I'm having a gun chambered in 6x284 Win. The neck reamer the smith is using is .267. I purchased neck dies from Redding w/ the bushing system. I have to form the cases from .284 Win cases. Was going to use a 6.5x 284 FL die then a 6x284 FL die. How do I know how much brass to trim from the necks? How much "neck tension" is adequate to hold bullets? Thanks in advance for any help. This is my first custom rifle and I want to do it right.

First, I doubt you need the 6.5 die to neck down the 6mm. Your regular 6 die should be fine.

Presume you don't mean "trim" but "turn" as to how much brass to remove from the neck. You'll trim to your normal "trim to" length. As for the neck turning, you want to end up with a minimum of .002" clearance with your seated bullet. Your finished round should have a neck diameter of .265" for that clearance. Subtracting the diameter of your .243 bullet, you'll have a total neck thickness of .022", divided by 2, so that your final neck wall thickness should be .011" in your turned neck.

As to grip on the bullet, it does affect accuracy as well as consistency of the load. About the best all around grip in my own experience, has been .002". You will want to experiment with your loads and your rifle so buy bushings from .261" through .264". In a brief experiment of my own I found a particular load I was using was most accurate with the lighter grips, about .001" and just enough to hold the bullet. However, this won't work well for recoil in a gun with more than minimum recoil as the bullets in the magazine will be affected by the recoil. .002" is about minimum for me, but these were seated in heavier recoilers in light rifles in my favorite caliber. After .003" I noticed a marked difference in accuracy in my experiment, and beyond .004" grip, it didn't seem to make much difference, either in accuracy or the amount of force required to seat the bullet.

Just an observation, but 6.5x284 brass would be my choice to form from. Particularly the Lapua version. FWIW, Dutch.

When I was building my first tight neck rifle I found that on Varmint Al's web pages he goes into detail on the correct way to fit the neck to the chamber. It is very helpful and worth checking out.