I just got a Browning X-bolt 7mm WSM, and i ma just wanting to know if i should use 140 grain or 160 grain bullets, or even 150's for hunting elk in eastern Oregon, i can only shoot a Spike since i can only go second hunt, i don't know if that makes any difference or not, maybe not as tough of skin as the older ones is my thinking. any info would be appreciated. Thanks.