Elmer Keith was probably the most to blame for the term "hard cast" because he was perhaps the most well known writer of the day, along with Phil Sharpe & a host of others although they did less writing.
The problem is, what Elmer was calling hard cast wasn't really hard at all compared to what many of us shoot today. His bullets most times were around 9-11 BHN using a lead/ tin mix, usually 16-1 or 20-1.
And so it goes, people call a bullet a hard cast & it can be anywhere from 9 up to 22 BHN or even harder if you decide to heat treat them in the oven, etc. Here you can exceed 30 BHN.
As Rey mentioned, and I happen to agree with, once you exceed around 1300 fps all you're buying is recoil. If you're using the correct bullet/alloy a cast bullet will pass through "almost" any animal you shoot with it, except for the megatron animals that some shoot from time to time.
In that case a better mouse trap (bullet) is needed, not a harder bullet, trust me. If you shoot a heavy for caliber bullet that's made out of the proper alloy for the game at hand you will seldom ever recover a cast bullet. Not saying it won't happen, just saying it's rare.
In over 50 years of shooting game up to the size of elk (many) moose & African plains game I've recovered maybe 4-5 bullets & these have been HP's. My solids have always exited, a few have went through big mule deer length ways.
My suggestion for HP's on deer if shooting for the shoulder would be to shoot a small HP at 3/4 throttle, say 1000-1200 fps in the 41, 44 or 45. Pushing a big HP faster just turns them inside out without the needed penetration & if you make a harder bullet you can upset the balance of hardness vs velocity.
The best way to over come this (my opinion) is 2 things. First is a gas check & the second is powder coating. This allows you to shoot a soft bullet faster than normal without leading but still, trying to use HP's on game bigger than deer or antelope size game is a slippery slope.
One exception is the big Miha 385 gr HP's running at 1000-1150 fps. Using the correct alloy at this speed, this bullet has taken some really big game like elk, moose & bison with complete pass throughs. Taking the next step up to the megatrons requires the high dollar bullets, so use them.
Another very good HP if using the medium HP pins in 45 caliber is that Miha 275 gr bullet like I sent to 45man, that bullet will handle game up to elk size.

Dick