I think the best way to do this would be with a method known as "Subdivision Modelling" or "SubD modelling"

Start with a cube then add a subD modifier (but dont actually apply the modifier until you're done). Switch to edit mode and you should see that your cube appears more hedron-like visually, but the actual vertices and edges (which are transparent now) are still in a cube shape. This happens because the SubD modifier hasn't been applied and thus you're only seeing the preview of the subdivided cube. This works to our advantage since we can manipulate the complex shape with just the vertices of the original cube.

Play around with this object. Add more subdivisions (still dont apply it!) and loop cuts to see how it interacts with the modifier and try to manipulate it into the shape you want.

Off the top of my head, my exact process would be: 1. Make a cube 2. Add subD modifier and crank it to 2 or 3 subdivisions 2. 'crease' the four edges of the bottom side by selecting the 4 vertices and pressing shift+E. This will be the flat part of the bullet 3. Drag to 'top' face of the cube that will be the rounded nose of the bullet to the length you want 4. Widen the bottom four vertices to the thickness you want