A normalized vector is a vector with a length of one. You already have access to those vectors, no need to normalize.
The bullet impact point is what seems funny to me. It draws a vector from the world origin to the impact point, and then to the reverse direction of the bullet.
A reverse of a vector can be found by multiplying the vector by -1.
Try:
FMath::RadiansToDegrees(acosf(Tank->GetForwardVector(), ActiveBullets[i]->GetActorForwardVector()*-1);