Cast bullets for hunting???????

The .300 WSM has a bit larger case capacity than is ideal for best efficiency with cast bullets, but it will work.  For deer I'd suggest the heaviest flat-nosed bullet you can load that will not result in the GC protruding into the powder space, and which will chamber easily with slight resistance, contact the rifling with light engraving, but in which you can extract a loaded round without debulleting to avoid spilling loose powder in the action. 

To enable bullet expansion alloy hardness should not exceed BHN15. Lyman No. 2 or Taracorp Magnum alloy expand somewhat and typically blow off the nose in large fragments, the bullet shank retaining about 50% of its weight like old Nosler Partitions or the RWS H-Mantel.

A softer alloy of BHN12-13 is better if you can use it. WW + 2% tin expands to 1.5 times its original diameter, and retains 2/3 of its weight, with penetration equal to factory SP loads at velocities from 1800-2000 f.p.s..

Harder alloy bullets such as linotype tend to “crumble” above 1800 fps rather than expand, and over 2000 fps they cause shallow wounds like a varmint bullet which wreck alot of meat and may fail on large animals due to poor penetration. 

Heat treated bullets “rivet” or flatten a bit at higher velocity, but stay together well and penetrate through and through from any angle.  They work best on large animals. For deer a HT solid bulllet should be  .35 cal.  or larger, be driven over 1800 fps and be shaped like LBT types with as large a meplat as will feed.

With a smooth barrel, a bullet which fits the throat well, and proper lubrication you should be able to maintain hunting accuracy with softer alloy to 2000 f.p.s. or so for “a few shots” before leading starts to degrade grouping.  If you keep velocity down to around 1800-1900 you should be able to shoot 20 rounds or so. With such loads cast bullet performance approximate factory soft points in a .30-30, .303 Savage or .30-40 Krag - it's a 150-yard load.

The slower magnum rifle powders are unsafe for reduced loads, so you must use a medium burning rate powder such as 4895, RL-15 or Varget, using an adequate powder charge which occupies not less than 2/3 of case capacity.  Use a hot primer such as the Federal 210, or Winchester WLR. 

In the .300 Win. Mag. I used 56 grs. of 4895 with good results with the RCBS 30-180FN cast of wheelweights for a guy who wanted to try cast in his deer rifle, but that was 20 years ago. It would be better to check with somebody who's tried this with current powders.