CBTO question

Tex S · August 22, 2021, 11:45 PM

Hi guys!

Cartridge is .223 Rem

I have the Hornady OAL gauge. Been a really long time since I have used it, and I can't remember how the math works out. Here is what I have...

The Hornady modified case measures 1.458 at the shoulder datum line.

My sized brass measures 1.455 at the shoulder datum line.

Obviously this means my sized brass is .003 shorter.

I would like to be .010 off the lands with a particular bullet, and when I touch the lands using this bullet I get a reading of 1.886 when the bullet is inserted into the Hornady modified case. Conventional wisdom says that I could load a round at 1.876 and be .010 off, but what about the 3 thou difference in my brass and the Hornady case? Would I add it to, or subtract it from the 1.876 number?

I'm thinking add it, and seat the bullet to where it measures 1.879.

Am I right?

Thanks!

nhyrum · August 23, 2021, 03:08 AM

When you're measuring with the bullet, are you measuring the overall length? Or using a comparator to measure to the ogive?

Either way, I set everything off what I measure using the modified case. Are you using a comparator to measure your shoulder bump when sizing? It could be you are over sizing your brass

Sent from my SM-G996U using Tapatalk

jetinteriorguy · August 23, 2021, 05:33 AM

The difference between the case lengths has nothing to do with it, only the setting on the bullet seating die determines the CBTO.

Tex S · August 23, 2021, 07:22 AM

Using a comparator to measure the ogive...

**Unclenick** · August 23, 2021, 09:31 AM

Quote:

I'm thinking add it, and seat the bullet to where it measures 1.879.

It's the other way around. You want your CBTO 0.003" shorter (1.873") with a shorter case head-to-shoulder dimension. When a rimless bottleneck cartridge is fired, the blow of the firing pin drives it forward until the case shoulder comes to a stop on the chamber shoulder. Your shorter case will go 0.003" further forward with the shorter head-to-shoulder dimension, putting the bullet 0.003" closer to the throat.

This all happens because, as shown in one of the Courtneys' papers, there is a delay of a couple of milliseconds after the primer is struck and before the pressure gets up enough to start moving the bullet. 0.003 inches is 0.00025 feet. Divided by 0.002 seconds of delay gives you 0.125 feet/second or 1.5 inches/second, so that's all the speed the cartridge needs to pick up from the firing pin to move it 0.003" forward while the powder is lighting up. It goes faster than that. Plus, the primer's gas backs it out of the primer pocket (unless you have strongly crimped primers) like a little piston and traps the case in that forward position while firing completes. This is all why we get pressure ring stretching in rimless bottleneck rifle cartridges; the case is forward in the chamber when the pressure sticks the case walls to the chamber, so the stretching has to be at the pressure ring to let the head move back into contact with the breech face. If cases stayed back in the gun during firing, the shoulder would blow forward instead.

cdoc42 · August 23, 2021, 09:36 AM

I agree with jetinteriorguy. You use the comparator to measure from the base of the case to the area called the ogive of the bullet to determine the "jump" the bullet makes to the rifling. A difference in the overall length of the case from base to mouth will reflect a different seating depth within the cases. It has nothing to do with the bullet jump.

Measuring the case base to the shoulder datum line tells you how much you resized the case from the fired chamber expansion.

**Unclenick** · August 23, 2021, 10:58 AM

Here's an exaggerated illustration to show what I am referring to. Unlike the rimmed and belted cases which have stops for the cartridge case ahead of the case shoulder meeting the chamber shoulder, a rimless bottleneck cartridge like the 223 Remington has its bullet jump determined by the distance from the shoulder datum to the bullet ogive's point of contact with the throat.

Tex S · August 23, 2021, 11:19 AM

Thanks, Nick.

I am going to create a spreadsheet where I can plug these numbers in for future reference so I don't forget again later down the road.

RC20 · August 23, 2021, 04:19 PM

For your case, what you want to use for a reference is an average of 5 FIRED cases on the shoulder. That tells you what your max chamber is.

Then up to you if you want to do a minimum bump back to .003 to .010 (I go for .004-005 as that accommodates some variant. There is one school of thought that says snug in chamber, slightly hard to close a bolt (if its a bolt gun). I don't agree.

Gas guns are an issue unto themselves and I make no statement in regards to them.

Pretty much then find the lands, set it back to whatever suits you to start, go from there. Any miner variation in shoulder then makes no real difference.

I log all my Ogive and as all bullets have different shapes, each is tested individually.

The Hornady case is to spec and good for close but its not the chamber cut in your gun.

cdoc42 · August 23, 2021, 05:44 PM

Unclenick, your fantastic explanation is the first time in 44 years that I have heard it.

Do I have this right? I measured the base to shoulder datum line on 6 fired cases of my .270. They measured 2.041". When I resized them, they measured 2.034". If I seat the bullet 0.015" from the rifling/leade, does that mean the jump is actually 0.007" closer than I think?

**Unclenick** · August 23, 2021, 07:06 PM

Did you determine the zero seating depth with a 2.041" case? If so, then yes. If you determined it with a 2.034" case, then no, you have it right. The OP used a Hornady gauge adapter. If you did the same, then how your 2.034" case compares to the adapter determines the jump adjustment.

Incidentally, if you are using the Hornady gauge, they offer the service of turning one of your cases into a gauge adapter. You can send them one that you resized the way you like to resize them and then use it with the gauge to get an exact location for the bullet with your case resizing practice. You can also drill them out and tap them yourself, which I have done, but found it a trial as the brass wants to be grabbed by the drill. You need a 7.3 mm tap drill and a 5/16-36 tap, IIRC.

cdoc42 · August 24, 2021, 11:26 AM

Unclenick, is there a difference between what you and RC20 said?

YOU: "Did you determine the zero seating depth with a 2.041" case? If so, then yes.(the bullet jump WILL be closer) If you determined it with a 2.034" case, then no" (It won't be closer)

RC20:"For your case, what you want to use for a reference is an average of 5 FIRED cases on the shoulder. That tells you what your max chamber is.

Then up to you if you want to do a minimum bump back to .003 to .010

Pretty much then find the lands, set it back to whatever suits you to start, go from there. Any miner variation in shoulder then makes no real difference"

Bottom line question: should the dummy round be seated in a fired or resized case?

Metal god · August 24, 2021, 04:05 PM

Never mind not feeling it right now haha

Tex S · August 24, 2021, 05:41 PM

Thanks for the help with my CBTO question. I have made a simple spreadsheet to calculate this data. I have attached a screenshot here. Does it look correct?

Bart B. · August 25, 2021, 12:35 PM

Remember the bullet's touch point on the throat rifling erodes further down the barrel about .001 inch every couple dozen shots.

Some competitors soft seat bullets so they get pushed back a few thousandths when chambered and fired. This is for single round loading only. It also aligns the bullet to the bore axis before firing.

August 22, 2021, 11:45 PM	#1
Tex S Senior Member Join Date: November 12, 2008 Location: Fort Worth, TEXAS Posts: 909	CBTO question Hi guys! Cartridge is .223 Rem I have the Hornady OAL gauge. Been a really long time since I have used it, and I can't remember how the math works out. Here is what I have... The Hornady modified case measures 1.458 at the shoulder datum line. My sized brass measures 1.455 at the shoulder datum line. Obviously this means my sized brass is .003 shorter. I would like to be .010 off the lands with a particular bullet, and when I touch the lands using this bullet I get a reading of 1.886 when the bullet is inserted into the Hornady modified case. Conventional wisdom says that I could load a round at 1.876 and be .010 off, but what about the 3 thou difference in my brass and the Hornady case? Would I add it to, or subtract it from the 1.876 number? I'm thinking add it, and seat the bullet to where it measures 1.879. Am I right? Thanks! Last edited by Tex S; August 23, 2021 at 07:22 AM.

August 23, 2021, 04:19 PM	#9
RC20 Senior Member Join Date: April 10, 2008 Location: Alaska Posts: 7,014	For your case, what you want to use for a reference is an average of 5 FIRED cases on the shoulder. That tells you what your max chamber is. Then up to you if you want to do a minimum bump back to .003 to .010 (I go for .004-005 as that accommodates some variant. There is one school of thought that says snug in chamber, slightly hard to close a bolt (if its a bolt gun). I don't agree. Gas guns are an issue unto themselves and I make no statement in regards to them. Pretty much then find the lands, set it back to whatever suits you to start, go from there. Any miner variation in shoulder then makes no real difference. I log all my Ogive and as all bullets have different shapes, each is tested individually. The Hornady case is to spec and good for close but its not the chamber cut in your gun. __________________ Science and Facts are True whether you believe it or not

August 23, 2021, 07:06 PM	#11
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,063	Did you determine the zero seating depth with a 2.041" case? If so, then yes. If you determined it with a 2.034" case, then no, you have it right. The OP used a Hornady gauge adapter. If you did the same, then how your 2.034" case compares to the adapter determines the jump adjustment. Incidentally, if you are using the Hornady gauge, they offer the service of turning one of your cases into a gauge adapter. You can send them one that you resized the way you like to resize them and then use it with the gauge to get an exact location for the bullet with your case resizing practice. You can also drill them out and tap them yourself, which I have done, but found it a trial as the brass wants to be grabbed by the drill. You need a 7.3 mm tap drill and a 5/16-36 tap, IIRC. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

August 24, 2021, 04:05 PM	#13
Metal god Senior Member Join Date: April 10, 2012 Location: San Diego CA Posts: 6,876	Never mind not feeling it right now haha __________________ If Jesus had a gun , he'd probably still be alive ! I almost always write my posts regardless of content in a jovial manor and intent . If that's not how you took it , please try again .

August 23, 2021, 03:08 AM	#2
nhyrum Senior Member Join Date: March 9, 2016 Posts: 588	When you're measuring with the bullet, are you measuring the overall length? Or using a comparator to measure to the ogive? Either way, I set everything off what I measure using the modified case. Are you using a comparator to measure your shoulder bump when sizing? It could be you are over sizing your brass Sent from my SM-G996U using Tapatalk

August 23, 2021, 05:33 AM	#3
jetinteriorguy Senior Member Join Date: April 28, 2013 Posts: 3,176	The difference between the case lengths has nothing to do with it, only the setting on the bullet seating die determines the CBTO.

August 23, 2021, 07:22 AM	#4
Tex S Senior Member Join Date: November 12, 2008 Location: Fort Worth, TEXAS Posts: 909	Using a comparator to measure the ogive...

August 23, 2021, 09:36 AM	#6
cdoc42 Senior Member Join Date: March 13, 2005 Location: Pennsylvania Posts: 1,687	I agree with jetinteriorguy. You use the comparator to measure from the base of the case to the area called the ogive of the bullet to determine the "jump" the bullet makes to the rifling. A difference in the overall length of the case from base to mouth will reflect a different seating depth within the cases. It has nothing to do with the bullet jump. Measuring the case base to the shoulder datum line tells you how much you resized the case from the fired chamber expansion.

August 23, 2021, 11:19 AM	#8
Tex S Senior Member Join Date: November 12, 2008 Location: Fort Worth, TEXAS Posts: 909	Thanks, Nick. I am going to create a spreadsheet where I can plug these numbers in for future reference so I don't forget again later down the road.

August 23, 2021, 05:44 PM	#10
cdoc42 Senior Member Join Date: March 13, 2005 Location: Pennsylvania Posts: 1,687	Unclenick, your fantastic explanation is the first time in 44 years that I have heard it. Do I have this right? I measured the base to shoulder datum line on 6 fired cases of my .270. They measured 2.041". When I resized them, they measured 2.034". If I seat the bullet 0.015" from the rifling/leade, does that mean the jump is actually 0.007" closer than I think?

August 24, 2021, 11:26 AM	#12
cdoc42 Senior Member Join Date: March 13, 2005 Location: Pennsylvania Posts: 1,687	Unclenick, is there a difference between what you and RC20 said? YOU: "Did you determine the zero seating depth with a 2.041" case? If so, then yes.(the bullet jump WILL be closer) If you determined it with a 2.034" case, then no" (It won't be closer) RC20:"For your case, what you want to use for a reference is an average of 5 FIRED cases on the shoulder. That tells you what your max chamber is. Then up to you if you want to do a minimum bump back to .003 to .010 Pretty much then find the lands, set it back to whatever suits you to start, go from there. Any miner variation in shoulder then makes no real difference" Bottom line question: should the dummy round be seated in a fired or resized case?

August 25, 2021, 12:35 PM	#15
Bart B. Senior Member Join Date: February 15, 2009 Posts: 8,927	Remember the bullet's touch point on the throat rifling erodes further down the barrel about .001 inch every couple dozen shots. Some competitors soft seat bullets so they get pushed back a few thousandths when chambered and fired. This is for single round loading only. It also aligns the bullet to the bore axis before firing.