I'm assuming a mature cow elk in KEntucky might be smaller than in Montana. Several years ago I chose to shoot a buffalo with a 44 mag 240grain Speer FMJ hotloaded and the results were bad! I head shot it at 35 yards and the first shot went through the skull and into the hillside just flenched like a bee sting and didn't hit the ground, 2nd shot missed the 1st hole by a couple inches through the skull into the hillside with the same reaction the 3rd shot was a little lower and further forward all within few inches of each other. This buffalo still standing decided enough was enough and left the area and about 3 hours later I found the buffalo still alive but bleeding out. It took another shot to finish it off. The energy was going through the buffalo with no expansion and this was a poor bullet choice. A year before that I shot another buffal with a 240grain hornady XTP same fps which dropped the buffalo in it's tracks because the bullet lodged inside the head and thus the energy was spent on the buffalo not the hillside. A 357 magnum can kill an elk but it's certainly not a good choice. I killed an elk with a 243 rifle which has much more power than a 357 magnum but nobody would recomend it for elk. Just don't rush the shot, get it in under 50 yards and make sure you put it through the vitals such as the lungs, heart, head not anywhere else. I don't know what to tell you about a hard cast bullet since it's not a good choice in small calibers and big animals. You need expansion but you also need penetration. I like XTP's but you have to choose what you believe in and know how the bullet works.