full sizing vs. neck sizing?

[anweis]

All else being equal (bullet, powder charge, primer, etc), i get 1" groups from new unfired cases (7mm Mauser) and 2-2.5" inch groups from neck sized brass reloaded after one firing. What does this mean?
It means that my load is excellent for new cases but i need to keep trying loads for the neck sized brass? Does it mean that i am not preparing the cases properly? Should i just get a full body die and full size?

[wncchester]

Are we to assume that your good groups are coming from new brass that you have not resized at all?  Or, were they FL sized or neck sized before using?

You don't say what brand of "neck" die you have.  It's likey that your neck die is sizing down much too far so when the expander button comes out the neck will get "bent", causing more bullet run-out.  AND/OR, the neck isn't being expanded quite enough, the "tension " is too high so the bullets actually act as an expander as they are seated.  If so, the case-to bullet junction is not supported when seating starts, so you get a lot of run-out.

Suggest you try a Lee Collet neck die, it leaves very straight necks that are big enough to allow bullets to enter properly.

[anweis]

new brass that you have not resized at all?  Suggest you try a Lee Collet neck die, it leaves very straight necks that are big enough to allow bullets to enter properly.

Yes, new brass, unformed, not resized at all gives me great accuracy. And yes, i am using a Lee collet die, and yes, last night i noticed that it was screwed in 2 turns past the point where the die touches the shellholder when the ram is completely up. The instructions call for 1-1 1/4 turn after touching the shellholder. You probably are right, i was pushing the shoulder of the case too much. I will go back and re-adjust the die, and see what happens. Any other opinion?
Thanks

[Wyo. Coyote Hunter]

  anweis, Sometimes new brass works well with a load from a fire formed case, sometimes not. Once the case is fireformed the capacity is a bit different. If you only neck size, it remains pretty constant from shot to shot. I usually necksize my cases. Sometims it is required to fireform the case then develope a load for the fireformed case. That sounds to me what you need to do. It is fustrating to have a good load in new brass that shoots poorly in fireformed cases. Been there done that. My pals .300 wea. is famous for this fault. While my old .300 win. accepts either with little problem. Just one of those deals with reloading.

[Siskiyou]

Just in case has copper accumulated in your bore from earlier firing?  I have seen this rear its ugly head when I was focused on load development and the solvent I was using at the time did not do a good job, or I failed to do a good job. 

Recently I was test loads and then switched to Barnes bullets.  Barnes recommends cleaning all accumulated copper fouling from the bore before firing their bullets to enhance accuracy.  I used Remington 40X and it must have worked.

I realize that two rounds is not a big test, but this was my bottom starting load in developing a load and only loaded two on the light end and increased the number of rounds as I increased the charge. 

[anweis]

Just in case has copper accumulated in your bore from earlier firing? 

Copper should not be a problem because i clean with a good cleaner every 10 shots or so. I can't remember the brand but it does smell like ammonia and it does get 5-10 patches blue. Nu brush, just lots of patches and soaking time.
I also wait and shoot slowly to prevent the barrel from being too hot.

What i do know for sure is that i clean the oil from the barel and shoot a 3 or 4 shot group (super light barrel!), and the first fowling shot from the clean dry cold barrel is exactly in the same place with the other 2 or 3.
 
I have 150 cases and i think that i messed up by
a) developing a load for the new unfired casses, rather than for for the fire-formed cases
b) screwed the collet die too much, and bumped the shoulder a bit too much on the cases

[Siskiyou]

After my little drop of in accuracy experience years ago I was puzzled.  I had been cleaning the barrel with the lowest bid G.I. bore cleaner.  I think that batch looked like water down latex paint, Hoppe’s #9, and other copper solvents.  At home I held my rifle with the butt on the floor in the shop and used a work light to examine the crown and the end of the bore.  I was surprised at how much copper fouling was visible.  Suggest you try this method of looking at the bore with a light at an angle.

I popped open a new can of lowest bid G.I. surplus bore clean, this batch was brownish color.  After making numerous passes with brush and patches I went to step #2.  Out came the Hoppe’s #9 and a new bore brush.  I made a few passes and then went to work with patches.  The results were the green residue from copper fouling.   A couple more sessions with #9 on a bore brush and the patches restored accuracy.

Out at the range I still find the G.I. stuff does a fair job if you can stand the smell.  When I get home the Remington 40X bore cleaner comes out.  It does a real good job.  Once I use up my life time supply of G.I. bore cleaner 40X.  I did give to quarts of it away.
In over fifty years of loading I have never had a load itself go totally out of whack.  A scope failed and bore fouling has been the cause of lost accuracy.

From reading your post you have a handle on the fouling.