Space invader barrier implementation algorithm

I've been struggling with implementing the space invader bullet to barriers. I wanted right now to blast a circle around, when the bullet hits the barrier and modifies the circle. As shown in the video, the algorithm looks wrong, http://youtu.be/VMBczCsP3J4 I have tried my best to debug it, but I've failed.

here is the algorithm:

// if there is intersection
    if (IntersectsWith((*it)->BoundingRect, barrierRect))
    {
        // subtract (*it) bullet position from barrrier's rect 
        int normX = (*it)->Position.x - barrierRect.x;
        int normY = (*it)->Position.y - barrierRect.y;

        // barrier image pixels 
        uint32* ptr = (uint32*)pixels;
        // height of the barrier
        int y = 31;

        for (int i = 0; i < 31; i++)
        {
            int pixelOffset = y + normX * 51;
            int color = ptr[pixelOffset];
            // look up the color if its not black, it should be green
            if (color != 0xFF000000)
            {
                // delete the bullet
                Bullet *bullet = *it;
                it = ship->Bullets->erase(it);
                delete bullet;
                ship->Canfire = true;
                isCollision = true;

                // blast a circle around that bullet hit position
                int radius = 9;
                for (int y = -radius; y <= radius; y++)
                {
                    for (int x = -radius; x <= radius; x++)
                    {
                        if (x*x + y*y <= radius*radius)
                        {
                            int j = x + normX;
                            int i = y + normY;
                            int pixelOffset = j + i * 51;
                            ptr[pixelOffset] = 0xff000000;
                        }
                    }
                }
                break;
            }
            // go up
            y--;
        }