a NA engine.
I also truck around to different elevations between 4000 and 13000. But what’s really interesting is that, at elevation, temperature has an enhanced impact on ballistics. For example, right now, with a freezing temperature, density altitude is 3800ft, even though I’m at 5000ft. At the height of summer, the DA gets up to 9000ft, even though I’m still at 5000ft. So, I can see a variance of >5000ft in my backyard, due to the extreme range of temperatures in my location. However, even at sea level, the DA can shift by a substantial amount during the year and few shooters are aware of its effects:
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BC is composed of the form factor and the sectional density. There are no other factors involved. If the form factor does not change, the BC will not change for a given projectile of a certain fixed size and weight. The form factor for bullets may change with different air densities if the bullets have marginal gyroscopic or dynamic stability, as the yaw may be different. There will normally be a change in group size with any change in stability, but it will depend on the bullet design and start stability, some may show small changes, others much larger changes. Pellets are somewhat different in that any change in yaw will usually give a wide variation in group size. This is because they are aerogyro stabilized with a very small value of aerodynamic overturning moment. Hence, aerodynamic drag should not vary greatly and neither will form factor.
What does change is air density, which will change the retardation of the projectiles, it is not a change in the BC causing the projectile to slow down at different rates at the different altitudes.
What does change is air density, which will change the retardation of the projectiles, it is not a change in the BC causing the projectile to slow down at different rates at the different altitudes.
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