bullet seating accuracy

Discussion in 'Reloading' started by fireworks, Aug 23, 2013.

  1. fireworks

    fireworks Member

    Messages:
    22
    Joined:
    Aug 18, 2013
    I am kind of new to reloading. I used to use a lee classic handloader but now have a press. With the handloader it was very time consuming to get perfect seat depths, not just because bullets are man made objects and therefore are not 100% uniform, but also the fact I was using a mallet to seat it.
    But even with the press and the die set tight I still manage to under seat or over seat and have to constantly "baby" the depth into place. How do you more experienced loaders get the depth perfect without growing older trying?
     
  2. MontanaRifleman

    MontanaRifleman Well-Known Member

    Messages:
    6,068
    Joined:
    May 21, 2008
    How are you measuring/defining your "depth"?
     

  3. fireworks

    fireworks Member

    Messages:
    22
    Joined:
    Aug 18, 2013
    I use a hornaday digital caliper and look for overall cartridge length off the lands.
     
  4. g0rd0

    g0rd0 Well-Known Member

    Messages:
    516
    Joined:
    Mar 9, 2012
    if you are useing the same bullet (and this can change from one lot number to another) than after you set your depth on your seat die you should be good to go.
    I use the Lee positive seat system it has no ability to crimp and the die is screwed all the way down to the shell holder. When locked in place all of my bullets are the same over all length
     
  5. MontanaRifleman

    MontanaRifleman Well-Known Member

    Messages:
    6,068
    Joined:
    May 21, 2008
    If you're using COAL to determine how far off the lands you are, you wont be able to that accurately with cup and core bullets. Cup and core bullets have variation in their shape from bullet to bullet. If you get your calipers and measure the OALs of the bullets, you will see they vary in length. They also vary in bearing surfaces, ogive, etc. I just measured 10 230 Berger hybrids and their OAL differed by as much as .012. I put them nose first into a sized case (this is approximately where the ogive engages the lands) and measure from the case head to the base of the bullet and that varied up to .008 A bullet comparator does the same thing.

    If you want to seat accurately and consistently "off the lands", then you need to use a bullet comparator and measure the seated ogive length of every round you seat and use a micrometer seating die to adjust for the differences. Some will also sort their bullets according to bearing surface as well. That's a lot of effort and I have never tried it and have been able to work up sub .5 MOA loads without doing either.

    Hope that helps and Welcome to LRH!
     
  6. boomtube

    boomtube Well-Known Member

    Messages:
    1,595
    Joined:
    Oct 8, 2007
    "How do you more experienced loaders get the depth perfect without growing older trying?"

    I suspect most experienced loaders know adjusting seating depth more precisely than maybe +/- 5 thou is usually meaningless.
     
  7. Mikecr

    Mikecr Well-Known Member

    Messages:
    4,264
    Joined:
    Aug 10, 2003
    5thou out would affect some of my loads.
    fireworks, you can seat to exact desired depths, but the trick is in measuring as such.

    You can't really measure by COAL even if you qualify bullets by same OAL, due to ogive radius variances. So you'd have to qualify both bullet OAL and bullet ogives before seating.
    A better measure is OgvOAL, using a comparator as mentioned. This at least removes bullet OAL as a factor.

    I do qualify ogive radius using Bob Green comparators(BGC) Bob Green New Products
    And for OgvOAL, I prefer a Sinclair 'nut' SINCLAIR HEX STYLE BULLET COMPARATORS | Sinclair Intl
    I seat bullets with under 1thou total variance(ES), and I measure every one.

    But the BGCs are expensive and I don't imagine you care to spend so much.
    Well the good news is, if you smart in your neck sizing and control over seating forces, you don't have to qualify bullets for seating on the money.
    I've done a lot of experimenting, and applied some fancy trig in a big program to figure this out.

    With a shorter ogive radius nose, your comparator hit's it's datum sooner, and so would the rifling(from the bullet base), but, the seater stem contacts sooner also. So such a bullet will seat deeper in bearing(in the neck), to provide the same comparator reading and same distance to the lands. The opposite holds for longer radius bullet noses.

    The variance left with this, is bullet bearing seated in the neck. I know folks will resist it, but bearing variances(seated or otherwise) are meaningless to internal ballistics.
    What is vital to perfect seating though, is rational and consistent seating forces(another thread).
    I'll say with high seating forces and variances to it, a seater stem wedges differently, affecting results.
     
  8. woods

    woods Well-Known Member

    Messages:
    965
    Joined:
    Jun 4, 2006
    Bingo! Nuff said

    What I always found incongruous is how the comparators from Hornady are .011" under caliber while the land size is typically .008" (from what I understand, correct me if incorrect) under.

    Example .308 caliber comparator

    [​IMG]

    Seems like if you were looking for a true distance to the lands a comparator that was land diameter would be more appropriate
     
  9. phorwath

    phorwath Well-Known Member

    Messages:
    4,896
    Joined:
    Apr 4, 2005
    Well they've got to be a little smaller than land diameter or the bullet would slide right thru the comparator - no?
     
  10. woods

    woods Well-Known Member

    Messages:
    965
    Joined:
    Jun 4, 2006
    No, the bullet is BORE diameter

    Example: bullet .308", groove diameter .308", land diameter .300"

    So if you made the comparator .300" then it would hit the bullet ogive exactly where the lands would hit it

    Right?
     
  11. phorwath

    phorwath Well-Known Member

    Messages:
    4,896
    Joined:
    Apr 4, 2005
    Yes, you're exactly right. My bad. My mind replaced groove with land. Sorry.

    I always knew my hornady comparators hit the bullet farther from the lands than necessary, and what seemed more desirable. Just never measured them.
     
  12. Mikecr

    Mikecr Well-Known Member

    Messages:
    4,264
    Joined:
    Aug 10, 2003
    It isn't that simple, and yet simpler still.
    Throats are cut at whatever angle the reamer provides. It could be from ~.5 to 15degs.
    Bullet noses come in various forms(tangent, secant, hybrid) and radius ~8 to 15degs.
    Various combinations change the contact points of both lands AND noses.

    You can order your reamer with a throat angle that matches a particular bullet nose.
    You can have a comparator made with the barrel stub and same throating from that reamer at barrel finishing. Only this would provide for true contact datum from a comparator.
    Change bullets and you negate the efforts.

    But it doesn't matter, as a relative datum is typically what we define while discovering most accurate seating distance -with any particular bullet. You can log it, and as long as you always use the same tool & method of measure, then you can reproduce that best seating distance.

    I don't pay any mind to where a Sinclair nut contacts w/resp to loaded contact. I'm satisfied as it get's that I can reproduce with the nut whatever I measured with it as best seating.
     
  13. woods

    woods Well-Known Member

    Messages:
    965
    Joined:
    Jun 4, 2006
    But the bullets don't really exert back pressure from engraving forces until the bullet actually hits the beginning of the lands

    So isn't that the true distance to the lands?
     
  14. boomtube

    boomtube Well-Known Member

    Messages:
    1,595
    Joined:
    Oct 8, 2007
    Barrels were origionally bored and then rifled. Bore diameter is, by definition, the size hole that gets precisely bored during manufactoring, groove diameter is to the bottom of the rifling. Ergo, a .30 cal barrel is bored to .300" and, with a normal rifling depth of 4 thou, the groove diameter is .308".

    Seating a precise amount off the lands isn't the goal, finding the best shooting jump to the lands is but that's a relitive thing,it matters not if we know to the last thou what it really is.

    A seating variation of +/- 5 thou rarely matters unless we're seating on a ragged edge of what works well..