AR's load data is hot, all of it is a little over book max so if your going to use it back off a ways for starting loads.
I've only played with one 338-378 and it had a 30" custome bbl. 119gr of AA 8700 under a 250gr SGK for 3100fps. This load is warm, primer pockets get lose after a couple of fireings but it was supper accurate and fast.