Cbto issues

savagelh

savagelh

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I was always under the impression that the ogive is where the bullet contacts the rifling so I'm a bit puzzled with the results of my measurements.

So I'm working with 280ai in a new build, I grabbed a bullet and checked the distance to the lands using my favorite method that has been pretty consistent thus far. I take a fired case, put some crazy glue in the case neck, place a bullet in leaving it very long and chamber it, I leave it in the chamber for a few minutes and the extract it and get my cbto measurement using the hornady comparator.

So I tried hornady 162 eldx as well as sierra 165 tipped gameking and was quite surprised that my cbto measurements were about .060 different. I did the same test a few times and got similar results. I guess going forward I will have to do my test for each bullet that I want to try. No big deal since it doesn't take long but I thought max cbto was max for any bullet.
 
Maybe part of the story? I am like you - I think the "ogive" is the point on the bullet where it gets to "groove" or "nominal" diameter. But nothing says that is the precise point that your lands are cut to - the actual point of contact with the bullet. So, it is as if it is the contact point to your lands that you are measuring, and therefore different for different bullets. I do not own a Hornady comparator tool - but I suspect the whole point of it might be to find the actual "ogive" on a bullet - evidently that is NOT where your lands touch that bullet.

I suspect it is "cool" to know where is the ogive on a bullet - but I think for setting actual bullet "jump" when firing it, needs to be in reference to where the lands are.
 
This isn't unusual. Different bullets will have different contact points depending on the profile of the bullet.
I make up a dummy round for each different bullet that I use.
A few have the same CBTO measurement for the same jump, but most are somewhat different.
 
You will hear or read of bench rest type guys "chasing the lands" - so measuring to your lands is not a "done once and forever" - was explained to me that the lands are eroded or burned away every time the rifle is fired - but I do not know if it is after 10 rounds or 100 rounds or 1000 rounds when that becomes measurable - no doubt is different barrel to barrel. So, it would require the lands to be exactly the same angle as your bullet shoulder, or greater, for the first contact to be at the ogive of the bullet - if the lands are cut at (or burned to) a lesser angle than the bullet, it will be the inner face of the lands that contact first, close to the face of the rifling, not at the bullet ogive.
 
Different bullets have different ogive profiles - eg tangent vs secant. This will affect your measurements. To boot, bullets from the same manufacturer can vary from lot to lot. Ask me how I know....
 
Also the comparator used is only good for building a reference point as it may not mesure any 'optimal' point on any given bullet
it compares measurements
 
OP - I am not a competitive target shooter, so might be very different for that kind of shooter with custom guns, which I do not own. As per a Barsness article, and I think as shown in an older Nosler manual - I ignore CBTO numbers - for better or worse. I find the lands by pushing bullet of choice into chamber - with a dowel, until I feel it on the lands - then another dowel or cleaning rod with squared tip down the muzzle until it touches the bullet tip. Make mark on rod. Push out bullet and make another mark on rod against closed bolt face. Is likely total heresy to measure to a bullet tip - I have measured like 20 from same box, and not precisely all the same - but like 0.005" at most difference. None of the bullet seaters here have been ground to only touch bullet on ogive, so seemed pointless to me to measure to there. As I mentioned to a "new" reloader sort of 0.020" to 0.040" bullet jump usually "good enough" to do pressure series - maybe someone can see difference by fine tuning bullet seating depth from that - I can not say that I ever did - so I generally load my rounds to be circa 0.030" less than the measurement - should be 0.030" "jump" - hence the variation among bullet length gets lost and I do not think it is of much significance, since the rest of my tooling can not deal with that, anyways. You can read the same thing on pages 40 and 41 of the Woodleigh reloading manual. If I do up a batch of 100 rounds, I want them to fit into the rifle / into the magazine the same to each other - so, from my point of view, "same" is good, and "not same" is less desirable. Many rifles here will get me 1" or 1.25" 5 shot groups on 100 yard target - is plenty "good enough" for any shooting that I do - and about 75% of that has not much to do with the load - many hours spent fussing about bedding, scope mounting, action screw torque, etc.
 
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I use the Erik Cortina method to find the CBTO that touches the lands.

I have only tried with two different bullet models, and never in the same session. So I don't know if the number resulting from one bullet model will match the number resulting from another bullet model.

As stated above, there are bullet shape reasons the number could be different.
 
POT - I too dont bother with CBTO, and those fancy comparator gizmos. I also use the dowel method. To address the variation in bullet length (common in lead tipped hunting bullets), the bullet I use for dowel measurement becomes my reference bullet. I mark it and measure it for length. I (hopefully) keep that bullet in the box and use it for further setups to get the correct COAL. If it gets lost or misplaced, I have the measurement to rely on - I can create another reference bullet.
 
So I had some time today. Tested a pile of different bullets, got a pile of different cbto lengths. I guess the comparator just doesn't do what I thought it did and it just measures an arbitrary spot on the ogive. No big deal, I now know that I will just have to measure different bullets each time I develop a load.
 
I don't know about other types of Bench Rest shooters but in Hunter Bench Rest, the reason "You will hear or read of bench rest type guys "chasing the lands" for this was quite simple

The chamber tolerances were extremely important, especially the necks.

When the chamber was reamed, the neck area was purposely cut several thousandths less than SAAMI spec.

We had to turn down the neck thickness so the cartridge would chamber.

We had dies that neck sized only and they had different inserts to match the dimensions of our chambers.

There were several reasons for this but neck tensions weren't nearly as heavy as those on cartridges used for hunting or casual target shooting.

As such, we always seated the bullets out longer than the distance from the bolt face to the beginning of the leades in these rifles.

The tension was often so light that the bullet could be pushed back into the case with thumb pressure.

We seated the ogive of our bullets full onto the leade and allowed it to push the bullet back into the neck in lieu of more neck tension.

This gave far more consistent results.

F Class shooters don't use this method and some of their groups are very impressive. So which is better??

The only reason I would turn down the necks of a cartridge is to relieve the tension holding the bullet in place, so I could seat the bullet deeper when camming the bolt closed on a round.
 
You will definitely see a difference in COAL when loading different bullets to max CBTO by using that method (I call it the loctite method since I use loctite in a once-fired case). All bullets will have a different ogive profile and as a result will contact the lands at a different COAL.

I usually load to max CBTO for my chamber/bullet combination, then subtract either 0.020" (lead core) or 0.050" (monolithic) from the resulting COAL as my starting seat-to depth.
 
Potashminer said:
You will hear or read of bench rest type guys "chasing the lands" - so measuring to your lands is not a "done once and forever" - was explained to me that the lands are eroded or burned away every time the rifle is fired - but I do not know if it is after 10 rounds or 100 rounds or 1000 rounds when that becomes measurable - no doubt is different barrel to barrel. So, it would require the lands to be exactly the same angle as your bullet shoulder, or greater, for the first contact to be at the ogive of the bullet - if the lands are cut at (or burned to) a lesser angle than the bullet, it will be the inner face of the lands that contact first, close to the face of the rifling, not at the bullet ogive.
Click to expand...

With my 308 target rifles, the throat erodes about 5 thou per season.
 
Potashminer said:
OP - I am not a competitive target shooter, so might be very different for that kind of shooter with custom guns, which I do not own. As per a Barsness article, and I think as shown in an older Nosler manual - I ignore CBTO numbers - for better or worse. I find the lands by pushing bullet of choice into chamber - with a dowel, until I feel it on the lands - then another dowel or cleaning rod with squared tip down the muzzle until it touches the bullet tip. Make mark on rod. Push out bullet and make another mark on rod against closed bolt face. Is likely total heresy to measure to a bullet tip - I have measured like 20 from same box, and not precisely all the same - but like 0.005" at most difference. None of the bullet seaters here have been ground to only touch bullet on ogive, so seemed pointless to me to measure to there. As I mentioned to a "new" reloader sort of 0.020" to 0.040" bullet jump usually "good enough" to do pressure series - maybe someone can see difference by fine tuning bullet seating depth from that - I can not say that I ever did - so I generally load my rounds to be circa 0.030" less than the measurement - should be 0.030" "jump" - hence the variation among bullet length gets lost and I do not think it is of much significance, since the rest of my tooling can not deal with that, anyways. You can read the same thing on pages 40 and 41 of the Woodleigh reloading manual. If I do up a batch of 100 rounds, I want them to fit into the rifle / into the magazine the same to each other - so, from my point of view, "same" is good, and "not same" is less desirable. Many rifles here will get me 1" or 1.25" 5 shot groups on 100 yard target - is plenty "good enough" for any shooting that I do - and about 75% of that has not much to do with the load - many hours spent fussing about bedding, scope mounting, action screw torque, etc.
Click to expand...

Pretty much how I've done it for years as well. - dan
 
Instead of using the glue, you could try seat the bullet into empty brass and see if you can easily push into the chamber, if not, seat few thousands deeper at least to the end, your measurement would be within a few thousands inch which is close enough for you to start your seating depth test.
 
CBTO makes a huge difference.
Different bullets will be different. Even Burgers in different lot numbers will be different.
The Hornady tool is useful but I just body size a case and then neck size just enough to let the bullet slip.
My worn Savage barrel comes in and out of tune in 0.007”. It shoots well, and when it does not I will end up back in the sweet spot with a 0.007” adjustment. This is why I won’t go back to a non micrometer seating die.
 
Potashminer said:
Maybe part of the story? I am like you - I think the "ogive" is the point on the bullet where it gets to "groove" or "nominal" diameter. But nothing says that is the precise point that your lands are cut to - the actual point of contact with the bullet. So, it is as if it is the contact point to your lands that you are measuring, and therefore different for different bullets. I do not own a Hornady comparator tool - but I suspect the whole point of it might be to find the actual "ogive" on a bullet - evidently that is NOT where your lands touch that bullet.

I suspect it is "cool" to know where is the ogive on a bullet - but I think for setting actual bullet "jump" when firing it, needs to be in reference to where the lands are.
Click to expand...

the ogive is not a particular or singular spot or position. The ogive is the curved area or portion
Finding a length in relation to the lands is by measuring a location on that ogive in a repeatable consistent manner

and yes your seating dies should contact the ogive and not the tip, optional cups are available
 
boxhitch said:
the ogive is not a particular or singular spot or position. The ogive is the curved area or portion
Finding a length in relation to the lands is by measuring a location on that ogive in a repeatable consistent manner

and yes your seating dies should contact the ogive and not the tip, optional cups are available
Click to expand...

When I was playing about that, I was quite sure that none of the seaters that I was using then, touched the bullet tip. Nor did they touch the bullet at the groove diameter - the contact to seat the bullet was somewhere in between. For sure on 6.5x55 (?) I had drilled out the "tip" area so that it did not contact the tip of the Norma "Golden Target" bullets that I was using - but I don't think the contact area went all the way to the groove diameter, though. Maybe was my dilemma - a "comparator tool" would likely measure from a specific diameter on the bullet, to cartridge case head, but none of my loading tools could match to that - and none of my rifles likely could show that difference, anyways. I presumed that if COAL differences of many thousandths - like 5 or more, did not make difference on 5 shot groups at 100 yards, I presumed (assumed) that much smaller increments would have no effect. Is no doubt to me that there are guns and shooters who can show that makes a difference though - I could not, with the guns that I have.
 
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