It turns out my neck tension is 0.002" (the diameter of the sized neck is 0.274" while the diameter of the neck with a bullet loaded is 0.276") so I will go ahead and try shooting it as is and hopefully will get excellent accuracy. Rufous.

My numbers were pulled from an imaginary case,...if that thew you off,..then I apologize. The calculation remains the same,..no matter the caliber or brass thickness. I simply plugged in some numbers to make my point.

Let me put it THIS way.

My sized necks on my 280AI are .308" and my seated round necks measure .309",..therefore my neck tenson is .001" and my groups are <.25moa OOPS,..now I am bragging

So when at the range with 4 presses:
1) decap
2) prime
3) resize
4) seat

I use the same case for all 5 shots, but after reading this thread, I see I may be getting a stiff neck.

I have done a bit more checking on my neck tension with my 6-284. As I said initially I am using Lapua 6.5-284 brass and necking it down. The outside neck diameter of my necked down case is 0.274" and the outside neck diameter of the case with a bullet seated is 0.276" which we are calling 0.002" of neck tension. The confusion is that when I run a fired case through my neck die and then seat a bullet into that case I find the same numbers but it is obviously easier to seat the bullet in the neck of the fired and resized case than in the neck of the new case which had just been necked down to 6mm from 6.5mm. So that is why I initially thought I had too much neck tension (based on past experience it seemed difficult to seat the bullets in the newly necked down cases, except for the example below with a previous 6-284). So far I am getting terrible accuracy. I am hoping that once I get my 100 cases fireformed the accuracy will improve as the neck tension will be less (at least it will seem less since the bullets will be easier to seat) and more consistant.

I remember when I had a previous 6-284. I was using 284 Win brass and necking it down to 6mm. It was very difficult to seat a bullet into those necks, so difficult in fact that the ogive of the Sierra 107 Matchking bullets would often look like it was collapsing somewhat right where the seater die plug (Redding cometition seater) was contacting the bullet.

I did not want to have to turn necks with this current 6-284. That is why I decided to use the high quality Lapua brass and have the gunsmith size the chamber neck based off of a loaded dummy round.

Any thoughts or comments? Thanks, Rufous.

Guys, I know some of you here have brushed across this but I didn't see anyone specifically cover it, so clear me up here if I'm missing something.



Measuring the outside diameter of the neck once the bullet is seated in it is NOT neck tension. Neck tension is certainly going to change with the thickness of the brass in the necks, but it will also change as the brass becomes work hardened. You can't measure neck tension by mic'ing the outside of the neck with a bullet seated in the case because that doesn't measure the elasticity of the brass and how hard the brass in the neck is trying to squeeze on the bullet. Generally you're going to have more of a problem with too little neck tension rather than too much neck tension. Too little neck tension obviously being caused by the necks getting hard or by being reamed out. All you're going to get by measuring the outside of the neck before and after the bullet is seated is the neck wall thickness.