Re: 6.5 X284 vs 300 ultra
IF you put a brake on the 300 it would tame it down considerably. I for one have shot 30 cal's most of my life and several years ago I won the I.B.S VA State Championship in Light gun using the 240 SMK. I shot a 4.040 group for a score of 50. Three shots were in 5/8 of an inch in the center of the X. The next day I shot a 5.2" with a 49 score.
I said that to say this, if you can reduce recoil the big gun always wins. I always shot with a tight grip and stiffened body so when the gun recoiled it didn't move.
Last year I shot a 5.5" group at my house using my 6.5x284 with the 140 Bergers
My choice between the 2 is definitely the 30 cal with 240 SMK
My final authority in all matters of faith and practice is the King James Bible. It's still the monarch of books!!