Well it is a shame you lost the deer. Hard to say what happened since the deer was not recovered. I will say if you shot a 300 grain Hornady XTP, they are known for taking many deer in the past and will do so in the future. I shoot them out of a number of rifles. I have not had the chance to try them on a deer yet.

If I had to pick a real knock them flat bullet it would be a Barnes Expander in 300 grain or a Nosler Partition in 260 grain. I just can not picture a deer taking a direct hit through the boiler room and making it very far without a lot of blood loss.