Calculating BC for an unknown bullet?

Coldbore 1.0 has tools to do this, if you are a Windows computer user, Download Coldbore, it's what i used to nail the Warner BC and it was backed up by Hornady using their doppler. I was within 1% of Doppler

But as noted, you need MV and information at both the shooter and target.
 
This actually isn't to bad of a start from a mathematical standpoint -
Wikipedia - Ballistic Coefficient.

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