Noob Question: Why does published BC vary so much?

[GettoPhilosopher]

This may be a complete noob question, but why do published ballistic values from official and/or trusted non-official sources vary so much?

I'm specifically talking about ballistic coefficients, though it's true of muzzle velocity as well (but I really need to get a chrono and get ACTUAL MV, I know). It seems like sometimes you look at 4 sources and get 5 BCs and MVs.

Example: Federal American Eagle .308 150gr FMJ-BT

http://www.sksboards.com/smf/index.php?topic=79382.0 (.456 BC, 2750 FPS)
http://www.snipercentral.com/forums/...p?f=14&t=37723 (.456 BC, 2820 FPS)
http://shootersnotes.com/battle-rifl...minal-effects/ (No BC, 2740 FPS)
http://www.warriortalk.com/showthrea...ct-on-Velocity... (.420 BC, earlier post guy says ~2663 FPS from 20")


XM193 is similar, with varying BCs and wildly varying MVs for the same length barrel.

So....why the variation?

[NotEnufGarage]

If you look on the Sierra website, you'll see that for many of their bullets the BC varies depending on the velocity. Generally it's higher BC for higher velocity.

It might be that the ammo manufacturers are basing their BC on how the ammo actually performs at various ranges and they calculate that back to BC. If that's the case, velocity plays a role, but so do altitude, temperature and humidity.

[Fjold]

BC changes in flight with velocity. If you start a bullet at 3,000 fps the BC will change as it slows down to 2,800 fps. Also any changes in ambient conditions such as barometric pressure, humidity, temperature, etc. will also change the BC.

[Munk]

Air-pressure is one of the biggest factors. Look at people running tests of ammo at various altitudes. you will see the shots string upwards closer to flatness as your altitude increases as a result of less interference from the air.

I'd wager that the different test locations for each manufacturer are at different altitudes.

[30Cal]

I think it's humorous that they publish it to three digits.