Skipper, I have found that in about any .338 the 300 Sierra Match King shoots well. They don't seem to mind to be a little off the lands, in fact mine in the lapua shoot better at least .060" off the lands. I feel this will work good with the freebore in a 338-378. So if it were me, I would be loading 300 SMK's at mag box length (3.800") over a charge of H-1000 and a CCI 250 primer. Work up your loads at 200 meters. I have seen them take this long to settle in. Then if you like the way they shoot, this could be used for hunting too and anything you shoot with it will be dead. Here is one after penetrating the shoulders of a bull elk at 526 yards.