RLD, keep in mind as your casting improves with experience that mold temperature is the leading cause of casting problems. That's not to say crank up the pot temp, with your alloy somewhere between 700 and 750 degrees is more than needed. Pre-heat your mold and cast at a faster pace to increase mold temp. Don't inspect your bullets as you cast, while your doing that the mold is cooling even more. If the mold gets too hot you'll see it quickly and slow your pace a bit.
The reason your bullets look like they do in the picture is because the mold temp is too far below the freeze temp of the alloy, the alloy flowing into the mold is actually freezing against the mold before it has a chance to properly fill out all the corners.