Calculating CBTO

[HighnDry]

My question is how do I recalculate the CBTO for my case after using the Hornaday OAL gauge?
Obviously, the modified Hornady case is not going to have the same dimensions as my fired case. Do you just add or subtract off the headspace on both cases?

 

[SDHNTR]

The cbto length is the cbto length. CBTO has nothing to do with headspace.

 

[HighnDry]

I understand, but how do you adjust CBTO length for your cartridge if you are using the generic Hornady case?

The reason I mention headspace, is that I read somewhere that you can use the Hornady headspace gauge on the two cartridges to give you an idea on how much the cartridges are off.

 

[Ram94]

Yes, you would add or subtract the difference in base to shoulder length between a fired case and your modified case. For example; If your modified case is shorter, you would add the difference once you get your measurement to calculate where your lands are on a fired case.

This is where making a modified case using an actual fired round comes in handy. Though not everyone has the tools to do so.

 

[HighnDry]

Got it- I think.
So the Datum line to the Ogive is constant in both the actual and modified case, so you take the difference from the base to the Datum line,

 

[Ram94]

Correct!

 

[sndmn11]

This is where making a modified case using an actual fired round comes in handy. Though not everyone has the tools to do so.

If a person were FL sizing their brass to reload, could they take the modified case and FL size it, then ream out the mouth so a bullet fits in? Would that be an apples to apples comparison?

 

[Ram94]

If a person were FL sizing their brass to reload, could they take the modified case and FL size it, then ream out the mouth so a bullet fits in? Would that be an apples to apples comparison?

In theory yes. My only concern would be overworking the brass every time it’s sized to get it down to wherever the mod case is if it started out shorter than fired brass.

But it really isn’t tough to do the calculation once you get your head wrapped around it.

 

[SDHNTR]

Ah, I see. Yes, I would just resize the modified case to your required headspace per your chamber, and then stretch out the neck. Or just use a body die. You're only doing it once, it shouldn't over work the brass.

 

[bsnedeker]

Man, threads like this always make me wonder why people use that piece of garbage when working up a load. It's SO much more accurate to just do this with an actual cartridge than use that stupid gauge. I played around with that thing when I first got into reloading and there is just so much room for error in that thing it is totally worthless in my opinion.

1: Remove firing pin and ejector from your bolt so your bolt will drop with zero effort...just gravity.

2. Figure out headspace: Take a fired piece of brass, put it in your press and adjust the die so it just BARELY pushes in the shoulders. See if this case allows the bolt handle to drop (it won't), adjust die a tiny bit to push shoulders in a bit further, size the case, put in chamber....lather, rinse, repeat until bolt drops with no resistance. You now know EXACTLY what your headspace is in your chamber and you have your die adjusted perfectly for your rifle.

3. Figure out your CBTO. Do the same as above, just with your seating die. Put the bullet in and seat it just a bit, see if you can drop the handle (you won't), adjust die to push bullet in a bit further, lather, rinse, repeat until the bolt drops. Measure CBTO. You now know your EXACT distance to the lands. Adjust your the die to push the bullet in to whatever distance off the lands you want, you are done.

4. Reassemble bolt. You are now done.

 

[ID_Matt]

Some of the above methods definitely will work. I really think it is a bit overkill. As long as you have a baseline to start from it shouldn't matter if it is off a few thousands of an inch unless you are actually starting your seating depth that close to the lands. My gunsmith will make me modified cases out of fired rounds but sometimes I don't even bother and just use the hornady case if that is what I have on hand because I never seat closer than .010 in a hunting rig.

 

[HighnDry]

Very good information.
Thanks all!

 

[Quant]

Accuracy One makes a seating depth comparator that measures from the bullet ogive to the shoulder vs the base of the case. It’s faster than using the caliper type comparators, and you don’t have to worry about the modified case being a different length than your fired/sized brass. Obviously, it doesn’t work for belted magnums.

 

[Hschweers]

Going to resurrect this old(ish) thread since it’s relatively on topic to my question. It actually opened up a lot more questions.

I don’t understand why headspace, shoulder bumping, etc are mentioned with CBTO? Please enlighten me.

CBTO is only measuring the distance from the base of the brass to the ogive of your bullet when it contacts the rifling, no? You could, in theory, have a case with a .1” case body, a 2.25” case neck, and get the same CBTO measurement as a properly proportioned case, right? All in theory, obviously that wouldn’t work in a chamber for a multitude of reasons.

I was under the impression that CBTO would be an accurate way to measure cartridge base to rifling regardless of bullet make/weight/design/etc. Your CBTO will be the same in that chamber. Why do the brass shape/measurements matter?

 

[Vern400]

Man, threads like this always make me wonder why people use that piece of garbage when working up a load. It's SO much more accurate to just do this with an actual cartridge than use that stupid gauge. I played around with that thing when I first got into reloading and there is just so much room for error in that thing it is totally worthless in my opinion.

1: Remove firing pin and ejector from your bolt so your bolt will drop with zero effort...just gravity.

2. Figure out headspace: Take a fired piece of brass, put it in your press and adjust the die so it just BARELY pushes in the shoulders. See if this case allows the bolt handle to drop (it won't), adjust die a tiny bit to push shoulders in a bit further, size the case, put in chamber....lather, rinse, repeat until bolt drops with no resistance. You now know EXACTLY what your headspace is in your chamber and you have your die adjusted perfectly for your rifle.

3. Figure out your CBTO. Do the same as above, just with your seating die. Put the bullet in and seat it just a bit, see if you can drop the handle (you won't), adjust die to push bullet in a bit further, lather, rinse, repeat until the bolt drops. Measure CBTO. You now know your EXACT distance to the lands. Adjust your the die to push the bullet in to whatever distance off the lands you want, you are done.

4. Reassemble bolt. You are now done.

I like your idea, and use it on headspace / die setting, haven't done it to find the bullet to land contact yet. I'm gonna try that. The fit of the cartridge to the chamber will never, ever lie. I like that.

 

[bmart2622]

Seems like there is the potential for error doing it that way as well. At the end of the day use whatever works and shoots well, if it shoots to your liking then it doesn't really matter if the bullet is seated .030 or .032

 

[Gila]

Going to resurrect this old(ish) thread since it’s relatively on topic to my question. It actually opened up a lot more questions.

I don’t understand why headspace, shoulder bumping, etc are mentioned with CBTO? Please enlighten me.

CBTO is only measuring the distance from the base of the brass to the ogive of your bullet when it contacts the rifling, no? You could, in theory, have a case with a .1” case body, a 2.25” case neck, and get the same CBTO measurement as a properly proportioned case, right? All in theory, obviously that wouldn’t work in a chamber for a multitude of reasons.

I was under the impression that CBTO would be an accurate way to measure cartridge base to rifling regardless of bullet make/weight/design/etc. Your CBTO will be the same in that chamber. Why do the brass shape/measurements matter?

As long as your brass is resized so it fits the chamber properly, yup it’s do it once until you get another barrel or use another rifle with a different barrel. If you want to play around with seating depth with different bullets then you just subtract from the CBTO which is measured out to lands contact.

 

[bmart2622]

It will change as the throat erodes

 

[Hschweers]

It will change as the throat erodes

Ah, haven’t considered this. For a 6.5 PRC with expected barrel life around 1,000 rounds, how quickly do you think it would change? Every 100 rounds? 200? 500?

 

[bmart2622]

Depends on the powder and powder charge but again Im not sure how drastic of an effect it will have on dispersion