Which area sould we write in the ref. values to calculate a bullet drag ?

for a basic calculus, I would take the cross sectional area.

But to be very accurate, should I take the : head surface (it's a smooth head, like a diabolo) + outline areas + cap area (botom, near the firing pin) ??

Hello,

Fluent is actually giving the calculated force. So, it's a matter of preference to use any of the areas. If the coefficient is to be compared to some data, then the same reference values (area, velocity, density, etc) should be used.

note: Setting rho = 2 should give the drag force, itself.