I would like to know your thoughts on this subject. Years ago when I was shooting handgun silhouette and casting for 7mm TCU, I believe I remember dropping the cast boolets into a 5 gallon bucket of water to (?) harden them?? Is hard cast just the lead / tin ratio difference ?
I have a Contender 14" in 45 Win mag and thinking about taking a Iowa deer with it. I'm thinking a 285 - 300 grain would would do the job? Thoughts?😎

So the hard cast terminology means contains antimony and quenched in water ? The 7 TCU was mostly cast with wheel weights 30+ years ago so not sure if they contained antimony back then. I agree that I don't want a hard bullet but wasn't sure what constitutes " Hard Cast "

BHN:

Pure lead: 5.0
40:1 lead:tin: 8.0
30:1 lead:tin: 9.0
20:1 lead:tin: 10.0
10:1 lead:tin: 11.0
water quenched WW: 18.0

WW: 1/2-1% tin (Sn), 2-3% antimony (Sb), 96.0% - 97.25% lead (Pb), 1/4% arsenic

When I lived in Illinois, there was a scrap yard in Genoa, Illinois that mixed a 94% Pb, 4% Sb, 2% Sn alloy that worked well for many uses. The owner was selling this in two-ton lots and we were able to buy the overrun of a lot. Unfortunately, he has retired and closed the scrap yard.

Dropped into ice-cold water, the bullets hardened to 20 to 22 Brinell which is linotype hardness. These were shot mainly for both rifle and pistol competition.

As cast, they were 14-15 Brinell which worked well for most informal pistol and rifle shooting.

In the world of commercial purveyors of lead bullets who service the mis-guided souls who can't/won't cast their own, "hard cast" has come to mean anything from 13-22bhn. There's no industry standard. They sell a lot because the misinformed public who dotes upon jacketed bullets intuitively think that "harder has got to be better" and trick themselves, mostly, when they seek out cheaper alternatives.

Hypo5.0
Your alloy goes not Have to be soft, though it can be ok 
in maybe 16+1 or 20+1. Remember that at high velocities,
you may get leading. In this event, use a gas check style bullet.

 The larger bore bullets such as you can shoot in that 45 Win Mag T/C will work just fine for your deer hunt.  Your bullet is already 45 cal, and will act like an expanded
smaller caliber 
Suggest you select a bullet in that 250 or so weight that has a Large flat nose.   
I have shot a number of large Wi deer with my M86 45-70
with a 385 gr Flat Nose gas check bullet at 1740fps.
It is very effective. One shot one Kill on all of them. They do not go far. If you must, say in a crowded area, to "drop them on the spot" , you can take a shoulder shot. The deer will be down "right now" and, with the medium hard No 2 level bullet
will not explode and will not blood shot much meat. 
Tha alloy can be like Lyman No 2 which has 90% lead, 5% Antimony and 5% tin.   
My alloy of choice is 90% COWW/10% Lino.  About the same hardness as the Lyman No 2

beltfed/arnie

Thanks for all the additional information and recommendations. Think I'll just cast up about 50 - 75 250 grain flat nose out of 20-1 , sight it in and go sit in the woods. I purchased some 255 grain Underwood " Hard Cast " off ammo seek to get some reloadable brass to start this project started.