entoptics
Well-Known Member
- Joined
- Jan 16, 2018
- Messages
- 878
Figured this might be a good resource, so I'll start.
If you contribute, please make it clear whether you collected quality atmospheric/elevation data for the shots. As an example of how much it affects the calculation using JBM's B.C. Calculator...
Pressure. ~0.002 change per 0.1 inHg
Elevation. ~0.002 change per 100 ft.
Temperature. ~0.001 change per °F
As you can see, just pulling data from a "nearby" weather station, using an app on your phone, may not be ideal. If that station is 200 feet different elevation, 5 degrees warmer, and 0.1 inHg different, that adds up to just over 0.01 difference in B.C. It's not a huge difference, but the errors can add up. I use a free barometer app on my iPhone and a Kestrel to get temp/humidity/pressure, within a few minutes of any recorded shot I intend to calculate the B.C. with. This is especially important if the temp is changing fast (early morning, late evening, etc).
Also, try and get as much spread as possible for your near/far distances. A bullet's B.C. varies with velocity, so the further apart you get readings, the better the "average" B.C. will be.
Be sure to set the furthest distance setting to something considerably further than you think you'll get a reading, as the LabRadar will stop a few yards past whatever you set V5 as. I can usually get at least a few readings past 100 yds with 0.243, 150 yds with 0.270, and 200 yds with 0.308. It varies with the bullet style of course.
So...my data.
0.284, 175 ELD-X, 7mm Rem Mag, 1:9.5 twist, quality atmospherics, near/far separation >100 yds. Average of 3 shots = 0.681. Surprisingly close to the 0.689 that Hornady quotes, despite being only recommended for 1:8.5 twist.
0.243, 103 ELD-X, 243 Win, 1:10 twist, quality atmospherics, near/far separation >100 yds. Average of 6 readings = 0.483. ~6% below Hornady's quoted 0.512, though not surprising, as they recommend a 1:8 twist.
0.243, 100 Sierra SPBT, 243 Win, 1:10 twist, quality atmospherics, near/far separation >100 yds. Average of 6 readings = 0.387. Surprisingly 8% lower than Sierra quotes for my velocity range.
0.277, 140 Nosler Accubond, 270 Win, 1:10 twist, quality atmospherics, near/far >100 yds. Average of 9 readings = 0.452. As is well documented, Nosler inflates their B.C., and my measurements are 9% lower than Nosler quotes. I will say though, I took these measurements over several months/conditions, and the Standard Deviation is only 0.005. The B.C. is inflated, but it's darn consistent.
0.277, 150 Nosler Accubond LR, 270 Win, 1:10 twist, quality atmospherics, near/far >100 yds. Average of 4 readings = 0.565. As above, inflated B.C. by 5%. Also very consistent like the AB. Too bad they shoot like crap in my rifle.
0.308, 208 ELD-M, 300 WM, 1:10 twist. Like an idiot, I never recorded these, as each time I calculated them, they were within ± 0.005 of the quoted B.C. of 0.690. I do wish I'd compiled the data though...
Anyway, hope somebody finds this information useful, and hopefully you all can add your data to get a good reference going. Perhaps if there's enough data, I'll compile it into a PDF or Spreadsheet.
If you contribute, please make it clear whether you collected quality atmospheric/elevation data for the shots. As an example of how much it affects the calculation using JBM's B.C. Calculator...
Pressure. ~0.002 change per 0.1 inHg
Elevation. ~0.002 change per 100 ft.
Temperature. ~0.001 change per °F
As you can see, just pulling data from a "nearby" weather station, using an app on your phone, may not be ideal. If that station is 200 feet different elevation, 5 degrees warmer, and 0.1 inHg different, that adds up to just over 0.01 difference in B.C. It's not a huge difference, but the errors can add up. I use a free barometer app on my iPhone and a Kestrel to get temp/humidity/pressure, within a few minutes of any recorded shot I intend to calculate the B.C. with. This is especially important if the temp is changing fast (early morning, late evening, etc).
Also, try and get as much spread as possible for your near/far distances. A bullet's B.C. varies with velocity, so the further apart you get readings, the better the "average" B.C. will be.
Be sure to set the furthest distance setting to something considerably further than you think you'll get a reading, as the LabRadar will stop a few yards past whatever you set V5 as. I can usually get at least a few readings past 100 yds with 0.243, 150 yds with 0.270, and 200 yds with 0.308. It varies with the bullet style of course.
So...my data.
0.284, 175 ELD-X, 7mm Rem Mag, 1:9.5 twist, quality atmospherics, near/far separation >100 yds. Average of 3 shots = 0.681. Surprisingly close to the 0.689 that Hornady quotes, despite being only recommended for 1:8.5 twist.
0.243, 103 ELD-X, 243 Win, 1:10 twist, quality atmospherics, near/far separation >100 yds. Average of 6 readings = 0.483. ~6% below Hornady's quoted 0.512, though not surprising, as they recommend a 1:8 twist.
0.243, 100 Sierra SPBT, 243 Win, 1:10 twist, quality atmospherics, near/far separation >100 yds. Average of 6 readings = 0.387. Surprisingly 8% lower than Sierra quotes for my velocity range.
0.277, 140 Nosler Accubond, 270 Win, 1:10 twist, quality atmospherics, near/far >100 yds. Average of 9 readings = 0.452. As is well documented, Nosler inflates their B.C., and my measurements are 9% lower than Nosler quotes. I will say though, I took these measurements over several months/conditions, and the Standard Deviation is only 0.005. The B.C. is inflated, but it's darn consistent.
0.277, 150 Nosler Accubond LR, 270 Win, 1:10 twist, quality atmospherics, near/far >100 yds. Average of 4 readings = 0.565. As above, inflated B.C. by 5%. Also very consistent like the AB. Too bad they shoot like crap in my rifle.
0.308, 208 ELD-M, 300 WM, 1:10 twist. Like an idiot, I never recorded these, as each time I calculated them, they were within ± 0.005 of the quoted B.C. of 0.690. I do wish I'd compiled the data though...
Anyway, hope somebody finds this information useful, and hopefully you all can add your data to get a good reference going. Perhaps if there's enough data, I'll compile it into a PDF or Spreadsheet.
