If you are contemplating using cast bullets in your .303 I would suggest you pm dodgyrog and purchase them from him. His bullets are the correct alloy(hardness ie: Lyman No2 not hard cast.) That will fill the grooves properly.
I cast my own bullets and I would not have hard cast bullets given even if they came for free. Actually that's not true I would melt them down and add soft lead to achieve a Lyman number 2 alloy.
I have been shooting cast loads for more years than I care to remember you want an alloy that will fill the groves hard cast is not the way to go you need to get them from Rog or cast your own.
I have used dodgyrog,s bullets in all my cast bullet calibers, and never had any issues. But nowadays as I said I usually cast my own I use range lead gathered up from the Clubs backstop.