What is a G7 Balistic Coefficient?

I am looking at the Norma site at a 6.5x55 Bondstrike round and they mention BC G1 of .629 and BC G7 of .313.
What is a G1 BC and a G7 BC?

Thanks
Mac

https://www.norma-ammunition.com/en-...-gr---20166382

https://en.wikipedia.org/wiki/Ballistic_coefficient

The G number is a ratio of how your projectile looks/behaves compared to a standardized shape.

The G1 shape is basically an old style canon projectile from the 1880's.

The G7 shape looks more like a modern projectile: long, boat tail, secant ogive. You generally get better results from ballistic calculators when using the G7 number.

BC is a measure of how well a particular projectile overcomes air resistance in flight. The higher the number, the better the performance of the projectile (the less it drops). If you are comparing BCs, it is important to compare like for like (G1 to G1).

Kestrel's explanation -
https://nz-kestrelmeters.glopalstore..._US&utm_term=b

G1 has been around for ever and the G7 is as ebf points out better suited for modern projectiles.
Most (all?) ballistic calculators now let you enter either.
One thing though, if using G7 remember to tell your calculator that you are using G7 otherwise you get seemingly poor results.

The Norma figures you have posted are quiet high/good, if actually totally true. There was a time when manufacturers were telling porkies re their projectile BC's.

Its not a gulfstream jet i can tell you that much

As a pretty close rule of thumb, if you only have a G1 figure and want a G7, use 55% of the known G1.

Order of best accuracy for modern sleek projectiles:

1. Custom drag model (applied ballistics)
2. G7
3. G1

In all reality unless you are doing serious stuff, any one works fine for basic hunting distances so long as you input data correctly.

Hornady would argue with Applied Ballistics (Litz) that their numbers are the bee's knees.

I always use Litz's number, when available.