Here's a quick example of how to estimate a G7 Ballistic Coefficient based on a bullets weight, caliber, and G7 form factor. Note, most long range bullets have G7 form factors between 0.9 and 1.0. So if you estimate a bullet to have particularly low drag, the form factor will be closer to 0.9, higher drag would be closer to 1.0. Of course there are many bullets outside this bracket but most bullets used in long range shooting are within it. Along the way to calculating BC, you have to calculate

Bullet Ballistic Coefficient: When It Matters for Hunters - Game & Fish