Why are my CBTO lengths different?

[punkwood2k]

Hey everyone.. First time posting a question, but I have lurked on this site forever and learned tons from it.. Please help me out with this one..

I've just started playing around with controlling my bullet jump. I bought the Hornady OAL gauge, and the Hornady Bullet Comparator, so I could figure the CBTO for my .308, and try loads at 0.010", 0.015" and 0.020" jumps to begin with..

My problem is that each bullet type / weight has a different CBTO when measured to the lands.. Now, please correct me if I'm wrong, but shouldnt the Base to lands measurement be the same for every bullet, since the chamber/throat/lands distance in the rifle dont change? No matter the length of the bullet, the ogive should be the same distance from the base of the cartridge, all that would change is the amount of bullet in and outside of the case neck..

I have measured each type of bullet a couple dozen times, and the measurements always come out the same for each specific bullet type, but CBTO varies by up to 0.055" between bullet types.

Am I understanding this wrong, or overthinking the problem?

Thanks.

 

[dustinoif3]

Think about it. Why do certain brands and types shoot better than others? If every manufacture made the same profile bullet per weight there wouldn't be anything that set them apart. So here's what I do if I want the same jump. I back my seating stem out and work it back down with the comparator. I learned the answer to your op the hard way. I was loading some 168 amax and was seating to x to ogive. I then switched to smk's without changing the stem. Started cranking then noticed something odd. The smk's ogive was way longer. Well it was due to the shorter nose on the bullet and if you've ever noticed your seating stem contacts out towards the end of the bullet. Nowhere close to ogive so you may end up with the same oal switching back and forth but in reality the round may be jumping more or less. Luckily I'm loading for a remi 700 so I'll never reach the lands. Lol

 

[dustinoif3]

To answer another part of your post I missed. Sometimes you don't seat to the same ogive length per profile. Some bullets like the amax are more tolerant to jump where as an smk likes being closer but it ultimately depends on the rifle. I've experienced with 168's and 175's both amax and smk. I've had great luck seating to the same ogive per bullet weights. I load for mag feeding so I'm limited. I'm jumping .101 and the rifle loves it

 

[Rprecision]

No each bullet has a different profile. If you take the Stoney point gauge a set it on the bullets the distance from base to gauge changes for each bullet.

You just have to take measurements for each bullet style and adjust.

No big thing

And BTW, I have found over the years that COAL adjustments can "tweak" a load a bit but don't expect them to make a load. Anymore I load to SAMMI COAL feed from mag and call it.

 

[punkwood2k]

I understand that each bullet has different profiles.. And how the different profiles will affect the COAL of the bullet, when measured from the TIP to BASE. But the CBTO distance from the BASE to the LANDS should NOT change, no matter how many different bullet styles you measure.. Because the lands of the rifle are ALWAYS going to be a fixed distance from the bolt face and cartridge base.. So no matter if your bullet is long or short, the ogive will always only engage the lands at one specific distance from the bolt face / base. Or at least it SHOULD, if the laws of physics are obeyed, and somehow I am breaking them. :-(

 

[Rprecision]

Define CBTO I may not be understanding what your talking about.

You lost me in your last post. Yes the throat is a specific distance from bolt face, while fixed it does Change with time (erosion)

Determining at what distance a bullet touches the throat at depends on bullet profile. It specific to the diameter of your throat vs. bullet profile. That specific diameter will be found at differing points on differing bullet profiles. It's not fixed

Now, if you had a modified seating die that used the exact diameter of your throat and basically copied your chamber then anything you seat would be the same.

But it's not that's why we use a gauge to compare where each bullet sits and adjust dies to accomplish our intended COAL

 

[damoncali]

I understand that each bullet has different profiles.. And how the different profiles will affect the COAL of the bullet, when measured from the TIP to BASE. But the CBTO distance from the BASE to the LANDS should NOT change, no matter how many different bullet styles you measure.. Because the lands of the rifle are ALWAYS going to be a fixed distance from the bolt face and cartridge base.. So no matter if your bullet is long or short, the ogive will always only engage the lands at one specific distance from the bolt face / base. Or at least it SHOULD, if the laws of physics are obeyed, and somehow I am breaking them. :-(

That's only true if the bullets contact the lands at the same point. A wadcutter will hit the lands at the beginning of the leade, while a skinny vld might not hit until the end.

 

[punkwood2k]

Define CBTO I may not be understanding what your talking about.

You lost me in your last post. Yes the throat is a specific distance from bolt face, while fixed it does Change with time (erosion)

Determining at what distance a bullet touches the throat at depends on bullet profile. It specific to the diameter of your throat vs. bullet profile. That specific diameter will be found at differing points on differing bullet profiles. It's not fixed

Now, if you had a modified seating die that used the exact diameter of your throat and basically copied your chamber then anything you seat would be the same.

But it's not that's why we use a gauge to compare where each bullet sits and adjust dies to accomplish our intended COAL

CBTO = Cartridge Base to Ogive measurement.. more accurate that measuring base to tip.

You say the diameter of a bullet isnt fixed? How so? .308 means that the bullets are .308" at the Ogive, right? I know the ogive location changes on different bullet types, but all bullets are .308" at the ogive / bearing surface.

I'm probably just overthinking all this, arent I

 

[damoncali]

CBTO = Cartridge Base to Ogive measurement.. more accurate that measuring base to tip.

You say the diameter of a bullet isnt fixed? How so? .308 means that the bullets are .308" at the Ogive, right? I know the ogive location changes on different bullet types, but all bullets are .308" at the ogive / bearing surface.

I'm probably just overthinking all this, arent I

The lands are less than .308 in diameter. There is a conical "ramp" going from the .308 freebore to the uncut lands (the leade). This is where the bullet makes contact. And where the bullet makes contact on the leade depends on the angle of the leade and the shape of the bullet.

 

[l115a3]

Your mearsure was right.

Different bullet has differ profile which lead to number of Ogive.

 

[punkwood2k]

The lands are less than .308 in diameter. There is a conical "ramp" going from the .308 freebore to the uncut lands (the leade). This is where the bullet makes contact. And where the bullet makes contact on the leade depends on the angle of the leade and the shape of the bullet.

Conical? I always thought it was straight with a sudden jump to the lands??... If it truly IS conical, that would explain the variances, and my confusion. That might be the piece of the puzzle I was missing.. thanks!

 

[damoncali]

Conical? I always thought it was straight with a sudden jump to the lands??... If it truly IS conical, that would explain the variances, and my confusion. That might be the piece of the puzzle I was missing.. thanks!

Yup- more info here: An Introduction to Rifle Chambers | Bison Ballistics

A typical leade angle is 1.5 degrees.

Edit: I just realized the images in that article is mislabeled. I'll fix it when I get a chance. Throat = freebore = the unrifled section where the bullet sits. Leade = the ramp that goes to the rifling with the 1.5 degree angle.