The insert will help a lot. I have an old split shot mold and even with a crude insert it cleaned up the pour a lot. Can't make shot out of it yet without the real insert but it helped to see what they are supposed to look like.

If you all you want to do is make sling shot ammo with it then find a piece of tin or aluminum that will fill the place the insert is supposed to go. Shape it a little and you should have some better results making them round without some much extra.