Trajectory.cs

public class Trajectory {
    public class PredictionResult {
        public bool InRange;
        public Vector3[] ShotVelocity;
        public float[] ShotImpactTime;
        public Vector3[] ShotImpactLocation;
    }

    public static PredictionResult ShootAtTarget(Vector3 start, Vector3 target, Vector3 startVel, Vector3 targetVel, Vector3 bulletAccel, Vector3 targetAccel, float bulletSpeed) {
        PredictionResult res = new PredictionResult();

        Vector3 deltaPos = target - start;
        Vector3 deltaVel = targetVel - startVel;
        Vector3 deltaAccel = targetAccel - bulletAccel;
        
        double[] impacts;
        if (deltaAccel == Vector3.zero) {
            double a = Vector3.Dot(deltaVel, deltaVel) - bulletSpeed * bulletSpeed;
            double b = 2.0 * Vector3.Dot(deltaVel, deltaPos);
            double c = Vector3.Dot(deltaPos, deltaPos);

            impacts = Solver.SolveQuadric(a, b, c);
        } else {
            double a = Vector3.Dot(deltaAccel, deltaAccel) / 4.0;
            double b = Vector3.Dot(deltaAccel, deltaVel);
            double c = Vector3.Dot(deltaAccel, deltaPos) + Vector3.Dot(deltaVel, deltaVel) - bulletSpeed * bulletSpeed;
            double d = 2.0 * Vector3.Dot(deltaVel, deltaPos);
            double e = Vector3.Dot(deltaPos, deltaPos);

            impacts = Solver.SolveQuartic(a, b, c, d, e);
        }

        if (impacts != null) {
            Array.Sort(impacts);
            int sol = 0;
            for (int i = 0; i < impacts.Length; i++) {
                if (impacts[i] > 0) {
                    sol++;
                }
            }
            if (sol > 0) {
                res.ShotImpactLocation = new Vector3[sol];
                res.ShotImpactTime = new float[sol];
                res.ShotVelocity = new Vector3[sol];

                int answer = 0;
                for (int i = 0; i < impacts.Length; i++) {
                    float imp = (float)impacts[i];
                    if (imp > 0) {
                        res.ShotImpactTime[answer] = imp;
                        Vector3 offsetHit = deltaPos + deltaVel * imp + (0.5f * targetAccel) * imp * imp;
                        res.ShotImpactLocation[answer] = start + offsetHit;
                        res.ShotVelocity[answer] = offsetHit / imp - (0.5f * bulletAccel) * imp;

                        answer++;
                    }
                }
                res.InRange = true;
            } else {
                res.InRange = false;
            }
        } else {
            res.InRange = false;
        }

        return res;
    }
}

Shouldn't res.ShotImpactLocation[answer] be something like this?

res.ShotImpactLocation[answer] = start + startVel + offsetHit;