I know this is a quirky question, but how accurate are the bullet manufacturers' ballistic coefficient data?

BC may be just one of many factors that affects a bullets flight, but it seems to be the one factor that handloaders and shooters can't verify for themselves.

I think most of them are exagerated to the high side. Someone linked to a magazine article where they were tested and most all were much lower than the published numbers.

One of the long range shooters here said he shoots 10 shots at the muzzle and 10 shots at 100 yds over the same chrono. and calculates the BC that way. There are programs for this calculation.

The BC of a bullet changes depending on the velocity, air temp,alt, and humidity. Sierra has a chapter on this in their manual. The manufactors list a BC based on sea level and "std atmospheric conditions" (what ever that is)
Unless you live and shoot under these conditions your BC will vary.

I've found, in general:

Nosler & Sierra seem to be right on the mark and jive with each other.

Hornady "underrates" theirs (especially in the case of the Interbond/SST's) a little relative to the above two.

Barnes exaggerates theirs a lot. To compare with the others, you need to knock about 10-15% off their published figures depending upon the bullet.

And how many times do I have to say it? As Ol` Joe mentions, the measured BC will change with conditions. A magazine test that tests under conditions with thick air is going to measure lower numbers than the advertised BC's which are for standard conditions. It does not mean the advertised BC's are incorrect--all it means is the author didn't correct the measured BC's to standard conditions!