weird neck tension-please advise

[rufous]

I just got my varmint rifle rechambered fro 243 Win to 6-284. I am using the Lapua 6.5-284 brass necked down to 6mm and the Sierra 107 Matchking bullet. This combo produced a loaded round neck diameter of 0.276”, while the outside neck diameter of the necked down case is 0.274” (apparently that would be called 0.002” of neck tension). The chamber neck diameter is 0.278”. It seems to me that the neck tension is higher than ideal for best accuracy even though it is only 0.002”.

I have noticed that when I run a fired case through my neck die and then seat a bullet into that case I find the same numbers (0.274” neck and 0.276” loaded round neck) 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 competition 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.

[jb1000br]

maybe the stress from necking down adds tension, and it is released when its fired??

JB [img]images/icons/confused.gif[/img]

[meichele]

Check for burrs on the inside and out side of the neck mouths. Also make sure that all necks are prepared the same. If there is powder residue on some but not others, if some have a little case lube in them and not others ect...