100% contact is more contact than 75% contact, I want all the contact I can get. and I want nothing between my chamber and case but air, clean air, air can be compressed, air is fluid, air flows.
Long story, I have a Model 70 Winchester chambered in 300 Winchester Mag, it came with the ugliest chamber I have ever seen, I called Winchester, I explained to them the ugliness of the chamber, in diameter, length and gouges, they instructed me to take the rifle to their warranty smith, I did, he said he was going to hone, polish and or ream the chamber, And I then ask “How will honing, polishing and or reaming going to make the chamber smaller”. Brilliant, his answer was brilliant, he said Winchester said he had to di it. Months later I went to the warranty smith to to check on my rifle, no rifle, I ask why? the warranty smith said the chamber was too large. He sent the rifle back to Winchester, Winchester and I had words, they accused me of of going from being difficult to impossible, I wanted a chamber that fit my dies or I wanted Winchester dies to fit their chamber. It has been 4 years since they returned the rifle ‘in a new box’ one day I will take the rifle to the range.
With all my barrels and not one barrel for a Winchester with a Magnum bolt face.