What are your favourite handloads or factory loads for these ultra-calibers? What kind of performance have you come to expect from them (i.e., How they work for you on game, the target groups you get, the speeds you get on your chronograph)?
My favorite load, so far, for the 30-378 Wby. Mag. is the 180 grain Nosler Partition, backed by 112.0 grains of Hodgdon's Retumbo powder. I use Federal 215 Match primers. According to my chronograph, I'm getting an average of 3400 ft/s, nine feet from the muzzle. It has averaged a little over an inch at 100 yards for 4, 3 shot groups off the bench. In my opinion, this appears to be a sensible maximum in my rifle.
This coming November will be the 30-378's maiden voyage for whitetail deer.
I haven't begun to reload for the 338-378 yet but plan to in the near future. Hopefully, I'll get a chance to hunt with it, too. The 250 grain Nosler Partition Weatherby factory load appears to be splendidly accurate for me. It has averaged an inch at 100 yards for 4, 3 shot groups off the bench.