CBTO

[Mike6158]

FWIW, with Hammers, I have never measured the CBTO as the uniformity from tip to tip is so good...

If you use your comparator on bullets from some bullet manufacturers, I won't name names but it starts with an H and ends with an ornady, doesn't have a consistent base to ogive distance. So, every bullet seated, with no change to the seating die, varies. a few thousandths in length.

To be fair the bullet I measured is a hunting bullet and most people just seat, dump, and squash when they load those. Most people as in new reloaders and "less picky" reloaders <-- is that even a thing?

 

[chav0_12]

I say oh-jive. But I have heard it pronounce ah-give, oh-give, and even ah-jive.
Ogive is actually the entire curved portion of the bullet in front of the bearing surface.
But, when saying you are measuring CBTO, it is usually referring to the distance from the case base (head of case) to the spot on the bullet where the ogive meets the bearing surface.

There are some variations between comparator inserts, so it might not be exactly at that point, as the comparator is actually slightly undersized from the bullet diameter, or else it would slide over the entire bullet. Like a 6.5mm comparator insert is actually probably .262"-.263".

I learned the word ogive when I was in the army going to EOD school learning about artillery rounds and their anatomy. It was definitely pronounced oh-jive. I also learned the word bourrelet, pronounced bore-lay, to describe the portion of the projection where it comes to its widest part (not including the rotating bands). This is what I believe we measure to, this is the point at which our comparators stop on the projectile.

 

[ntsqd]

I'm going to toss this out there for consideration. Maybe it's viable as-is, maybe it's carp, maybe it's workable with some tuning. Maybe it's a great idea that will never happen (most likely as I seem to be really good at those).

To Lance's and other's point about the tool being necessarily slightly smaller than the bearing surface diameter I wonder if there is, or should be a standard dimension for each bore size? That way all tools would yield the same measurement. Given that no two barrel makers use exactly the same land and groove dimensions I suspect that the standard would need to be a percentage rather than a set distance. Say it's a 2% reduction of bore diameter. That yields these CBTO Bushing ID's from .22 to .30 (I'm not as conversant in the larger bores, so I stopped there but the list would go past what I've posted):
.2195
.2381
.2519
.2587
.2715
.2783
.3018
I'm not saying that it should be 2%, just that this is what that would look like. I suspect that the tolerance on these dims should be in the +/-.0002 range if not BASIC. Those are expensive holes. There's some further refinements needed to make this std a functional one. Perpendicularity of the ends of the bushing must be an extremely tight spec and so will need to be the dim for the edge breaks.

The potential problem that I see with the Whidden tool isn't one of their making. It is that it is possible to use either end of the bushing in question, and the transitions from bore to end on all of mine are rather inconsistent. Since that determines where the actual end of the bore starts it will introduce some variability in the measurements from one session to another. I suggest that the end used on each bushing be marked so that it is always the end used.

 

[Rflshootr]

Me play
Oh jive
Sabot is Sa BOW
FYI

And a new little ditty.....Me play shootin a wabbit with a sabbit and when I hit one Oh I jive.....and iffin he ain't dead I just hit 'em with a hammer.

 

[Buzzsaw]

well, i'll catch HE77 from many especially from ALL the doodad makers who make tools to measure these lengths. I think .20 thou is the thickness of a piece of paper. Not an engineer
I'm old and have been reloading forever. I too went through the OCD era of my reloading. IT's all pretty much BS. Your rifle will shoot or it won't, PERIOD. People continue to buy 1 MOA rifles and try to get them to shoot .25MOA. STOP!!! You're chasing unicorns. Hunters should load for FUNCTION, NOT gnat hair accuracy.
For those who continue to unethically shoot game at stupid ranges only to hit them (if at all) in the ***, only for the animal to get away and die a cruel death being eaten alive by predators.

OPPS got off track. Do what you want, but, seat to functioning magazine length, leave the minutia to the benchrest folk.

 

[C-130 Dude]

This has been discussed in the past. I just want to see what the current membership thinks. CBTO stands for Case Base To Ogive as a measurement having to do with cartridge length. What is an Ogive? We can debate pronunciation too. That's always fun too. No cheating and looking back a few years for past discussions!

Do you have any idea what you just started???

 

[Mikecr]

I feel the closer you get to measuring were the bearing surface junction with ogive starts theres less variance,

There would likely be less variance in bullet there, but far greater variance in measurement.
This due to the shallow angle of contact. The slightest ogive radius variance, or the slightest change in contact force, would dramatically change the reading.
This is why a high contact(on the nose) seating stem is a very bad idea.

Depending on the ogive type/radius, and the leade angle, actual land contact is quite a bit different than bore diameter.
We can have gizzys made with barrel finishing, using the same reamer with cut off barrel stub, to provide the same leade angle as our initial barrel finish, as chambered. This could provide a same contact point for our tool, -but only initially. Once you fire the gun, it all changes.
And again with angles, your measure with this would be precarious to force, even if you pre-qualified ogive radius with a Bob Green Comparator (BGC).

I do qualify all bullet ogives, and I use the simple Sinclair 'nut' for CBTO measure. This just works fine for me.
I initially measure CBTO to land touching, using the old cleaning rod method, and log that as touching.
For seating testing, I start 5thou off touching and work back from there. That works fine for me also.
When I discover the optimum seating depth for chosen bullet, I log it, and reproduce that initial CBTO (not land relationship) for the accurate life of that barrel, with that bullet.

While I'm not changing bullets, and not relying on boosted starting pressure from land contact, I don't ever have to mess with seating adjustments again. I could wiggle seating for another ~100 or so shots of life at the end, but that's tail chasing and I am already working on a replacement barrel by then.

 

[Old Rooster]

I'm going to toss this out there for consideration. Maybe it's viable as-is, maybe it's carp, maybe it's workable with some tuning. Maybe it's a great idea that will never happen (most likely as I seem to be really good at those).

To Lance's and other's point about the tool being necessarily slightly smaller than the bearing surface diameter I wonder if there is, or should be a standard dimension for each bore size? That way all tools would yield the same measurement. Given that no two barrel makers use exactly the same land and groove dimensions I suspect that the standard would need to be a percentage rather than a set distance. Say it's a 2% reduction of bore diameter. That yields these CBTO Bushing ID's from .22 to .30 (I'm not as conversant in the larger bores, so I stopped there but the list would go past what I've posted):
.2195
.2381
.2519
.2587
.2715
.2783
.3018
I'm not saying that it should be 2%, just that this is what that would look like. I suspect that the tolerance on these dims should be in the +/-.0002 range if not BASIC. Those are expensive holes. There's some further refinements needed to make this std a functional one. Perpendicularity of the ends of the bushing must be an extremely tight spec and so will need to be the dim for the edge breaks.

The potential problem that I see with the Whidden tool isn't one of their making. It is that it is possible to use either end of the bushing in question, and the transitions from bore to end on all of mine are rather inconsistent. Since that determines where the actual end of the bore starts it will introduce some variability in the measurements from one session to another. I suggest that the end used on each bushing be marked so that it is always the end used.

Being a machinist you bring up a good point!
Would plaining the bushing on a flat surface with light sand paper remove the variance?
Edit to add: Crocus cloth?

 

[ntsqd]

That would depend on the actual bushing. If you mark one side and always use it then it may not matter.

 

[C-130 Dude]

That would depend on the actual bushing. If you mark one side and always use it then it may not matter.

Aren't all Whidden bushings marked for reference? Their neck bushings are.

 

[Mike6158]

In consideration of Mr. Buzzsaw I will say this, I have been working up a load for my 7mag. using a 150 gr Barnes TTSX and H1000. I found "a" node. Started adjusting seating depth and found a good range for that. I've used the Barnes in the past and it does a nice job when you hit the right spot. That said, the hunts here in TX, where I go, are either high fence, feeder, 40 - 100 yards, shoot from a box "hunts" (it's not hunting, never has been). Or, if I can get access, I have a place where I can sit on the ground and watch a feeder or trail. One moa groups at 100 are more than sufficient. I got curious about how well the bullet would fly downrange so I started tuning for the best 100 yard group I could get (.3 ish). I haven't shot that load at 300 (longest range I have) yet. It's almost 20 degrees hotter than it was when I started. I went through a s-tonne <-- metric of bullets, powder, and primers and I can either start over or wait for temperatures in the mid 80's. Yes... I tried shooting when it was a 100 degrees but everything shifted and I don't want to go through anymore "node finding" exercises with hunting bullets that do well at the ranges I have to shoot at. If I was going to stretch the 7 mag's legs, I would keep going. And... to be honest. I probably will keep going. Note where I said "I got curious". I get that way sometimes.

I don't use the magazine my Sendero came with. I load 1 and shoot. At the ranges I shoot at my Nightforce will let me pick flies for the target.

So yeah, it's probably all pixie dust and dreams but I enjoy it.

 

[C-130 Dude]

In consideration of Mr. Buzzsaw I will say this, I have been working up a load for my 7mag. using a 150 gr Barnes TTSX and H1000. I found "a" node. Started adjusting seating depth and found a good range for that. I've used the Barnes in the past and it does a nice job when you hit the right spot. That said, the hunts here in TX, where I go, are either high fence, feeder, 40 - 100 yards, shoot from a box "hunts" (it's not hunting, never has been). Or, if I can get access, I have a place where I can sit on the ground and watch a feeder or trail. One moa groups at 100 are more than sufficient. I got curious about how well the bullet would fly downrange so I started tuning for the best 100 yard group I could get (.3 ish). I haven't shot that load at 300 (longest range I have) yet. It's almost 20 degrees hotter than it was when I started. I went through a s-tonne <-- metric of bullets, powder, and primers and I can either start over or wait for temperatures in the mid 80's. Yes... I tried shooting when it was a 100 degrees but everything shifted and I don't want to go through anymore "node finding" exercises with hunting bullets that do well at the ranges I have to shoot at. If I was going to stretch the 7 mag's legs, I would keep going. And... to be honest. I probably will keep going. Note where I said "I got curious". I get that way sometimes.

I don't use the magazine my Sendero came with. I load 1 and shoot. At the ranges I shoot at my Nightforce will let me pick flies for the target.

So yeah, it's probably all pixie dust and dreams but I enjoy it.

Pixie dust. Dreams........and down the rabbit hole we go...

 

[Jasbo]

I'm going to toss this out there for consideration. Maybe it's viable as-is, maybe it's carp, maybe it's workable with some tuning. Maybe it's a great idea that will never happen (most likely as I seem to be really good at those).

To Lance's and other's point about the tool being necessarily slightly smaller than the bearing surface diameter I wonder if there is, or should be a standard dimension for each bore size? That way all tools would yield the same measurement. Given that no two barrel makers use exactly the same land and groove dimensions I suspect that the standard would need to be a percentage rather than a set distance. Say it's a 2% reduction of bore diameter. That yields these CBTO Bushing ID's from .22 to .30 (I'm not as conversant in the larger bores, so I stopped there but the list would go past what I've posted):
.2195
.2381
.2519
.2587
.2715
.2783
.3018
I'm not saying that it should be 2%, just that this is what that would look like. I suspect that the tolerance on these dims should be in the +/-.0002 range if not BASIC. Those are expensive holes. There's some further refinements needed to make this std a functional one. Perpendicularity of the ends of the bushing must be an extremely tight spec and so will need to be the dim for the edge breaks.

The potential problem that I see with the Whidden tool isn't one of their making. It is that it is possible to use either end of the bushing in question, and the transitions from bore to end on all of mine are rather inconsistent. Since that determines where the actual end of the bore starts it will introduce some variability in the measurements from one session to another. I suggest that the end used on each bushing be marked so that it is always the end used.

Redding suggests Bushings be mounted with their engraved dimension facing downward in the Die; that is, toward the shell holder. This could be a basis for consistency.

 

[ntsqd]

Aren't all Whidden bushings marked for reference? Their neck bushings are.

Marked for size likely, marked for which side to use? No idea.

Redding suggests Bushings be mounted with their engraved dimension facing downward in the Die; that is, toward the shell holder. This could be a basis for consistency.

That will work!

BUT, to my original point that is only good within your set of tools. For instance say you have a rifle built and the builder finds some recommended load(s) for it. Their CBTO number(s) will mean nothing to your tools. If a standard is developed then their numbers will mean something to your tools because they're all conforming to that standard. OR say that your reloading skills are now up to speed and you want to take over the loading that a friend has been doing. Same thing, his tools are different and his CBTO numbers are useless to you. In both cases you'll need to measure with your tools and record your measurements. It's not the end of the world to do this, but wouldn't be beneficial if you didn't have to? Imagine a world where no one's scale weighs the same. This is a similar thing.