Anybody shoot a .270 x 300 weatherby mag or 270 STW?

I built a .270 x 300 WBY Mark V with a 26" stainless fluted factory wby 270 barrel. Tight neck, and seating bullet .005" off the lands, (no wby freebore).

My first three shots were .56" with 150g Nosler B.T. and 76g RL22 @ 3270 fps.
Next test 3 shot group to speak of grouped .22" with 150g Nosler B.T. and 70g RL22 @ 3047 fps.

The only problem I have found is with: (79g RL22 with 150 B.T. @ 3464 fps), the pressure is too high and primer pockets start to blow.

I can't get velocities equal to my 7mm STW, must be due to the bore is smaller, the 140g bullet friction is greater than 7STW, and less burn volume builds pressure quicker. Is this true? anyone with experience on this one?

I also tested AA8700 powder: 140g Nosler Bonded, 95g =3370fps. and 89g = 3100 fps. 1" groups.

Thanks for any other traded experience.