Why does OAL to Ogive vary?

Discussion in 'Big Stuff -- 6.5mm, 7mm, 30 Cal' started by Papa Charlie, Oct 8, 2017.

  1. Papa Charlie

    Papa Charlie

    Joined:
    Jun 10, 2017
    Messages:
    251
    Ok this has been bothering me for some time. I have been taught to measure for every type of bullet and do so religiously. I measure the throat depth on my rifle for each bullet to determine the seating depth. I seat at 0.015” OTL.

    I am getting a new barrel from Benchmark Barrels. I plan to use the Berger 185 Juggernauts and the 200.20X as my primary rounds for F-Class TR. In discussing the throat depth with Benchmark and Berger it was decided to load a dummy round with the 200.20X and cut the throat for that round.

    While doing the setup I measured the case and the bullets carefully to decide on the depth of the bullet. I was concerned with the possibility of compressing the powder so wanted to take it all into account.

    The thing that has always bothered me and really got me to thinking is that when measuring the shell and bullet for the length to the Ogive why does the end measurement change between different bullets when measuring to a single rifles lands? Using the same comparator, shouldn’t the end result of the measurement of the round come out the same regardless of the bullet? We are measuring a length form the base of the case to a constant (the comparators ID) which represents the rifles lands.
     
  2. XTR

    XTR

    Joined:
    May 15, 2011
    Messages:
    2,950
    The difference in caused by differences in the shape of the nose. In particular the rate of taper from the bearing surface to the point at which it contact the lands, and your measuring tool. This rate of change will also effect the distance from the full diameter to where it contacts the lands, keep in mind there is a leade angle on the lands so a different angle on the nose can change the point at which the bullet touches.

    Another thing to remember is that the point at which it contacts the lands is likely not the same point at which it contacts your tool, but it's a reference that you can use.

    Interestingly enough Berger 215Hybrids, 200Hybrids, and the 200.20X bullet all have the same BTO measurements in my barrels (close enough to be identical accounting for lot to lot variances)
     
  3. hrlincoln

    hrlincoln

    Joined:
    Mar 13, 2017
    Messages:
    80
    The comparator provides consistent measurement to a point on a bullet's surface that corresponds to the ID of the comparator insert. Where (ie. the length at which) this diameter occurs on a bullet's surface depends on the design/surface curvature of the bullet.

    The measurement you obtain with the comparator does not represent the rifles lands per se. It represents a reference length/value which we take when a particular bullet's forward advancement is stopped by the lands. Where a particular bullet first encounters contact with the lands depends on that particular bullet's profile, not to mention the condition of the lands.

    Berger QR sheet attached for dimensional comparison...hope this helps.
     

    Attached Files:

  4. Richard Coody

    Richard Coody Silver $$ Contributor

    Joined:
    Feb 19, 2017
    Messages:
    651
    what they said

    when you measure with a comparator you are measuring a point on the curve of the ogive not the actual lands. different bullets have a different curve to the ogive so you have a different reference point

    want better consistency with the same bullet. sort by base to ogive. of course my experience is bergers are very consistent
     
  5. CTK

    CTK Gold $$ Contributor

    Joined:
    Jul 5, 2016
    Messages:
    457
    I frequently get a seated comparator measurement that is a few thou longer than the rest of the batch. I put it back in the sizing die and get the same. I guess this is evidence that the comparator measures at a different spot than the sizing die hits when it pushes back the bullet? Does it make sense to dial in a few thousands more and seat this bullet to match the rest? Sometimes in the 5 thou range.
     
  6. Papa Charlie

    Papa Charlie

    Joined:
    Jun 10, 2017
    Messages:
    251
    Thanks for the measurements, wish it had the measurements for the 200.20X (30417).

    I get the comparator is not the exact same as the lands but it is a representative of the lands and there in concept should be a consistent measurement from one bullet to another. You are still measuring the distance from the base of the case to the point (representative of the Lands) on the bullet that matches the diameter of the comparator. So theoretically this measurement and distance should result in the same measurement each time.

    Maybe I am missing something here.
     
  7. hrlincoln

    hrlincoln

    Joined:
    Mar 13, 2017
    Messages:
    80
    I agree that you are measuring the distance from the base of the case to a point on the bullet's surface that matches the ID of the comparator. Consider two separate diameters that we're talking about.

    1. Bullet diameter of the nose that the comparator is measuring to and,
    2. Bullet diameter along the nose that first contacts the lands.

    The centerline distance/length between these two diameters can vary from bullet to bullet depending on design resulting in different CBTO measurements . Maybe a custom bullet maker could chime in.

    Afterthought...call Berger and get the dimensions for the 200.20X
     
    Last edited: Oct 8, 2017
  8. Bigguy1951

    Bigguy1951

    Joined:
    May 20, 2014
    Messages:
    120
    The reasons above cover most of the issue. I will try to explain what I do to minimize what you are asking about. First I measure each bullet with a Sinclair bullet comparator, granite base and dial indicator. I work at this and often measure each bullet twice. I usually end up with 5 containers each .001 different in length. Usually 2 of the containers will hold the majority of the bullets. I use this tool because it sorts the bullets closer to the ogive than my other tools and gives me a fairly uniform set of bullets.

    Then I will take a container of sorted bullets and measure each bullet with the same tool that I am using to measure the OAL to Ogive. This tool measures at a different point but is the tool that tells me if I am going to be happy with the loaded cartridge. Again I will have 2 or 3 containers when finished, usually 2. I use the majority container first and seat them as testing shows they should be seated, the other container will be either a .001 longer or shorter. If longer, seat them .001 longer, shorter bullets are seated .001 shorter. Doing this puts the ogive in the same point, more or less. It can vary depending on how closely my first measurement came to the true ogive. Remember the goal is to get the bullet to seat the same each time.

    Down to the last major thing that messes up your efforts. The bullet seating stem is a measuring tool in a sense. It is fixed diameter and will always stop at that diameter on the bullet. The problem is that the bullet is not always the same size and like the previous two measurements you get differences in the length of the bullet from its base to where the seating stem contacts it. If important I will seat .002 short of goal and then adjust the seater micrometer and finish up.

    Lastly differences in seating resistance that you feel in the handle of your press will also change seating depth. Bullets that seat with more resistance tend to be longer as brass is springy and it will push down some when seating. When the brass springs back up the cartridge will be longer because the bullet didn't get as far into the neck.

    Hope this was clear but I am sure that there is some mud floating around in it.
     
    pacificman likes this.
  9. XTR

    XTR

    Joined:
    May 15, 2011
    Messages:
    2,950
    What kind of shooting are you doing? Benchrest?
     
  10. Bigguy1951

    Bigguy1951

    Joined:
    May 20, 2014
    Messages:
    120
    Yes and no. Our small club shoots competition twice a month. The first shoot is "long-distance" with targets from 300 to 700. The second shoot is more bench rest, shooting groups from 100 to 300. I don't always win. But I always have a great time.
     
  11. XTR

    XTR

    Joined:
    May 15, 2011
    Messages:
    2,950
    I ask because you are doing a sorting regimen that is awfully time consuming if you were shooting a discipline that shoots lots of rounds.

    Shooters participating in NRA HP events (that is sling and F class) typically won't get much of a return on the investment of that kind of time, particularly at long range. In mid range F Open where you need a 1000-60+ score to win, maybe, but even then I doubt it if you have a load that jumps at least .015. If you are loading within .005 of the lands then I can see where you would need to be that precise because variations might result in some significant velocity changes.

    New shooters see this type of post and think they need to do all of this stuff to shoot well, and at least in F-TR or HP shooting it's just not so. The only thing I do to my bullets is point them. I don't sort brass, bullets or primers, and with a properly developed load I have never felt that my bullets limited my position on the leader board. That's not to say that I've never had a load that shot like buckshot, but the solutions were never in the minutia.

    Another example. This summer I shot an XTC match and shot rather poorly at 600 yards. My scorer was watching me police up my brass and noted to me that I was shooting LC brass and that if I sorted it I might get better results on the target. I had to laugh to myself. This is the same mixed lot of LC that I've shot in the 223 bolt gun that I shoot in mid range F-TR, that rifle has won a lot of mid range matches, and even with the worst load I've ever shot from it it was capable of cleaning a HP target. I don't think sorting is going to make a difference in an AR on a target with a 10 ring that is 4x as large, the point being, I'm sure he believes he needs to sort his brass.
     
    barefooter56 and milanuk like this.
  12. barefooter56

    barefooter56

    Joined:
    Nov 10, 2014
    Messages:
    565
    This may help explain :
    http://www.bergerbullets.com/effect...coal-and-cartridge-base-to-ogive-cbto-part-1/
    and
    http://www.bergerbullets.com/effect...coal-and-cartridge-base-to-ogive-cbto-part-2/
    And when consulting the QUCK REFERENCE SHEETS :http://www.bergerbullets.com/pdf/Quick-Reference-Sheets.pdf
    ALWAYS KEEP THIS IN MIND:
    Our Quick Reference Sheet (QRS) presents the details of each bullet in a convenient format. Keep in mind these numbers are for reference and do not reflect exact numbers for every lot.
    Take care,
     
  13. joshb

    joshb Gold $$ Contributor

    Joined:
    Oct 5, 2014
    Messages:
    2,878
    I'll add something else to this seating discussion, as to bullet quality. I have Bergers and Sierras for my 223 and 6.5-47. I don't compete, I just like to shoot and take friends shooting. I bought a ton of Sierras at a great price. Call me cheap and selfish, but I don't like watching my Newby friends train with my Bergers. While trying to find a good load for the Sierras, I started to try to seat them at touch. The shells came out of my die with base to ogive measurements that varied by 3-6 thousandths. I tried to sort some. I wound up with 11 piles and a bunch of "way offs". I took my biggest pile, set my die and started loading. I still had variances of 2-4 thou. in base to ogive. It seems these "cheap" bullets are very inconsistent. Duh! Old lesson relearned. Can't make a silk purse out of a sows ear. I sorted some Bergers. I got 4 piles. They seat in my dies with 1-2 thou variance. That may be neck tension differences. Ya get what ya pay for. I'll be jumping the Sierras. How much? I'll never really know cause I'm done wasting my time trying to get them to the same specs.
     
    msinc likes this.
  14. ImBIllT

    ImBIllT Silver $$ Contributor

    Joined:
    Jun 1, 2008
    Messages:
    250
    The seating die doesn't contact the bullet where the comparator does. The result is that different ogive shapes yeild different seating depths with the same die adjustment.
     
    joshb likes this.
  15. BP1

    BP1 Gold $$ Contributor

    Joined:
    Nov 8, 2015
    Messages:
    376
    They are the same in my chamber also. I read this and had to check and I’ll be darned same measurement.
     
  16. msinc

    msinc

    Joined:
    May 14, 2011
    Messages:
    1,063
    Papa Charlie could you clarify something please??? Are you asking why does case head or base to ogive vary????? Or are you asking why is OAL {base to bullet tip} different, but base to ogive is not????
    You threw in that "OAL" thing...OAL is base or case head to bullet tip and really only matters if you are trying to fit the loaded round into a certain magazine. OAL is not a good thing to worry about as far as accurate bullet seating goes because: A. the tips of bullets can and do vary by quite a bit sometimes, especially BTHP match bullets, and B. who cares where the very tip of the bullet is in relation to anything {except the front of the magazine}.
    Just trying to avoid you getting a bunch of confusing answers if the answers don't really match the question. I know a lot of shooters call base to ogive "OAL" and to them it is because that is what they are concerned about and it is all they are measuring, but when you are asking a question it is always best if you clarify.

    joshb, I also am cheap and selfish!!!! If somebody want to shoot Bergers....they need to buy and load them theirself!!!!! They don't get mine.
     
    Last edited: Oct 17, 2017
  17. Papa Charlie

    Papa Charlie

    Joined:
    Jun 10, 2017
    Messages:
    251
    To clarify:
    1) The distance from the breach to the Lands is constant. Assuming the same gun each time.
    2) The diameter of the lands is constant.
    3) The same comparator is used each time.

    Therefore, if you load a bullet to contact the lands, shouldn't the length from the case base to the ogive of the bullet be the same regardless of the bullet configuration each time?

    I know that it doesn't but can't seem to wrap my head around this.
     
  18. XTR

    XTR

    Joined:
    May 15, 2011
    Messages:
    2,950
    You are not measuring it where it contacts the lands, you are measuring it where your tool contacts the bullet at the diameter of the tool.
     
  19. msinc

    msinc

    Joined:
    May 14, 2011
    Messages:
    1,063
    How much "difference" are you getting???? and what are you seating the bullets with??? I agree, it should be a constant, but {again, not knowing what you are calling different} there are a few things that come into play I can think of....I sometimes have trouble hitting dead same with an L.E. Wilson straight line seater. I used to think it could be air trapped in the die between the bullet and seater stem. I can have a charged case in the die and put a bullet in the die and it will sit there on air. I know that is not the way you are supposed to load with those dies...but it will do it. When used properly the air is between the bullet and stem. There could be a little {very little} spring back if the case needs annealed. The bullets themselves are swaged to size/shape using automated equipment, most of which appears to be hydraulically powered {at least what I have seen}. i understand what you are saying, I mean I usually get mine within a thou or two....how much difference do you see???? Then again, I am not trying to set a record either.
     
    Last edited: Oct 17, 2017
  20. hogpatrol

    hogpatrol Gold $$ Contributor

    Joined:
    Apr 28, 2009
    Messages:
    3,910
    Are you taking in to account the lead angle? I haven't worked the math but that may have something to do with bullet nose contact distance from the bolt face.
     

Share This Page