I would think the harder the lead the less chance to mushroom on game if bone is hit,and using higher speed in m/l less chance to lead the barrel.I have tried his 10 BHN and have no problem in loading them down the barrel..
Below is the guy statement who cast them between the two choices soft or hard REALS
These hard REALS should leave you two holes, and assuming you go with your hunting (upper) load, and don't hit serious bone (if you are somone who subscribes to the "break both shoulders" school of thought, I would suggest our minie instead of the REAL or any other bullet style) you will be fine.
This BHN 10 REAL should completely penetrate, and leave an exit wound at the yardages you list and at the 90 grain load. Possibly even at the 80 grain load. My Hawken is listed for 90 max as well, but the difference in velocity has hit the law of diminishing returns by then (more powder for not much more umph).
The rifling is correct in both your rifles for the R.E.A.L.
The main difference in the two 50 cal REALS we make is that the pure lead soft one will mushroom out substantially on impact, and though you will get better than round ball penetration, you might hit one shoulder or nick a rib and a soft pure lead slug will DEFINITELY mushroom out a bit at that moment, starting to careen off target. Longer bullets can also have this problem, due to deflection altering their course.
Though it will tear up the medium through which it passes, the frontal resistance will stop the slug FAST, especially if it encounters another rib on the way out (your one-hole experience, likely). Worst of all, it makes a heart hit into a possible one lung hit if the angle is not good, again making it a one-hole, deer still running for a mile (or 2), nightmare tracking experience.
These harder slugs will not deform as easily, making an exit hole almost 70% more possible in a whitetail deer. They retain their weight well also, as they will not expand and fragment as easily (a round ball in pure lead can splatter flat open when it hits a solid bone).