Wednesday, January 11, 2006
Ballistic Coeffiecents
We had a couple of very mild weekends recently so I took the time to test the ballistic coeffiecents (BC) of several pellets through two of my target rifles. I have read that bullets and pellets have differenct BCs when fired from different rifles. That didn't seem too intuitive to me since ballistics should be about the pellet shape, size and weight.
Since I shoot mostly springguns I tested the Crosman Premier 7.9 and JSB 8.44 pellets in my HW97 and TX200 MkI to see what differences I could find. Since I had only one chronograph I had to shoot strings of 10 shots at 1 yard and 50 yards and then perform some averaging to get the average BC. I actually tried two methods of calculating the BC. First I sorted the 10 velocities at both distances from highest to lowest and then paired up the results highest 1 yard and 50 yard and calculated the BC. I continued this until I got to the lowest velocities and then averaged the calculated BCs. Next I averaged the velocities at 1 yard and 50 yards and used the two averages to calculate an average BC. Both BCs came out to the same answer so I figured it must be pretty close to the actual BC.
The two test rifles are different power levels, the HW97 shoots around 13.5 ft lbs while the TX200 is a 12 ft lbs version. The results of my tests surprised me a bit. The BC for the Crosman Premier came out to be 0.026 in my HW97 while it calculated to 0.020 in the TX200. The JSB had a better overall BC with a 0.030 in the HW97 and 0.028 in the TX200. I also tested some Daystate 8.44 pellets (which are made by JSB) in the TX200 and they calculated to 0.026.
I thought this to be pretty interesting. I am not sure why the difference, perhaps others know more about this than I but I would have to suspect diffences in twist rates in the barrel, height of the rifling or maybe the difference in power level of the rifle causing the diffences in BC. I am not sure what causes the difference but there is definitely a difference. In dicussions with other shooters I have heard that there are BC differences between guns of the same type and power levels.
I have been using 0.025 as a general BC for the Premier 7.9 pellet for years but now I know that in my HW97 I need to use 0.026. I started wondering just how much difference this little amount would make in the actual point of impact. I brought up my STP trajectory program (which has a BC calculator in it) and punched in the numbers. The difference between 0.025 and 0.026 at 50 yards was 1 click on my Bushnell Elite 6-24x scope which amounts to about 1/16th of an inch. However if your rifle is down in the 0.020 BC range the difference is more like 1/4".
One thing to note about the BC, it will vary some with environmental conditions, mostly atmospheric pressure if memory serves me correctly, but the differences usually isn't more than a click or so at 50 yards. The bottom line is that if you are anal, like me, about being precise on your calculations, getting an accurate BC is important. That means testing every rifle/pellet combo and calculating the BC for that combo. It may also mean retesting when ever you get a pellets from a different die or batch to ensure that the BC hasn't changed. Remember in all of this, no matter what you calculate for the BC or trajectory, it still all has to be proven at the range.
Since I shoot mostly springguns I tested the Crosman Premier 7.9 and JSB 8.44 pellets in my HW97 and TX200 MkI to see what differences I could find. Since I had only one chronograph I had to shoot strings of 10 shots at 1 yard and 50 yards and then perform some averaging to get the average BC. I actually tried two methods of calculating the BC. First I sorted the 10 velocities at both distances from highest to lowest and then paired up the results highest 1 yard and 50 yard and calculated the BC. I continued this until I got to the lowest velocities and then averaged the calculated BCs. Next I averaged the velocities at 1 yard and 50 yards and used the two averages to calculate an average BC. Both BCs came out to the same answer so I figured it must be pretty close to the actual BC.
The two test rifles are different power levels, the HW97 shoots around 13.5 ft lbs while the TX200 is a 12 ft lbs version. The results of my tests surprised me a bit. The BC for the Crosman Premier came out to be 0.026 in my HW97 while it calculated to 0.020 in the TX200. The JSB had a better overall BC with a 0.030 in the HW97 and 0.028 in the TX200. I also tested some Daystate 8.44 pellets (which are made by JSB) in the TX200 and they calculated to 0.026.
I thought this to be pretty interesting. I am not sure why the difference, perhaps others know more about this than I but I would have to suspect diffences in twist rates in the barrel, height of the rifling or maybe the difference in power level of the rifle causing the diffences in BC. I am not sure what causes the difference but there is definitely a difference. In dicussions with other shooters I have heard that there are BC differences between guns of the same type and power levels.
I have been using 0.025 as a general BC for the Premier 7.9 pellet for years but now I know that in my HW97 I need to use 0.026. I started wondering just how much difference this little amount would make in the actual point of impact. I brought up my STP trajectory program (which has a BC calculator in it) and punched in the numbers. The difference between 0.025 and 0.026 at 50 yards was 1 click on my Bushnell Elite 6-24x scope which amounts to about 1/16th of an inch. However if your rifle is down in the 0.020 BC range the difference is more like 1/4".
One thing to note about the BC, it will vary some with environmental conditions, mostly atmospheric pressure if memory serves me correctly, but the differences usually isn't more than a click or so at 50 yards. The bottom line is that if you are anal, like me, about being precise on your calculations, getting an accurate BC is important. That means testing every rifle/pellet combo and calculating the BC for that combo. It may also mean retesting when ever you get a pellets from a different die or batch to ensure that the BC hasn't changed. Remember in all of this, no matter what you calculate for the BC or trajectory, it still all has to be proven at the range.
