I have a load worked up for my 300 Win Mag using the 180 Barnes TSX. From a zero of 250 yards I had to go up 102 clicks to be rezeroed at 965 yards. This was a bit crude but it shows the BC to be more like 0.410 rather than 0.55 which is what Barnes has published. I had thought earlier that the BC would be 0.510 based on some I had done out to 600 yards but adding another 365 yards gave me much more drop than expected. The only way the drop makes sense is if the BC is much lower than I had thought. Have any of you done more extensive testing than I on this bullet to establish the true BC? Thanks, Rufous.
That is pretty typical but I would not say it is all the bullets fault.
Often the click value of a scope is not what it is listed and it takes more or less to cover a desired correction.
First thing to do is figure out exactly how far the point of impact is shifted by each click and then you will be able to get a better idea of exactly how much correction you are dialing into your scope.
Allen Precision Shooting
Home of the Allen Magnum, Allen Xpress and Allen Tactical Wildcats and the Painkiller Muzzle brakes.
Something you guys are forgetting here- the BC changes constantly with velocity. That's why you get a lower BC with the added distance. It probably started out near the advertised BC (most companies are optimistic about their BCs) and then the first range you shot (600) the bullet maintained enough velocity to keep a higher BC then over the next 365 yards out to the final disatnce of 965 the velocity dropped off enough to lower the BC even further.
Look at Sierra's website and you'll notice they give three BCs for every bullet.
The only way you are going to get a true BC is to shoot it over a chronograph at the muzzle and at say 100,300, 600 and 1000. Then you can get an accurate BC. The "reverse engineering" method you are using will give you an estimate but is tough unless you know velocity.
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!
Maybe people will start paying attention when I say not to believe Barnes' advertised BC's.
Rick Jamison tested the 180 TSX on his Oehler and found it to have a BC 12% lower than the 180 AccuBond and 19% lower than the 180 Scirocco. Of course Barnes advertises it higher than both...as if its less aggressive nose and big hollow point on the front can defy the laws of physics somehow. [img]images/icons/rolleyes.gif[/img]
Give one of the plastic tipped bullets a try at that range and see how many fewer clicks it takes....
esay way to compare the truest of bullet BC is to shoot same distance same ( closely same velocity as possible ) another bullet of same class weight as 180 SMK or 175 SMK ( Sierra BC are quite right ) and compare both trajectory and BC
BC is from now a very good way to sale bullet but real BC value are sometime disapointed for real shooter , in a confortable living roon all is laways OK but oon the range .....