Improper bullet choice always fails.

Based on any reliable data I can find, you can reliably load a 150gr partition (in .300 RUM) down to 3400 and as high as 3900. Either way or anywhere inbetween it is a horrible choice.

As our beloved BJ says, "plastic is fer wimmins pantees". Pantee tips will explode. Lighter partition style bullets are moving WAY to fast to stay together. Barnes X expand as low as 2200 (per Barnes) and you will end up with four petals and a stub at 3400. That RUM is just too darn fast for expansion on NA game sized animals.