Bullet Seating Depth / Comparator Confusion

Discussion in 'Reloading Forum (All Calibers)' started by Phil3, Dec 30, 2009.

  1. Phil3

    Phil3

    Joined:
    Sep 30, 2008
    Messages:
    1,004
    I am trying to use the Sinclair bullet seating measurement tool, and am very confused. See http://www.sinclairintl.com/product/6803/Bullet-Seating-Depth-Tools-OAL. This is for an AR15, Krieger barrel, twice fired Winchester brass, Sierra 52 grain HPBT MatchKing.

    I followed the directions and I have determined the distance from the bolt face to the base of the bullet (1.516"). You are now to add that distance to the length of the bullet as measured from base of bullet to ogive (.262" using Sinclair comparator). That totals 1.778", but what value is this number?

    Is the process to load rounds using the same bullet and then adjust that dimension to affect bullet jump (i.e. reduce OAL by .010" to increase jump by .010")?

    I am not sure of my calculations though. I thought that on AR15s, getting close to the lands would be near impossible, since to do so, the OAL would exceed what could fit in the mag. If, with bullet touching the lands, the bolt face to bullet base is 1,516", and the bullet is .716" (tip to base), that is a total of 2.232", which would easily fit in the mag. Also, some factory ammo is longer than what I measured.

    Where have I gone astray?

    - Phil
     
  2. fdshuster

    fdshuster Site $$ Contributor

    Joined:
    Jul 30, 2007
    Messages:
    4,080
    phil3: Go to this sites home page, click-on "technical articles" ( a receiver pictured), scroll-down to near the bottom (wind flags shown) and click-on the Stoney Point information for the chamber over-all-length gauge. The Sinclair is basically the same tool.
     
  3. Mikem

    Mikem

    Joined:
    May 13, 2006
    Messages:
    405
    Phil3, send me a PM or e-mail.
    Mike.
     
  4. Joe O

    Joe O

    Joined:
    Dec 2, 2009
    Messages:
    156
    I too found the procedure confusing,using the Sinclair OAL gage,I thought that once i made up a dummy round with one bullet,say a Sie 95gr Match,touching the lands,I could measure with a comparator,then seat a different bullet,say a 80gr PBT,adjusted to the same orgive dimention as the 95gr.Tech at Sinclair says I need to get the OAL for each bullet.I get the OAL part,but when setting up bullets to be x # of .000 off the lands,why couldn't that be determined for all different bullets,once you have established the measurement of one bullet,regardless of weight or shape,by using the comparator only?
    Hope someone can clear this up.
     
  5. fdshuster

    fdshuster Site $$ Contributor

    Joined:
    Jul 30, 2007
    Messages:
    4,080
    The shape, or profile of each bullet will most likely be different. For example: a Sierra 55 gr. #1390 hpbt GameKing will have a "rounded" ogive profile. A Sierra 69 gr. #1380 hpbt MatchKing will be somewhat "pointier". And the Sierra 80 gr. hpbt MatchKing will be very "pointed", almost like a VLD. When I am starting with a new chambering, I line up all the different types of bullets I intend to use and take a base-to-ogive-touch dimension for each and record those measurements. The seater die is then adjusted so I have the same dimension on the loaded round. Your example of the 95 gr. Sierra hpbt MatchKing, 6mm is another example. One of my most accurate bullets in my 1-8 twist 6BR, but the 95 gr Berger VLD is also a "winner", so seated-to-touch lengths are recorded for each, and they are different.
     
  6. rocketron

    rocketron Site $$ Contributor

    Joined:
    Dec 5, 2009
    Messages:
    324
    this is how use mine. take one case and one bullet measure the length of the bullet (base to tip) then use the seating depth tool and get the mesurement from case head to bullet base. add the two together. then seat the bullet to that length in the case. this load is now touching the landes. now use your comparator to get the base to ogive mesurement. add to the measurement for jam or subtract for jump.

    i think you will find that if you measure the bullet with the comparator (base to ogive) and add it to the seating depth tool measurement it will be the same as the measurement you get with the comparator from above.
     
  7. fdshuster

    fdshuster Site $$ Contributor

    Joined:
    Jul 30, 2007
    Messages:
    4,080
    joe o: A very thorough explanation of how the shapes of bullet ogives effects bullet seating depths, (along with the leade angle of the throat), is contained in "The Benchrest Shooting Primer", page 317, an article by Jim Borden dated October 1992, titled " Bullet Ogives and Throat Contact". At the beginning of the article he speaks of bullet comparators as they relate to ogive shapes, and says, " They are often misused by shooters who believe that once an ogive to cartridge base dimension is established, using a comparator, that all future loads with various bullets can be set up to that same dimension.. This is not the case".
     
  8. CanusLatransSnpr

    CanusLatransSnpr

    Joined:
    Nov 4, 2009
    Messages:
    921
    Frank is 100% right! For every different bullet you intend to use, you have to measure the OAL for each different bullet using the Stoney Point or Hornady OAL gauge. For example, I shoot Nosler 55gr BTs and Sierra 55gr BKs through my 22/250. The OAL for the Noslers is 2.016" and for the Sierras it is 2.024". They are both 55gr bullets but, they are not made the same and are not shaped the same. As Frank told me one time, you have to record the lot # for a particular bullet also, because the shape of a particular bullet from lot to lot may vary. Another thing, don't confuse OAL with COAL. They are not the same. COAL is measured from base of case to tip of bullet, OAL is measured from base of case to where the bearing surface on the bullet and ogive come together. The OAL measurement requires a bullet comparator. Bullet comparator will not work for cartridges loaded to fit and feed from magazines. I either helped you or confused you even more but, I hope I helped!

    Mike
     
  9. Mikem

    Mikem

    Joined:
    May 13, 2006
    Messages:
    405
    I have lists of the S.P. (Stony Point) reading to the lands, taped to the wall above my bench, one for every rifle, listing every bullet I have in stock, plus the date.
    The date is important, as these reading will change with the amount of round fired. I recheck them about every 250 rounds, as the numbers will grow (get higher) as the lands are eroded from use.
    If you dont do something like this, in 6 mo, you could have a Jump, when you think you have a Jam. ;)
    I check 3 bullets from each batch, and take an avg, the bullets and there readings will vary a bit.
    Mike.
     
  10. fdshuster

    fdshuster Site $$ Contributor

    Joined:
    Jul 30, 2007
    Messages:
    4,080
    mike: We are taking different "paths" to the same destination. Each bullets length from the base of the case, to the ogive point-of-contact is recorded, and updated (as you also do) in the log book I keep for each of my rifles. I also include the adjustemnt required with my Forster micrometer Ultra BR seaters. Ex: To touch, lower .055" from 0/0 reference. Instantly available settings for any and all bullets used with that particular chambering. Except as it relates to magazine length limitations ( AR-15), I could care less about cartridge o.a.l., to me it just confuses the issue. With the vast differences in bullet o.a.l. it has little meaning anyway.
     
  11. Phil3

    Phil3

    Joined:
    Sep 30, 2008
    Messages:
    1,004
    Thanks for the many replies...I now "get" it. I even tested my results by polishing a fired case to make sure it fit into the chamber with little resistance, and then inserting a bullet just barely into the neck. I chambered the round, pushing the bullet against the lands, and also into the case (until case bottomed out in chamber). Carefully tapped the round out with a wooden dowel down the bore, and measured the round. About .010" shorter from base to ogive than the Sinclair measurement, but suspect I may have knocked the bullet back into the case a bit, when knocking it out with the dowel. At least I know I am using the Sinclair tool correctly, and now have a good understanding of what is going on with bullet jump.

    - Phil
     
  12. Joe O

    Joe O

    Joined:
    Dec 2, 2009
    Messages:
    156
    Mike
    I believe I get it,as far as averaging three or four bullets with the OAL gage,to establish the base to ogive distance.Having done that with one bullet (gr),why couldn't that dimention be used with a different bullet using only the comparator to get the base to ogive for that bullet.The OAL would be different,but the COAL would be the same.WHere have I gone wrong?
     
  13. fdshuster

    fdshuster Site $$ Contributor

    Joined:
    Jul 30, 2007
    Messages:
    4,080
    joe o: The ogive shape (curvature) of any given bullet is based on the radius of the secant. The secant is based on a number (like 6) being multiplied by the bullet dia.,(like .243). 6 would be the ogive shape that is the most "rounded", and with the older soft point designs is the most common. That radius would be 1.458", so a circle drawn with that radius would have the same curvature as a "6" secant bullet. At the other extreme, the new VLD bullets may have a secant of 12 ( or even higher, I don't know), so 12 x .243" would be 2.916", a much larger circle with a "flatter" circular line, giving the VLD bullet an ogive taper that is sharper. Because of these different secant's bullet ogives (shape) varies a lot, one of several reasons why they all have different seating depths, and overall lengths from the base of the case to the bullet ogive point-of-contact. ;)
     
  14. Joe O

    Joe O

    Joined:
    Dec 2, 2009
    Messages:
    156
    Thanks Frank.I wish the light would come on.LOL At some point,say at the .243 dia the bullet will contact the lands.I know that this will occur further back from the nose of the bullet in a VLD profile,than ,say in a hunting bullet,but their base to orgive should be the same,even though the OAL would be different.That's why I started the Comparator question.I know I'm missing something.I need to play around with the OAL gage,different bullets,and the comparator.I'll get it figured out sooner or later.Sorry for the agrivation,and Thanks again for your patient explanation.
     

Share This Page