Re: true bc of barnes 180 tsx?
I'll piggyback on what Chris Matthews stated. The G1 BC advertised by the bullet manufactures were taken using an average test velocity. Depending on your load and the velocity of your bullet, it may be higher or lower and will change over distance.
The best things to do is to chronograph your load and get an average velocity at the muzzle from 7 to 10' away. Then move your chronograph out to exactly 100 yards and get an average velocity reading there. Any good ballistic program will let you plug in those numbers and get your true BC for your bullet load combo. Then depending on the bullet type, convert the G1 BC to a G5 or G7 BC
Then move the chronograph out to 2, 3 and 400 yards and get your average velocities as well as how much bullet drop there was. You can then start to work with your ballistic program to get a very accurate track of your bullet. I like to track and record how much actual drop in inches my bullets experience at 2, 3 and 400-yard distances.
I use the RSI ballistic lab and it has features that most ballistic programs only dream of. One of those features is the “Estimate Drag Coefficient”. This feature allows you to compare your actual range data to all the different drag models and determine which drag model best matches your bullet. Once you determine the best drag model for your bullet, it will show you all the different BC changes your bullet experiences as it’s velocity decreases.
I use two CED Millennium chronographs when I’m working on my range data. One chronograph is always at the muzzle and the other one at precise 100-yard increments. Once I had my range data at 300 and 400 yards, the RSI ballistic lab predicted my bullet velocities out to 700 yards within a few feet per second. 700 yards is the farthest I’ve been able to use my second chronograph with my .308. This method has worked very well for me.
Hope it helps
Distance is not an issue, but the wind will make it interesting!