Using The Bob Green Bullet Comparator

Discussion in 'Reloading Forum (All Calibers)' started by kvd, Jun 16, 2017.

  1. kvd

    kvd Silver $$ Contributor

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    The Bob Green Bullet Comparator (BGC) has quickly become indispensable in my handloading process. This device allows one to compare bullets from where the measuring stem in the tool makes contact near the tip of the bullet to the point on the ogive that contacts the same diameter as the lands of the rifling for a given caliber. This measurement is much more useful than measuring bullets with a typical comparator that measures from a point on the ogive (hopefully the same diameter as the lands) rearward to the base of the bullet. My feeling is that measuring the backend of a bullet is not the best practice if trying to establish a critical dimension involving the front of the bullet.

    I reestablish the distance to the lands frequently via the Alex Wheeler stripped bolt method as shown in his excellent youtube video. The bullet used for this becomes a reference standard of sorts. I zero the indicator dial on the BGC using this particular bullet and proceed to use this setting on the tool to compare all the bullets required for the reloading session to within ± 0.001 inch or less of the reference bullet. The dial indicator is capable of measuring to 0.0001 inch so even finer sorting could be done but a maximum of plus or minus a thousandths of an inch works for me. Only by sorting in this way can one be assured that bullets seated with the same seating die adjusted in or out as required be the expected distance from the lands.

    Using this tool has been very eye opening to me. There is typically at least a 0.005 to 0.006 inch extreme spread in the distance from the measuring stem in the tool to land diameter contact on the ogive of the bullets in a box of 500. This translates directly to the same error in seating the bullet a known distance from the lands if using one setting on the seating die for all bullets. Using the Bob Green Bullet Comparator helps mitigate this random seating depth error.

    Happy shooting!

    Ken
     
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  2. T-shooter

    T-shooter

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    I watch a video on the Bob Green tool. The I.D. measured .298" for a 30 caliber. My Hornady 30 caliber insert also measures .298". Am I missing something about this tool besides being a higher quality piece? I try seating the same way, a similar bullet checked in the bore as a reference and then deduct whatever jump I want. I seat bullets with the tool to within .001". Is there anything wrong with what I am using and would the Bob Green tool improve accuracy of measurement by any noticeable amount? It's only measuring from one diameter, right?
    [​IMG]
     
  3. Ned Ludd

    Ned Ludd Gold $$ Contributor

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    A typical caliper insert seats and measures seating depth from a single point on the bullet ogive. This is different than what Bob's Comparator accomplishes (described in more detail below). According to your picture and post, you're using Hornady comparator inserts. The Hornady inserts I have for both .223 Rem and .308 Win are quite a bit smaller in diameter than other inserts I have used from Sinclair. For that reason, the Sinclair inserts seat on the bullet much closer to the ogive/bearing surface junction (i.e. closer to the case mouth); the comparable Hornady inserts seat farther out on the ogive. In the cartoon below, the blue arrows illustrate two different caliper insert contact points. In reality, the Hornady inserts I have seat quite a bit farther out on the nose than is indicated in the cartoon. For this reason, when you use the Hornady inserts you are already measuring seating depth at a point farther out on the bullet ogive closer to where the seating die stem actually contacts and pushes the bullet during the seating process. When using a comparator insert with a larger diameter hole that is closer to bullet diameter, such that it seats on the ogive only barely above the bearing surface junction, the distance between the insert contact point and the seating die stem contact point is longer and seating depth measurements will show greater variance due to bullet length variance in the same region.

    The shorter the distance between where the seating die stem contacts the bullet and the caliper insert used to measure seating depth contact the bullet, the less bullet length variance in that region will affect the measurement and the more consistent your seating depth will appear to be. Ideally, we'd like the seating die stem to contact and "push" the bullet during the seating process just barely above the point where we measure seating depth; in other words with minimal distance between the two contact points. However, there are a number of technical reasons seating dies are not constructed in this way. We would also like the caliper inserts to seat (i.e. measure) at the exact point that first contacts the rifling as the bullet enters the bore. Because the Hornady inserts seat farther out on the bullet ogive and closer to the seating die stem contact point, bullet length variance in this region affects seating depth measurements less. However, there is a potential trade-off. Measurements may appear more consistent, but you're also not measuring seating depth at the exact spot on the bullet that will first contact the lands. In contrast, seating depth measurements taken using a caliper insert with a larger hole that seats closer to the bearing surface/ogive junction probably seat closer to where the bullet actually first contacts the rifling. All things considered, I use the Hornady inserts and I've never had any reason to think seating depth irregularities are a problem with it. I actually like the fact that they seat farther out on the ogive for several reasons.

    Bob's tool uses a different approach to improve seating depth consistency. Specifically, you use Bob's Comparator tool to sort bullets into groups that simply have minimal length variance between the seating die stem contact point and the caliper insert contact point (shown as "critical distance" in the diagram below in the green box). If the length between those contact points is very uniform, so should the seating depth be. In effect, Bob's tool measures off the ogive and is improving the "bullet" by allowing you to sort bullets into more consistent groups, as opposed to altering the measuring process, or by moving the seating die stem closer to the caliper insert contact point. In that regard, you might also benefit from better bullet ogive uniformity when using Bob's tool. Arguably, it doesn't measure all possible bullet dimensions, so there is probably a limit to the benefit, but it's in the ogive region, which is certainly one important area in terms of drag and BC.

    The bottom line is that if you already experience very uniform seating depth with no special "extra" attention, you may not notice a big difference using bullets sorted with Bob's tool in terms of actual seating depth consistency. You may still benefit from improved bullet uniformity in terms of consistency of exterior ballistics, but unless the differences are very large (which they won't be in this case), it is harder to definitively quantify the benefit other than by keeping track of your scores over some period of time.

    [​IMG]
     
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  4. kvd

    kvd Silver $$ Contributor

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    Thanks for the more detailed explanation gstaylorg. Sorting bullets is certainly a good use for the tool. I like to use it to insure that the bullets I'm about to use are similar in "critical distance" to the one I just used to find the distance to the lands. Attached is a picture of the tool from Bob Green's website.

    Ken

    308_comp_38aa.jpg
     
  5. Jet

    Jet Gold $$ Contributor

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    I use Bob Green's tool and have also found it a valuable tool. Bearing surface is the first sort I do followed by the BGC. Like you state, a typical box of 500 bullets will have some variance and it is critical to identify these and create sub lots to load with.

    Good Shooting

    Rich
     
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  6. T-shooter

    T-shooter

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    Thanks very much for the detailed explanation. I measured my .223 and .308 inserts and both are .010" (measured using a caliper) smaller than the bullet O.D. I may hone the .308 insert out closer to the bullet diameter, say .302-.304" or whatever size I can find that's close. I have a .3075" hone but it's a bit too large. I've never sorted bullets by bearing surface or overall length but I do seat using the tool to seat within .001". I use a Redding Competition seater with the micrometer dial. Normally I set it a bit long and seat the bullets, then measure and adjust. The Lee press (cast iron) will give a bit with a little more force and allow an extra .003-.004" seating depth without resetting the seating die. The 3 primary .308 bullets I use are Hornady 168g Match, Sierra 168g M/K, and Hornady 208g ELD. Both 168g's seat almost exactly the same, the 208g takes about 1-1/2 turns out on the seating dial. I have checked all the bullets (probably should do them with different lot numbers) in the chamber and set them to .025" jump where they all seem to do well. With the test dummy rounds made to touch the rifling, the measurements on the Hornady tool are all within .002".

    There are a lot of differences in bullets. These are 2 different lots of 168g Match measured with the Hornady comparator. Notice the large difference in the nose/ogive (.021") and bearing surfaces (.024").
    [​IMG]

    Same with the .223 75g BTHP.
    [​IMG]
    Since these seem to bee all over the place, what do you feel is the most important for accuracy, the bearing surface length or ogive length, as long as they are seated so the bullet jump is consistent? And you also mentioned the difference between the contact point of the seating stem and the comparator tool which may indicate a difference in the curve of the nose. Is that something you measure too?
     
  7. BoydAllen

    BoydAllen

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    Another approach that can be used is to seat all bullets to a longer length and then sort them into groups for their final seating based on how much farther bullets need to be seated into their cases. For instance, assuming that seating .010 long (seater setting for one round) would result in all bullets being longer than desired, one could then use the regular Hornady "headspace" tool and insert to sort them into groups and then adjust your seater for each group. This should take care of the variations in stem contact to rifling contact distance. In reality this would take no more time than sorting bullets, and not require additional equipment.
     
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  8. mikecr

    mikecr

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    Regardless of merchandising or misunderstandings, the BGC does not establish consistent CBTO. That's not what it's for.
    The BGC is purely for comparative measure of ogive radius variances.
    Once you've established bullet groups matching in ogive radius, i.e. qualified your datums, further measures like CBTO become credible.
    But you still have to measure CBTO for every round loaded to establish matching CBTO(which is affected by seating forces).

    Keep in mind also that a given CBTO is usually taken from a point different than actual land contact, which is affected by throat angle to ogive angles,, not just diameters. And bearing does not begin anywhere near land contact.

    There is also a gualifying measure prior to ogive radius comparison with a BGC. That is, bullet diameter at bearing end/ogive begin. Without considering this, you could match ogives with bullets that don't actually have matching ogives. That doesn't change CBTO directly, but it would affect meplat trimming taken from a high ogive datum.
    Meplats should always be trimmed from ogives to obtain same meplat diameters,, not from bases merely to get same OAL..
     
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  9. kvd

    kvd Silver $$ Contributor

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    T-shooter,

    It's not so much the length but also the radius of the ogive. A fact we simply can't get around is that we must make physical contact with the bullet somewhere on the ogive in order to seat a bullet. I have seen what you mention in 0.284 bullets as well. Consider this - if the length of the bearing surface and the length of the ogive to tip have the greatest variance but this same degree or magnitude of variance is not seen in the overall length of the lot of bullets measured, then the radius of the ogive must exhibit the same variance or even greater since a long bearing surface/short ogive to tip combination would require a more drastic curvature to end up with the same length bullet. What the BGC is helping with is that it shows this variance compared to the other bullets in the lot sampled and allows one to group similar bullets together and compensate for it. The tool will help insure that all the bullets will contact the seating stem at roughly the same place on the ogive and thus push in the same distance. This in turn enhances the chance that your CBTO measurement will be the same for all rounds without individual adjustment (but I still check it anyway just to verify).


    Ken
     
  10. kvd

    kvd Silver $$ Contributor

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    Certainly another way to go about it. Thank you for the comment.

    Ken
     
  11. kvd

    kvd Silver $$ Contributor

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    Good insights. You've obviously given this subject some thought. I appreciate your sharing this.

    Ken
     
  12. Ned Ludd

    Ned Ludd Gold $$ Contributor

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    You're correct about the mechanism by which it works, but it's stated purpose from Bob's website (below) is specifically for improving seating depth consistency. Because we typically measure seating depth via CBTO measurements, I thinks it's completely valid to say that Bob's tool does, in fact, aid in establishing consistent CBTO. I would also add the caveat that obtaining some benefit from this tool with regard to establishing more consistent seating depth would, of course, be dependent on bullet ogive length variance as a primary cause of seating depth inconsistency.

    "Bob Greens custom shop is now offering one of the most interesting accuracy enhancing products to come along in some time. The B.G.C. Bullet Comparator. This new design came about as a result of customers seeking to understand why different lots of bullets would not shoot as well as others. What we found was, especially regarding VLD bullets, the point on the bullet where the bullet seater makes contact and the point on the bullet where the lands make contact would vary from bullet-to-bullet and from lot-to-lot. With this device you are able to sort bullets into sub-lots, therefore keeping the loaded rounds seating depth very consistent in seating depth without constantly changing your bullet seater."
     
  13. mikecr

    mikecr

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    I agree that ogive radius variance is a hard contributor to CBTO variance. This I can calculate and show on paper.
    However, it's my belief that seating force variance is a larger contributor. Even with matched ogives, varying seating force directly varies CBTO. So in practice, what I might put on paper about expected CBTO goes out the window..
    The forces wedging tool contact datums, are just not the same as forces seen with seater plug contact during seating, nor tool contact forces while measuring CBTO.

    I have a couple BGCs(24 & 26cal), various base to ogive tools(BTO), things I've made for myself over the years, and custom software. My perspectives are not just from math. My standard CBTO tolerance is under 1thou. That is, when making 50 rounds with a target CBTO of 1.985", I make 50 at 1.985 (not 1.986 or 1.984). It's not difficult for me to SEAT like this, but there were prerequisite efforts to get there.
    I pre-seat with a loadcell instrumented mandrel to match seating forces into necks with a standard light carbon film. This, before seating actual bullets. The necks were partial length sized for 1thou interference after springback -for a length that provides a pre-seating force match. On the bullets; for non-BR bullets, like Berger or Lapua, I've qualified ogives using a BGC. For BR bullets, like Jayner or BIB, I don't bother. All my bullets are also WS2 coated, which provides an insanely low friction coefficient. In 26cal, I've trimmed/pointed meplats on qualified ogives.

    I'm just reminding here that qualifying ogives with a BGC is useful but not a cure-all shortcut to reaching target CBTOs.
    You should logically remove assumptions, and always verify.
     
  14. Bart B.

    Bart B.

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    What about the leade (throat?) angle? Depending on it's angle and the distance it has from bore to freebore diameter, plus the bullet diameter and ogive shape, leade and bullet touch points will be all over the place.
     
  15. mikecr

    mikecr

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    That's true Bart. Fortunately, results override.
    We set CBTO where testing proves best, and reproduce local measure of it.

    Land relationships change a tiny bit with each shot, but I haven't seen this matter provided I'm off the lands(OTL) to begin.
    Some rely on ITL or jam, likely for the high starting pressures in underbores. And for some cartridges this is worthwhile to manage. I doubt so with hunting capacity cartridges. I believe I could make any standard or overbore cartridge shoot it's best while OTL, with the right testing and powder.
     
  16. ckaberna

    ckaberna Gold $$ Contributor

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    This came along at a great time. For about a month I have noticed my groups would have 2-3 touching at 100 yrds. then one or two would be out by .5"-.7" very randomly. This has been going on since trying to develop a load for my new creedmoor. I weigh sorted two lots of 142 gr. SMK's trying to solve the issue. Newbie mistake of mixing two lots. Never again.
    Yesterday while loading i noticed my seating depth varied by as much as .010" CBTO, again randomly. So then I got to measuring the bullets, these things had three different distinct groups with the ES @ .036" OAL. I remeasured my distance to lands on the rifle using one from the shorter group and one from the longer group. The differences in the two were .017". Wow. I am relatively new to reloading and this has become an obsession (almost OCD). I am using the redding comp. dies/seater and was somewhat frustrated about the different contact points between the stem and the hornady comparator but seems Bob's tool may help with this issue. I did manage to get all sets of loaded ammo to the same CBTO length by adjusting micrometer on the seater but I had to adjust my targeted depth to match the shortest. Not to mention time consuming. Maybe this will help with the grouping issues if it ever stops raining.
    I know a lot of you have years experience in this but I am having a hard time understanding how neck tension or "friction" would cause inconsistencies with seating depth. You are pushing two rigid objects (case/bullet) in a press, if the handle goes to the same place every time how can it not push the bullet to the same point every time? I am using a rockchucker single stage with .002 neck tension and do not see how the press could flex which might cause the difference in seating depth.
    Thanks,
    Kevin
     
  17. jsthntn247

    jsthntn247

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    Could you not run a uni throater in a Hornady insert and make it measure where the bullets would contact the lands?
     
  18. Patch700

    Patch700

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    Sounds good on paper but you are still left with a couple problems... The first being that for obvious reasons the point at which the bullet would contact the lands is going to be nowhere near where the seating stem would contact.
     
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  19. ckaberna

    ckaberna Gold $$ Contributor

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    I actually turned a insert for the comparator and bored it to .263 to get as close a possible. Only issue, it is so close that the bullet tends to want to "stick" in it.
     
  20. T-shooter

    T-shooter

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    So basically the tool is giving you a second point on the ogive to compare and/or comparing the distance from the seating die insert to the (.298" I.D.) comparator. I don't see why it's necessary to achieve consistent seating depths. Seating from the larger diameter of the tool instead of the seating stem diameter should be a better way. You can get consistent seating depths and consistent distances to the lands. I use a cast iron Lee single stage press with a 1-1/4" ram and a Redding Competition micrometer seating die. Still even using graphite in the neck, if I seat until I just detect the ram bottoming out, then press down with maybe 4-5 lbs force, the bullet seats about .003" deeper. I also tried putting 15-20 lbs on the handle and it went in another .003". The press uses quick change inserts but they are always screwed in tight, not to where they just index against the locating pin. I seat with the bullet usually .002"-.003" from where I want it and then apply a bit more pressure and remeasure. Normally once does it unless I get a bullet too far out of specs. Occasionally I'll get one that goes to the desired setting with very light pressure. Sometimes one may stop .010" out instead of the .003" and even with heavy force, I can't reach the correct depth, so those are used for other purposes. What I'm getting at, it all my rounds are within .001" when measured from the comparator I.D. (0.298") to the lands. I know this adds an extra step or two and will not work in a turret press without doing a final resizing later. I measured both .308 seating dies (stems) I have and they contact the bullet about .100" different in distance.
     

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