Thanks for the responses
Looking like it will be the A- Frames, based on the fact that there seem to be are far more sources of factory loaded A-Frames than anything else.
Understandably loads like this are a limited production run with Federal and not always available from Federal. If I can't run down some Federal, one of the other sources mentioned here should work. A bit if internet research shows that both Norma and Sako also load the 300 A-Frame in 375 H&H, and surely one of those should shoot well if I can't find the Federal. Anything Norma is hard to find near me, so that cuts out the 350 Woodleigh PH ammo, although if I was prepared to hand load for the 375, I would certainly try the 350.
My (untested ) personal thought is that the A-Frames will probably not open as wide as TBBC, 300 Woodleighs, or 300 Oryx. I'm being a bit simplistic, but less expansion should help penetration.
And thanks for the info about the "trampoline effect". Makes sense to me.
Thanks again.