CBTO Different?

[Lefty57]

Can someone explain to me why some bullets just will not group.(other than"they just won't")
Am I correct in saying regardless of COL if the CBTO is the same that the bullet is entering the rifling at the same measurement. I am comparing all high BC bullets, all the same wt. .284 -- 175ABLR - 175EH - 175ELD-X
Maybe I was better off when I did not try to understand ballistics, seems this loading can drive a man crazy trying to understand.

 

[asd9055]

My understanding, CBTO needs to be measured for each lot of bullets. Friction is also different and so is cg, and other details, so every time you need to take all that into account.

 

[Jakeweb09]

Are you saying you're setting all the 175 class bullets to same cbto? You need to check where every type of bullet touches the lands. Not just same weight. A 175eldx will be significantly different then a 175 elite hunter

 

[Chesterrose79]

Every bullet manufacturer has a slightly different bullet shape even if the weight is the same. Also different amount of bearing surface which changes things. Have you measured the velocity difference between the bullets?

 

[med358-boise]

You have asked two very different questions. Taking the second question first about CBTO. The answer is basically- no. Now let me explain, why.

Assumption for the purposes of the explanation is that every bullet produce of a given make,model, and weight is produced perfectly and their is zero mfg variances.

1) The point where the bullet contacts the lands is where the bullet diameter matches the bore diameter. So on a .308 caliber rifle the bore diameter is typically .300.
2) Most people use a bullet comparator tool to measure CBTO. But the vast majority of bullet comparator tools do not use bore diameter for the inside diameter of their "30 cal" insert. They might use .293 or some other number.
3) The ogive curve is different for every bullet, so using the example above, the distance between the .293 point on the bullet to the .300 diameter point will be different because of the shape of the ogive curve which means the distance from the case base to that .300 diameter point will be different.

A few nomenclature problems exist in this area. Case Base To Ogive (CBTO) is a horrible acronym but it is the acronym we are stuck with. It is horrible because 1) Ogive means the curved portion of the bullet is does not mean the point where the the bullet has a diameter equal to the typical bore diameter for that caliber but the shooting community uses "ogive" in this context to imply a point on the bullet and further since the objective is to measure distance to the lands, shooters imply that "ogive" is referencing the bore diameter point on the bullet when it is not. 2) The commonly used tool to measure CBTO is a bullet comparator tool and every bullet comparator tool maker will have a different inside diameter (ID) for a given caliber and even with the same tool mfg tolerances will vary the ID. 3) Shooters use CBTO to get a consistent measurement of how far the bullets are seated from the lands but that is not what is really being measured. What is being "created" is a referential index number based on the specific bullet comparator being used to facilitate consistent seating depth because bullet lengths are not as accurately controlled in the production process and what is also being created is referential number to compare bullets.

 

[lancetkenyon]

CBTO is only one factor in where the bullet will touch the lands. Just because you seat all your bullets to the same CBTO, they will not all be the same distance from touching the lands. Bullet ogive shape and leade angle play a big part in this.

Here is a prime example of how various bullets will vary for CBTO and COAL touching the lands in a brand new chamber.

 

[med358-boise]

Your first question about why do some bullets don't shoot well in a given rifle but will shoot well in others and vis-a-versa is harmonics and variances between bullets and rifles and how that leads to consistency of all variables that impact accuracy and the manner in which the bullet exits the barrel.

 

[lancetkenyon]

As for "why won't a bullet group well"....I see you are shooting NABLRs. Is that the one in question? (I have my reasons for asking)

 

[Lefty57]

You have asked two very different questions. Taking the second question first about CBTO. The answer is basically- no. Now let me explain, why.

Assumption for the purposes of the explanation is that every bullet produce of a given make,model, and weight is produced perfectly and their is zero mfg variances.

1) The point where the bullet contacts the lands is where the bullet diameter matches the bore diameter. So on a .308 caliber rifle the bore diameter is typically .300.
2) Most people use a bullet comparator tool to measure CBTO. But the vast majority of bullet comparator tools do not use bore diameter for the inside diameter of their "30 cal" insert. They might use .293 or some other number.
3) The ogive curve is different for every bullet, so using the example above, the distance between the .293 point on the bullet to the .300 diameter point will be different because of the shape of the ogive curve which means the distance from the case base to that .300 diameter point will be different.

A few nomenclature problems exist in this area. Case Base To Ogive (CBTO) is a horrible acronym but it is the acronym we are stuck with. It is horrible because 1) Ogive means the curved portion of the bullet is does not mean the point where the the bullet has a diameter equal to the typical bore diameter for that caliber but the shooting community uses "ogive" in this context to imply a point on the bullet and further since the objective is to measure distance to the lands, shooters imply that "ogive" is referencing the bore diameter point on the bullet when it is not. 2) The commonly used tool to measure CBTO is a bullet comparator tool and every bullet comparator tool maker will have a different inside diameter (ID) for a given caliber and even with the same tool mfg tolerances will vary the ID. 3) Shooters use CBTO to get a consistent measurement of how far the bullets are seated from the lands but that is not what is really being measured. What is being "created" is a referential index number based on the specific bullet comparator being used to facilitate consistent seating depth because bullet lengths are not as accurately controlled in the production process and what is also being created is referential number to compare bullets.

You have asked two very different questions. Taking the second question first about CBTO. The answer is basically- no. Now let me explain, why.

Assumption for the purposes of the explanation is that every bullet produce of a given make,model, and weight is produced perfectly and their is zero mfg variances.

1) The point where the bullet contacts the lands is where the bullet diameter matches the bore diameter. So on a .308 caliber rifle the bore diameter is typically .300.
2) Most people use a bullet comparator tool to measure CBTO. But the vast majority of bullet comparator tools do not use bore diameter for the inside diameter of their "30 cal" insert. They might use .293 or some other number.
3) The ogive curve is different for every bullet, so using the example above, the distance between the .293 point on the bullet to the .300 diameter point will be different because of the shape of the ogive curve which means the distance from the case base to that .300 diameter point will be different.

A few nomenclature problems exist in this area. Case Base To Ogive (CBTO) is a horrible acronym but it is the acronym we are stuck with. It is horrible because 1) Ogive means the curved portion of the bullet is does not mean the point where the the bullet has a diameter equal to the typical bore diameter for that caliber but the shooting community uses "ogive" in this context to imply a point on the bullet and further since the objective is to measure distance to the lands, shooters imply that "ogive" is referencing the bore diameter point on the bullet when it is not. 2) The commonly used tool to measure CBTO is a bullet comparator tool and every bullet comparator tool maker will have a different inside diameter (ID) for a given caliber and even with the same tool mfg tolerances will vary the ID. 3) Shooters use CBTO to get a consistent measurement of how far the bullets are seated from the lands but that is not what is really being measured. What is being "created" is a referential index number based on the specific bullet comparator being used to facilitate consistent seating depth because bullet lengths are not as accurately controlled in the production process and what is also being created is referential number to compare bullets.

Thanks i do understand the way you explained...you are correct, i was understanding that the ogive measurement was at the point where diameter measured the size i was loading, had i thought at all that can't be because the bullet would completely bury in the comparator.

 

[Lefty57]

As for "why won't a bullet group well"....I see you are shooting NABLRs. Is that the one in question? (I have my reasons for asking)

Yes in my 7PRC, i was already told(after i had the bullets) how finicky ABLR's can be. I also shoot the 142's in my 6.5 PRC with good accuracy.
I know part of the problem is 7PRC SAAMI spec reamers have .250 freebore ....most tips don't have enough BS left if you try to touch.
Thanks