that is incredibly interesting considering that the 40gr hornady Vmax loaded in most factory 5.7 casings have a BC of .200 yet a 55gr sierra gameking intended for 223 and 22-250 has a BC of .185.
That has nothing to do with the 5.7x28 case. Both those ballistic coefficients are terrible, and it's likely comparing bullets of different shapes. Considering all these bullets are .224 diameter, they could all potentially be used interchangeably, and if you compare similar shaped bullets and constructed bullets, heavier ones will always have better BC's. Of course if you compare a 40 grain boat tail to a 55 grain flat base, there are going to be numerous differences. And the boat tail has the potential for a higher BC despite the flatbase being slightly heavier.
You do realize you could load the Vmax into .223 and the gameking in the 5.7, right? It doesn't change the factuality of my statement, simply because you found a bullet with a higher BC in 5.7 than a bullet in .223 doesn't change that the 5.7 is generally loaded with low BC and low SD bullets. Of course you could find a lower BC bullet for the .223 as you could for the 5.7
The original AP ammo in the 5.7 ranged in weight from 23-31 grains.
Also note I did type 'generally'
Of course you could probably load a 90 grain boat tail into the 5.7 and get a decent BC and SD but it's going to be pushed SLOW and it's going to need a custom barrel. GENERALLY 5.7 as well as .223 bullets tend to have low BC's and SD's. Since 60+ grain bullets are more common in the .223 it generally is loaded with slightly higher SD and BC bullets. But that doesn't stop someone from loading a 30 grain flat base in their .223 either.