I have two 357 Mag handi's that are now maximums for Indiana deer. I'm shooting the 180 SSPB bullets out of the maximum. My opinion your asking too much of the magnum with 180 grain bullets. The maximum and the 180 are a wonderful marriage. For a magnum at 75 yards I would use a 158 grain gas check or 158 soft point jacketed, avoid the hollow point. I don't use 2400 so can't say anything about that.
Water filed containers are the worst thing to test bullets with. Bullets over expand when shot into water, water tension is a lot stronger than most people understand. A better test is damp condensed paper like phone books, magazines or newspapers. Google ballistic buffalo. A pile of sand isn't bad either.
Just my two cents.