case neck and bullet runout

Discussion in 'Reloading' started by dmax1800, Nov 28, 2013.

  1. dmax1800

    dmax1800 Well-Known Member

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    Since I recently found out that I had .005 to .008 bullet runout on some 300 win mag rounds I loaded in Sep, I have become interested in bullet runout. I'm loading for hunting, but I want THE most accurate round I can load. A 1/4" inch improvement at 100 yard groupings is important to me. Just the way I am.

    I've squared the RCBS dies, got a Lee decapping die, moved the expander ball all the way up in the FL sizing die, put the bullet straight up in the case before seating it and rotated the bullet 180 degrees after seating the bullet half way. I tried some 270 win fireformed brass and loaded dummy rounds. I got less than .001 bullet runout, but the runout measured on the neck about 1/8" from the mouth was .002 to .003.

    How could I get so little bullet runout but have a significant case neck runout??? Which is more important measure for accuracy: bullet runout or case neck runout or both? If case neck runout is important, what can I do to correct it?

    Thanks.
     
  2. Mikecr

    Mikecr Well-Known Member

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    You might pick one set of conditions for anyone to reply on.
    Is your question for 300WM, or 270Win?
    What are the conditions for either of interest?
     

  3. Firearrow

    Firearrow Well-Known Member

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  4. dmax1800

    dmax1800 Well-Known Member

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    My question is about the 270 win loads. As I stated in my original post, I'm interested in knowing 1) why there can be more runout in the case neck than in the bullet; 2) which is the more important measurement, case neck or bullet runout for accuracy; and 3) if case neck runout is important, what can be done to correct it.
     
  5. Derek M.

    Derek M. Well-Known Member

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    You can turn the necks, both inside and out. I think the runout of the finished loaded round is a function of case neck runout for the most part. Both contribute so correct both the best you can. Even with a properly prepped case, including inside and outside neck turning, you can still seat a bullet with a bit of runout. One measure I took was to eliminate the expander all together and opened up the die neck to .003 less than a loaded dummy round.
     
  6. Derek M.

    Derek M. Well-Known Member

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    I also anneal after each firing for the most part. I've had no problem with consistent, repeatable accuracy.
     
  7. woods

    woods Well-Known Member

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    If you had thick and thin side on the brass of your neck that measured .015" and .013" respectively, you could have zero runout on the bullet and .002" runout on the case neck brass

    But runout on the case neck is of little consequence compared to bullet runout

    You could outside neck turn and solve the case runout but haven't accomplished much IMO
     
  8. Mikecr

    Mikecr Well-Known Member

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    WOODS explained it well for a SAAMI (typical hunting) cartridge/chamber.

    An unlucky friend had lugs badly lapped in a gun(unknown to him), and afterwards his go-to ammo started throwing shots all over the place. His ammo could show low runout(depending on gage), but the case heads were not square, as seen by rolling the cases across glass. After further work on the action and re-fireforming his ammo performed well again. I can only imagine his off-square case heads were setting a skew to cases chambered. And so I can imagine that crooked cases in general could cause throwing shots.
    So I say the best we can do is make fully straight and square ammo, and use this in a precision fit chamber.
    This is best seen with a v-block type gage, measuring TIR off necks AND bullet bearing.
    But, with generous neck clearances and runout known to be neck thickness variance alone, a measure off bullets will suffice.
     
  9. Firearrow

    Firearrow Well-Known Member

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    I was wondering about this. Thankyou for pointing it out. I had sorted all my brass be weight and neck runout and thought that the brass that was less than .002 would be perfect and the brass that was .005-.006 would be not so perfect.

    Well after I loaded up my test loads I found out that was not the case. It seemed some brass just produced low bullet run out regardless of what the neck was, and then there was the brass that wanted to have more run out.

    I will play with the expander die and mandrel when it shows up on Tuesday and see if it helps.