I use a .25/06 for long range varmint (goats etc) shooting with 115 gr Nosler ballistic tips.
The stated B.C is 0.453, however I shot these bullets at a muzzle velocity of 3161 average and a 100 yard velocity of 2896 average. when I put those figures into a ballistics computer the B.C worked out at 0.350. Anyone help?

How many shots did you fire to get those averages?

The reason I asked is that I suspect the BC calculation is very sensitive to small changes in the velocity data. How different would the velocity loss have to be to get the expected BC? Maybe 30 fps? Just guessing.

I would think the most accurate way to do this would be to use two chronographs (one at the muzzle and one at the target).

Mark you are correct the most accurate way is with 2 chrono's. The published B.C is always going to be a little off from your testing unless you test in the same conditions the manufacturers do. Altitude plays a big role in B.C #'s as well as air temps. Barrel ondition is another factor but how large I can't tell you.
There are many factors that change a bullets B.C and the only true way to measure it for yourself to get good respectable results is with two chrono's peferably oehler chrono's since they tend to be more precise than shooting chronies.

Thanks for the help, I fired 5 shots at each range. I moved my shooting chrony out to 100 yds after my muzzle velocity shots. I could just make out the reading through my VX2 6-18. It was quite cold, around 6 deg celcius and I was probably only 50 feet above sea level.
Thanks

A small variation in data will result in quite a change in the BC. On the other hand your MV is instrumental and not calculated to the muzzle no doubt and that would make it worse.

It seems that most of, but not all, the published data is high. Many Speer bullet data seems that way too. It may be a competative thing as we may buy bullets based on the best BC.

This bothers me as I like to calculate BC data also.

Plus 6 Celcius is not cold. Minus 25 Celcius is cold!