Hello all. I am new to the forum and have been loading for some years now. I just started using a Hornady compensator to take the next step in shooting. Here is my frustration. See picture below. I took measurements on a Hornady 178 gr A-max ( CBTO 2.2615) and a Sierra 190 gr MK (CBTO 2.2855) for my Bergara 308. Understanding that the COL will change with the bullet but the CBTO should remain within .001-.002 or am I missing something. The picture shows the compensator line. Any advice would be greatly appreciated.
CBTO HELP!!!!
- Thread starter Obowhunter
- Start date
Ned Ludd
Silver $$ Contributor
When you take cartridge base-to-ogive (CBTO) measurements using a calipers and a comparator insert, the tool seats on the ogive to where the bullet nose diameter matches the hole diameter of the comparator insert. This will not necessarily be the exact same spot on two different brands of bullets. Exactly where the insert seats (i.e. how far down the bullet ogive, or nose) depends on the ogive radius of the bullet. Different bullets will reach the diameter of the comparator insert hole at different distances above the bearing surface. This is because the ogive radius for two different brands of bullets can be very different. There can/will be variance in the ogive radius even within the same Lot# of bullets, although it's usually markedly less than between two different brands of bullets as shown in your picture. Because the diameter of the bullet nose at a given point above the bearing surface is dictated by the ogive radius, the observation you have made also means that the points at which two different bullets first contact the lands at "touching", and therefore how much bullet nose extends into the bore past the contact point cam also vary. So don't be concerned. The bottom line is that what you are experiencing is normal, even expected.
Likewise, COL measurements can and will vary with different brands of bullets, or even bullets from within the same Lot#. This is because the bullet nose lengths above the comparator insert contact point can/will also vary. Unless you have length-sorted bullets to a very small increment, it is therefore not totally uncommon for two loaded rounds to different COL measurements, even if they had the same CBTO measurement. Again, you'd generally expect this difference to be larger for two different brands of bullets, but not necessarily zero even for bullets from within the same Lot#.
Because of the [typically much larger] bullet length and ogive radius variance between two different brands of bullets, the key is produce as consistent as possible CBTO measurements for a single specific bullet type, as determined from measurements made using a tool such as like the Hornady OAL gauged or stripped bolt method for finding the distance to "touching" the lands. As indicated above, CBTO measurements made at "touching" for two different brands of bullets, or even two different length bullets from the same Lot# can and will vary. So it's important to find the distance to "touching" with every different brand of bullet you test. A measurement made with one brand may be very different than another, as illustrated very nicely in your picture above.
Even for a single Lot# of bullets, I set aside 10 bullets chosen at random (i.e. unsorted), label them #1 to #10 on the base with a Sharpie, then use the entire set of 10 bullets to measure the distance to "touching" with a Hornady OAL gauge, as well as COL. I then then take the average of the 10 measurement values to begin the reloading process (i.e. CBTO and COL at "touching"). I label the set of 10 bullets with the bullet info (i.e. Lot#, bullet type/weight, etc.) and keep them as my "measurement set" for that particular Lot# of bullets for as long as I am shooting them. That way, when I need to re-optimize seating depth, or make some comparative measurements to evaluate land erosion, I am using the exact same set of 10 bullets every time for the life of that Lot# of bullets. Some might consider taking 10 measurements every time excessive. It could certainly be done with fewer bullets than 10; for example 3, 5, 7, or whatever number someone felt comfortable doing.
Likewise, COL measurements can and will vary with different brands of bullets, or even bullets from within the same Lot#. This is because the bullet nose lengths above the comparator insert contact point can/will also vary. Unless you have length-sorted bullets to a very small increment, it is therefore not totally uncommon for two loaded rounds to different COL measurements, even if they had the same CBTO measurement. Again, you'd generally expect this difference to be larger for two different brands of bullets, but not necessarily zero even for bullets from within the same Lot#.
Because of the [typically much larger] bullet length and ogive radius variance between two different brands of bullets, the key is produce as consistent as possible CBTO measurements for a single specific bullet type, as determined from measurements made using a tool such as like the Hornady OAL gauged or stripped bolt method for finding the distance to "touching" the lands. As indicated above, CBTO measurements made at "touching" for two different brands of bullets, or even two different length bullets from the same Lot# can and will vary. So it's important to find the distance to "touching" with every different brand of bullet you test. A measurement made with one brand may be very different than another, as illustrated very nicely in your picture above.
Even for a single Lot# of bullets, I set aside 10 bullets chosen at random (i.e. unsorted), label them #1 to #10 on the base with a Sharpie, then use the entire set of 10 bullets to measure the distance to "touching" with a Hornady OAL gauge, as well as COL. I then then take the average of the 10 measurement values to begin the reloading process (i.e. CBTO and COL at "touching"). I label the set of 10 bullets with the bullet info (i.e. Lot#, bullet type/weight, etc.) and keep them as my "measurement set" for that particular Lot# of bullets for as long as I am shooting them. That way, when I need to re-optimize seating depth, or make some comparative measurements to evaluate land erosion, I am using the exact same set of 10 bullets every time for the life of that Lot# of bullets. Some might consider taking 10 measurements every time excessive. It could certainly be done with fewer bullets than 10; for example 3, 5, 7, or whatever number someone felt comfortable doing.
Last edited:
Your comparitor measures to an arbitrary point depending on the point form of the bullet. Where it strikes the ogive curve depends on the construction of the point. An extreme example, a tangent ogive would likely have less likelihood of consistency than a spire point because there's more area where it's damned near the same diameter.
Unless both bullets claim to have the same nose profile design, they're not going to be the same.
Unless both bullets claim to have the same nose profile design, they're not going to be the same.
RegionRat
Gold $$ Contributor
Here is another explanation that helps without getting into complex sketches of the differences in bullet shapes where they touch the comparator tools versus the cut rifling in the throat.
If the comparator tool diameter were 0.300” and the bullets are seated to an equal setting for that tool, then the only way they would be the same within the gun is if the ramps formed by the cut rifling were cut out to form a shoulder at 0.300”with no ramps. In reality, the rifling just below the 0.300 bore is a ramp from that 0.300 out to 0.308”, and that ramp can vary with the reamer.
If you think of the comparator tool like a section slice line on the bullet profile, the different types of ogive values touch that 0.300 slice point at different angles.
If the angle is very shallow, you get a different outcome than one where that angle is very steep and blunt.
So now if we introduce the ramps formed by where the rifling is cut, bullets will touch the rifling sooner or later depending on that angle you imagined above, and that means that it no longer touches at the 0.300 diameter in the gun like it does on the comparator tool, just based on the ogive shape.
In the gun, those touch point differences also vary between guns based on different reamer angles. It takes math out to several decimal points to calculate these differences, or sketches with very high resolution to be able to show them.
The difference between a bore of 0.300 and the 0.308 diameter is a ramp height of only 0.004” per land on a side. When you consider the ramp angles are cut shallow, you can imagine why the different ogive surfaces do not touch at the same point along the bullet length in the rifle as they do on the comparator tool.
ETA: If a hypothetical comparator tool were to be cut exactly like your bbl throat, then you would be able to use it as a way to measure different bullet ogives and get an answer that translates to the gun.
If the comparator tool diameter were 0.300” and the bullets are seated to an equal setting for that tool, then the only way they would be the same within the gun is if the ramps formed by the cut rifling were cut out to form a shoulder at 0.300”with no ramps. In reality, the rifling just below the 0.300 bore is a ramp from that 0.300 out to 0.308”, and that ramp can vary with the reamer.
If you think of the comparator tool like a section slice line on the bullet profile, the different types of ogive values touch that 0.300 slice point at different angles.
If the angle is very shallow, you get a different outcome than one where that angle is very steep and blunt.
So now if we introduce the ramps formed by where the rifling is cut, bullets will touch the rifling sooner or later depending on that angle you imagined above, and that means that it no longer touches at the 0.300 diameter in the gun like it does on the comparator tool, just based on the ogive shape.
In the gun, those touch point differences also vary between guns based on different reamer angles. It takes math out to several decimal points to calculate these differences, or sketches with very high resolution to be able to show them.
The difference between a bore of 0.300 and the 0.308 diameter is a ramp height of only 0.004” per land on a side. When you consider the ramp angles are cut shallow, you can imagine why the different ogive surfaces do not touch at the same point along the bullet length in the rifle as they do on the comparator tool.
ETA: If a hypothetical comparator tool were to be cut exactly like your bbl throat, then you would be able to use it as a way to measure different bullet ogives and get an answer that translates to the gun.
Last edited:
Ned Ludd covered it really well. I would only add . . .
What's important is to get consistent seating depth where the base of the bullets are all at the same depth, or very close. Because CBTO's can vary by several thousandths, it can be challenge to do this when your seating stem makes contact at a very different place than the comparator insert used to measure the CBTO's.
Of course, different bullets will require different seating depths as well in order to produce accuracy or precisions with any of them.
Last edited:
Coyotefurharvester
Silver $$ Contributor
Yes, answering your question. An individual lot of the same bullets is fairly easy to seat within 0.001-0.002 cbto of each other and yet have different col. Always remembering you are using a comparator and your measuring tool will have a +,- factor.
Thank you for all the information it has been helpful. Although wouldn’t the A-Max bullet with a decreased angle profile have the bigger CBTO. In this case the 190gr MK has a steeper bullet nose angle than the A-max. With that said shouldn’t the 190 MK have a smaller CBTO measurement due to contacting the lands sooner than the A-max? In this case it is the opposite.
A-max is 2.2655 less angle.
Match king is 2.2855 steeper angle.
See pics above
Again thank you all for the responses.
A-max is 2.2655 less angle.
Match king is 2.2855 steeper angle.
See pics above
Again thank you all for the responses.
The angle is only part of the equation as you also have differences in bearing surface and boat tail lengths.
BoydAllen
Gold $$ Contributor
We work out our seating dept for a particular bullet based on working with the chamber and the actual bullet and then we need a way to record that measurement. In the past the overall length of the loaded round was the reference but people came to understand that the tips of bullets are not uniform enough to be reliable for setting up a seating die, so someone made a tool to take measurements off of the ogive, near where the rifling would first make contact. These tools were called ogive length tools which made it easy for new loaders to think that their purpose was/is to measure the length of a bullets ogive, which of course is incorrect. They are meant to measure loaded ammo for purposes of record keeping so that dies may be reset more accurately than if measurements are taken off of bullets tips, properly called meplats. These tools do not duplicate rifling, partly because barrel dimensions vary as do throat configurations. They allow the user to set a seating die based on a particular make and model of bullet. Even with the same rifle, different bullets (mfg. and specific bullet) will be different distances from or into the rifling even though the same comparitor is used and the same ogive to head measurement used. Even when used properly these measurements are only of transient value given that barrel throats wear which requires that bullets be seated longer and longer, as throat wear occurs, to touch the rifling.
Yes. Not to confuse matters but a surprise is when it's different on the SAME brand. Or I should say don't be surprised.
Absolutely. Sometimes moving from one box of a given bullet to another of same brand and weight will produce a different measurement and more than you would expect in some brands.
Lands have nothing to do with CBTO. It's cartridge base to ogive, not ogive to lands (OTL).
If steeper 'angle' means more curvature, your tool would come to rest on the nose further from bearing (and further from cartridge base).
The Amax is 9.6cal secant
The MK is 7cal tangent
Fewer cals = more curvature
Consistent seating with the Amax may mean getting a VLD seater stem.
But you may be alright with simple die adjustment.
Ned Ludd
Silver $$ Contributor
It really shouldn't be all that surprising. Within a single Lot# of bullets, it is normal to find OAL variance, sometimes significant. For example, in my hands a typical Lot# of Berger bullets might exhibit around .015" to .020" OAL variance. A few extreme outliers might bump that up even a bit more. Additional measurements reveal that the majority of the OAL variance resides in the nose segment of those bullets. Because the bullet diameter at the top of the bearing surface is effectively a constant, any nose length variance also means ogive radius variance. It's basically just a function of how they're made and the tolerances involved.
Fortunately, we can deal with these variances pretty easily in terms of the reloading process with careful measurements and by length sorting bullets. The thing I find most frustrating is when I sort bullets by OAL into length groups, and find the distribution to be relatively uniform across all but the very longest and shortest length groups. That only happens occasionally, but I'd much rather find a typical Gaussian distribution with the largest numbers of sorted bullets bullets near the middle two or three groups. Having a larger number of sorted bullets near the middle sorted length groups makes it much easier to load and shoot bullets from pretty close to the same length group for a while before I run out of those two or three length groups. Although not a deal-breaker, it's a little messier when they're spread out fairly equally across 6 or 7 length groups.
Upgrades & Donations
This Forum's expenses are primarily paid by member contributions. You can upgrade your Forum membership in seconds. Gold and Silver members get unlimited FREE classifieds for one year. Gold members can upload custom avatars.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.
Click Upgrade Membership Button ABOVE to get Gold or Silver Status.
You can also donate any amount, large or small, with the button below. Include your Forum Name in the PayPal Notes field.
To DONATE by CHECK, or make a recurring donation, CLICK HERE to learn how.