In the worst case you will get a .308 entry hole and a .308 exit hole. Plenty to kill a deer IMO. I have it on good authority that any number of Roosevelt elk were killed in western Washington with .30-06 GI ball ammo during the depression. That said, 18-20 bhn are actually brittle...if you whack one with a hammer you will discover just how frangible they are.
Of the most concern to me if I had your rifle and was trying those hard bullets...would be the accuracy question. Hard bullets need to fit the throat...soft bullets give you a little wriggle room if they are not perfectly dimensioned for the throat.
All in all, given the fact good serviceable hunting bullets are available now for the most part, I'd recommend any other bullet over what you have.
If you'd like to pursue cast bullets, bless you...it's so easy and cheap to get started...why not jump in?
Help is here if you ask for it...more help than you ever wanted, LOL.
