Ccrider
Gold $$ Contributor
Sorting bullets: Seating Depth vs OAL
- Thread starter Ccrider
- Start date
Ned Ludd
Silver $$ Contributor
The real question is about the return on investment of time. Everyone must choose for themselves how much time they're willing to spend doing things such as pointing bullets, and whether it's worth the amount of time it takes. Trimming meplats will lower bullet BC slightly as compared to the reported [box] value, subsequently pointing them will then increase it above the box value. However, pointing bullets is not solely about increasing the BC; it's also about making the BC more consistent/uniform.
Whether one trims first then points, or points length-sorted bullets without trimming them first, it's worth doing a little testing to determine whether what was done actually achieves the desired goal. Only then can one truly know how something works in their own hands. My thought on this has always been that I need to be thinking very critically about whether a certain step in the reloading process is actually worth my time if I can't shoot the difference. Only by testing can I really know whether a given process really provides some sort of quantifiable benefit.
Pointing bullets is a process where it may be difficult to actually quantify the outcomes beyond the most basic pointed versus un-pointed stage. It is quite easy to observe and quantify a difference in BC between pointed and un-pointed bullets of the same Lot#. Even at distances of only 300 yd, it can be observed that pointed bullets require less elevation to center on target than do un-pointed bullets. However, when you get further into the details of exactly how the bullets are sorted/trimmed/etc. before pointing, it may not be so easy to accurately quantify differences between various approaches. Having done these sorts of tests/experiments from time to time in the past, I settled on sorting bullets by OAL, then pointing without trimming first. The pointed bullet meplats end up a little more jagged and perhaps not quite as "pretty" as if I had length-sorted and trimmed them before pointing, but the process improves BC and consistency on the target, which is all I'm really after. It's absolutely worth the effort for someone that is interested in pointing bullets to do a little experimentation as you suggested, because then they will know what works best in their hands, develop a better feel how each stage in the process contributes to the final product, and learn whether subtle differences in how the process is carried out such as length-sorting, trimming, etc., make a significant difference in their final product. Then they can make a more informed decision about how much time they're willing to invest and what the return on that investment of time will be.
FWIW - a LabRadar can be a useful tool in these kinds of tests. The velocity drop over distance data can be used at online ballistic calculators such as JBM (https://www.jbmballistics.com/cgi-bin/jbmbcv-5.1.cgi) to estimate BCs for bullets straight out of the box, or that have been modified by some pointing process in order to make a comparison between different pointing approaches. The main caveat to the use of LabRadar velocity drop data is that it is measured over a relatively short distance. In my case, I use the velocity drop at the 55 yd LabRadar factory preset, which would obviously not provide same resolution that a much larger velocity drop over several hundred yards might generate. Nonetheless, the values I have obtained using this approach with numerous un-pointed bullets are very close those provided by the manufacturer. Further, pointed bullets display the expected ~3-6% increase in BC as determined using LabRadar velocity drop. My take is that if nothing else, the LabRadar velocity drop data and bullet BCs estimated using it are reasonably close to the true values, and are sufficiently "solid" to be used for comparative purposes, especially side-by-side comparisons made on the same day.
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rwj
Gold $$ Contributor
Trimming doesn’t change the ogive radius, only its arc length…
Ned Ludd
Silver $$ Contributor
Never said it did. Bullets of different nose length by definition have a different ogive radius, i.e. shortest vs longest from a single Lot. If you trim a bunch of bullets of different starting lengths all to the same length, just so you can point and shoot them together, they will not necessarily end up with the same ogive radius, even though their lengths may be uniform. Length sorting first will improve that.
rwj
Gold $$ Contributor
Please clarify what is meant by the statement “The issue associated with trimming bullets all to the same length before pointing is that they will not have the same ogive radius”
If the bullets are formed in the same die, how could they have a different ogive radius (beyond normal manufacturing variation)? The formed bullets OAL can vary based on the length of the incoming jackets.
Ned Ludd
Silver $$ Contributor
In my hands with the extreme longest and shortest bullets within a given Lot#, the primary length difference occurs within the nose region. That means the ogive radii will likely not all be the same, because if they were, the meplats of the longest bullets would have to be noticeably smaller in diameter than the shortest bullets. In other words, if we're only talking about a difference in arc length rather than ogive radius, longer bullets would be expected to have a longer and "pointier" nose if the arc length was simply extended as compared to the shorter bullets. Except that they aren't. This concept is easiest to visualize when comparing the very longest and shortest bullets from a given Lot#. That suggests to me that the ogive radii of longer bullets in a batch isn't necessarily exactly the same as for the shorter bullets.
This notion is also supported by the observation that when I seat bullets from the shortest versus longest length groups within a single Lot# of bullets, I have to change the seating die micrometer to achieve the same CBTO measurement, sometimes by as much as a few thousandths. Further, it is not too difficult to visualize that the caliper insert tool doesn't appear to seat in exactly the same spot relative to the top of the bearing surface with bullets from the longest and shortest length groups. This suggest variance in the diameter of the bullet nose at different points along the ogive for bullets of different nose length. One could argue that the primary difference is nose length variance between the critical contact points (i.e. where the seating die stem and caliper insert each contact the bullet ogive) rather than ogive radius/diameter at a given point on the ogive, but I believe both change as bullet nose length changes. I thought this notion worth mention because the topic of this thread is about seating depth and OAL.
As far as I can tell, these kinds of bullet length variance within a single Lot of bullets are due to "normal" manufacturing variance and tolerances. If the manufacturers' machines and dies operated perfectly, there would be little to no bullet dimensional variance at all. Yet we know this is not the case. In any event, these kinds of length variance can affect seating depth as measured using CBTO, and therefore will need to be dealt with if one's reloading OCD happens to extend that far (mine does), which brings me back to the original topic of seating depth and OAL. When attempting to maintain the the most consistent seating depth possible, even little things that affect seating depth by a thousandth or two can become additive. Thus, trimming a really long bullet to the same length as a really short bullet before pointing them both is not the best approach, IMO. I don't think the points end up as uniform, and as I've already explained, I believe doing so unnecessarily introduces nose length and/or ogive radius variance. Both these issues can readily be "tamed" by simply sorting bullets into length groups prior to trimming/pointing.
Now if you want to debate whether these differences are large enough that anyone can easily shoot the difference, that's another discussion entirely. As reloaders, many of us do things routinely where it is questionable whether we can reliably shoot the difference, or at least reliably quantify the difference. Nonetheless, we do them anyhow, because we believe they help, and they give us more confidence behind the rifle on the firing line, which is never a bad thing. I am also of the mind that the simplest approach is usually the best. At least, it typically requires only a minimal effort to determine whether it is effective or not. For anyone struggling to generate uniform seating depth in their loaded rounds, sorting bullets by OAL prior to any other modification is a very simple approach that can be tested to determine whether it has a beneficial effect on maintaining uniform seating depth. If it doesn't work in a particular case, one can move to other possible solutions with a minimum of expended time and effort.
Edited to add: for the purpose of this disccussion, I just tried a simple experiment. I lined up two bullets from the shortest and longest length groups from a particular Lot#, with the shortest of the two in front so I could visually compare the ogives. The ogive radii were clearly not the same, and it wasn't difficult to see the difference. To my eye, the noticeable difference started about halfway between the top of the bearing surface and the meplat, which would put it in a region that could affect seating depth via the seating die stem contact point. In fairness, a sample size of one isn't very meaningful, but I don't have the motivation right now to compare several bullets from each group. But what I observed from the pair of bullets fits with what I have observed many times in the past regarding having to change the seating die micrometer in order to achieve the same seating depth with bullets from different length groups. YMMV.
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Jeff Porter
Gold $$ Contributor
As good as they are , even the best factory made bullets on high speed machines will give variable bullet OAL in BTHP bullet due to the variables in the process where the jacket is drawn over the core to the hollow point tip. A few years back I measured some .223 bullets from Nosler, Sierra and Hornady. Caliper for OAL, comparator for Base to “Ogive” and calculating the difference as Ogive to tip. (Ogive in quotes because it is the reference diameter of my comparator insert on the profile of the bullet and not the true Ogive )
The OAL in all three varies by 0.010-0.015”. The base to Ogive varied less than 0.002”
Inconsistencies in the OAL were mostly from the Ogive to tip.
The difference this makes in the BC and the center of pressure in flight has been shown to be a factor by Litz and others. The actual effect it has can be lost in other factors but it is an attribute that can be measured and controlled easily with a caliper, so we do.
The draw shape or profile of these factory bullets will vary from lot to lot and machine set up to machine set up. Most at fairly close but you can see evidence of the tooling wearing over time and when a new tool is being used if you buy multiple lots of the same bullet over time. They all will meet the QC required but if you are thinking every bullet is just like the others, they are not.
Custom bullet makers tend to have more consistency and control over these variables.
The OAL in all three varies by 0.010-0.015”. The base to Ogive varied less than 0.002”
Inconsistencies in the OAL were mostly from the Ogive to tip.
The difference this makes in the BC and the center of pressure in flight has been shown to be a factor by Litz and others. The actual effect it has can be lost in other factors but it is an attribute that can be measured and controlled easily with a caliper, so we do.
The draw shape or profile of these factory bullets will vary from lot to lot and machine set up to machine set up. Most at fairly close but you can see evidence of the tooling wearing over time and when a new tool is being used if you buy multiple lots of the same bullet over time. They all will meet the QC required but if you are thinking every bullet is just like the others, they are not.
Custom bullet makers tend to have more consistency and control over these variables.
Dave M.
F-Open Class shooter (284 win, 6dasher, 6.5-7PRCW)
Chapter 3, Page 83 in Bryan Litz’s book “Modern Advancements in Long Range Shooting, volume II”
Dusty Stevens
Shiner
How bout losing bc because of the shorter length?
Per my previous post, to get consistent seating depth, I sort from the seating stem contact point to the base. And I do see significant difference on the targets related to the difference in that measurement. Here's an example how some loads that I fired this weekend are pretty much like I've had in the past:
I was at the end of one sorted batch of 169 SMK's and had a small amount that was .010 difference between the seating stem contact point and the base (.858 - .848 - .010, see the green part of the highlighted lines of data in my spreadsheet below). Note that though there was a .010 difference, but the CBTO is the same . . . meaning, the base of the .858 projectile was ~ .010 deeper in the case than the .848 projectile due to no adjustment to the seating die. I also started a new larger batch that is .008 longer than the .848 batch that I've been running and made seating die adjustment to get the base of the projectile to the same place as I've had with the .848 projectiles.
Also below are the targets that I used and you can see on the last set of targets (only 3 used) the last two target were from the deeper seated rounds. Of those last two targets, the very last one I mounted my MagnetoSpeed for velocity recording, and was surprised as chrono seemed to actually improve the deeper seated rounds results, though there was the expected vertical POI shift that I usually get and why my POA was lower.
Yes it does " point up" the bullet. : https://www.whiddengunworks.com/bullet-pointing-die-system-2/
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