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All About Ballistic Coefficient
By [url="https://www.airgundepot.com/admin-article.html"]Airgun Depot | Published Hard Air Magazine, or you will need to do your own testing.
To figure out the BC of your pellet of choice you will need either a chronograph or a Doppler radar and since you are more likely to own a chronograph than a radar that is what we will cover.
The basic process is to measure the velocity of the pellet at two distances and then plug the numbers into our ballistics calculator. If you have only one chronograph place it a couple of yards in front of your shooting bench and shoot a number of pellets through it. (If you are using a PCP either tether it or refill to the same pressure frequently so that you get the most consistent results possible.) Take the average of the shots. Now move the chronograph down range and repeat the process. (Don’t shoot the chronograph!) Measure the distance between the two locations and put the distances and the velocities into our ballistic calculator and there you go!
If you have two chronographs it is a bit easier. First, make sure they are calibrated by putting them end to end and firing some pellets through them. The velocities shown by the second chronograph should be just a little lower than the first. Then switch the two chronographs around and check that the results are the same. Now the process is the same as with one chronograph, but you don’t need to take the average of a number of shots through the first chronograph and compare it to an average of shots through the second. Instead, plug in the two velocities of each shot.
Can bad BC be good?
As we saw above, a good BC means that a projectile will have a longer effective range, retaining both power and accuracy at longer distances, but is this always a good thing? While you might think the answer is an obvious yes, the answer is actually a definite no! In fact, one of the real advantages of an airgun over a firearm is that pellets lose their energy very quickly, making them much safer to use in urban environments. This is not to say that you don’t need to be conscious of your backstops or what is behind your target because you sure do, but consider this example: A 7gr Hobby pellet traveling at 800 fps at the muzzle putting out over 10 fpe only retains about 1.5 foot-pounds of energy (for comparison, the Red Ryder BB gun puts out about 1fpe) at 100 yards while a 22lr bullet can retain up to 15fpe at 1000 yards. If you are pesting in your backyard, for instance, or in a barn or a feedlot, the fact that the pellet is going to quickly shed energy is a very good thing.
How do you get the best possible BC with your airgun?
Shoot slugs! Because of their shape and sectional density, slugs have a much better BC than pellets do, but they also have drawbacks. First, they don’t have a flared backside s they need gyroscopic stabilization and many airguns, especially springers and CO2 guns, don’t have the power to shoot them fast enough to get sufficient spin stabilization to make them accurate. Second, they are more expensive. Third, their better BC makes them less fit for shooting in urban environments. Fourth, most slugs aren’t as accurate as pellets.
One last consideration is the fact that a good BC does not mean that the pellet is going to be more accurate"that has to do with a lot of other factors.
Some Complications
Now that you have the gist of ballistic coefficient and how to calculate it we’ll add the fine print.
As mentioned above, there are many factors that affect BC so that it is never exact. For example, BCs change with velocity, so a listed BC of a bullet or a pellet is only really meaningful if you know at what velocity the projectile was measured. Bullets tend to have a better BC the faster they are going and a worse BC as they begin to slow down, but a pellet’s BC is generally the opposite. A pellet is going to slow down a lot faster in the 1000-800 fps range, and thus have a worse BC than it will in the 600-400 fps range. For example, JSB Beasts in .22 has a BC of 0.038 when measured with a muzzle velocity of 920 fps. This is a great BC for a pellet! However, starting with a muzzle velocity of 790 fps they have a BC of 0.054, which is the best we’ve ever seen!
Other factors such as temperature, elevation above sea level, barometric pressure, and even the idiosyncrasies of individual barrels also affect BC, but these are minor enough that we won’t go into them here.
One thing that complicates BC calculations is that they are done in relation to a standard model. For bullets the two most common standards are the G1 and G7 models, but pellets are so different that they have their own standard, the GA. A ballistic standard such as the G1 or GA is a model of a projectile shape from which the drag and BC can be calculated. The G1 is a model of a short, flat-bottomed bullet, while the G7 is for longer boat-tailed bullets. The GA is a standard domed pellet.
All About Ballistic Coefficient
By [url="https://www.airgundepot.com/admin-article.html"]Airgun Depot | Published Hard Air Magazine, or you will need to do your own testing.
To figure out the BC of your pellet of choice you will need either a chronograph or a Doppler radar and since you are more likely to own a chronograph than a radar that is what we will cover.
The basic process is to measure the velocity of the pellet at two distances and then plug the numbers into our ballistics calculator. If you have only one chronograph place it a couple of yards in front of your shooting bench and shoot a number of pellets through it. (If you are using a PCP either tether it or refill to the same pressure frequently so that you get the most consistent results possible.) Take the average of the shots. Now move the chronograph down range and repeat the process. (Don’t shoot the chronograph!) Measure the distance between the two locations and put the distances and the velocities into our ballistic calculator and there you go!
If you have two chronographs it is a bit easier. First, make sure they are calibrated by putting them end to end and firing some pellets through them. The velocities shown by the second chronograph should be just a little lower than the first. Then switch the two chronographs around and check that the results are the same. Now the process is the same as with one chronograph, but you don’t need to take the average of a number of shots through the first chronograph and compare it to an average of shots through the second. Instead, plug in the two velocities of each shot.
Can bad BC be good?
As we saw above, a good BC means that a projectile will have a longer effective range, retaining both power and accuracy at longer distances, but is this always a good thing? While you might think the answer is an obvious yes, the answer is actually a definite no! In fact, one of the real advantages of an airgun over a firearm is that pellets lose their energy very quickly, making them much safer to use in urban environments. This is not to say that you don’t need to be conscious of your backstops or what is behind your target because you sure do, but consider this example: A 7gr Hobby pellet traveling at 800 fps at the muzzle putting out over 10 fpe only retains about 1.5 foot-pounds of energy (for comparison, the Red Ryder BB gun puts out about 1fpe) at 100 yards while a 22lr bullet can retain up to 15fpe at 1000 yards. If you are pesting in your backyard, for instance, or in a barn or a feedlot, the fact that the pellet is going to quickly shed energy is a very good thing.
How do you get the best possible BC with your airgun?
Shoot slugs! Because of their shape and sectional density, slugs have a much better BC than pellets do, but they also have drawbacks. First, they don’t have a flared backside s they need gyroscopic stabilization and many airguns, especially springers and CO2 guns, don’t have the power to shoot them fast enough to get sufficient spin stabilization to make them accurate. Second, they are more expensive. Third, their better BC makes them less fit for shooting in urban environments. Fourth, most slugs aren’t as accurate as pellets.
One last consideration is the fact that a good BC does not mean that the pellet is going to be more accurate"that has to do with a lot of other factors.
Some Complications
Now that you have the gist of ballistic coefficient and how to calculate it we’ll add the fine print.
As mentioned above, there are many factors that affect BC so that it is never exact. For example, BCs change with velocity, so a listed BC of a bullet or a pellet is only really meaningful if you know at what velocity the projectile was measured. Bullets tend to have a better BC the faster they are going and a worse BC as they begin to slow down, but a pellet’s BC is generally the opposite. A pellet is going to slow down a lot faster in the 1000-800 fps range, and thus have a worse BC than it will in the 600-400 fps range. For example, JSB Beasts in .22 has a BC of 0.038 when measured with a muzzle velocity of 920 fps. This is a great BC for a pellet! However, starting with a muzzle velocity of 790 fps they have a BC of 0.054, which is the best we’ve ever seen!
Other factors such as temperature, elevation above sea level, barometric pressure, and even the idiosyncrasies of individual barrels also affect BC, but these are minor enough that we won’t go into them here.
One thing that complicates BC calculations is that they are done in relation to a standard model. For bullets the two most common standards are the G1 and G7 models, but pellets are so different that they have their own standard, the GA. A ballistic standard such as the G1 or GA is a model of a projectile shape from which the drag and BC can be calculated. The G1 is a model of a short, flat-bottomed bullet, while the G7 is for longer boat-tailed bullets. The GA is a standard domed pellet.
