Measuring CBTO - What is wrong with my approach?

[asd9055]

There you go again, making things too simple and not selling any new tools....whats a matter you?
In all seriousness...It works...I used the second method before...first 1 is new to me...but I like it
Thanks for sharing...

 

[epoletna]

After reading that long article from Precision Rifle Blog, I will propose a perhaps radical approach to seating depth.

It is this: perhaps we don't need to measure the CBTO precisely to start.

We know a general seating depth for the caliber in question from published sources and measuring commercial rounds. And the PRB article makes it sound as if deeper seating depth generally is better, so why not start from the published data and seat shorter and shorter until we find the best ES? Of course this will vary from rifle to rifle, but for load development in an individual rifle this would seem like an acceptable way to start establishing seating depth in a specific cartridge for a specific rifle/barrel.

If I read the PRB article correctly, shorter (deeper) seating is generally better up to the point where the bearing surface goes below the case neck junction. So does it really matter where the rifling starts, or where the bullet meets the lands? As long as we start short of that point and continue going shorter until we find the best ES, we should be able to establish the best seating depth for that rifle/cartridge combination. If we load for more than one rifle in a specific caliber, we would want to repeat this for each rifle, but does it really matter where the lands start if we are looking to find a starting point for seating depth?

I think I am going to start seating shorter to see if that changes ES.

 

[Mark37082]

I came away with similar thoughts from reading this article. I will still want to know where to start, but I will be increasing the jump from a near jam to .01 to start with.

 

[epoletna]

Where to start? Why not start with published COAL and seat deeper and deeper from that point? Do you really need to know where the lands start? Really? Especially if they erode over time, in which case all your carefully measured data from when the barrel was new goes out the window?

 

[Mark37082]

One reason is the significant amount of variance in the bullets I am testing. I am seeing .12 variance in COAL when measuring and setting the same CBTO for different bullets. I'm not sure how much difference this will make, but it seems to be somewhat material to me.

 

[DoneNOut]

 

[esshup]

The only problem that I see with using a case to hold the bullet is if the bullet is jammed into the lands and is held by the lands for a couple of thou or more as the case is extracted from the chamber then the bullet releases, the measurement won't be consistent.

A person cannot use COAL to measure bullet jump to the lands because the tips of the bullet aren't exactly the same and the seating die doesn't push on the bullet tip, it pushes on the ogive of the bullet.

In regards to Eric's video, it's just a matter of semantics. i.e., tomato, tomatoe. He IS chasing the lands in my way of thinking. I define the lands as where the ogive of the bullet touches them, which he calls jam. As the lands erode they move further away from the bullet so he chases them by seating the bullet deeper into the chamber as they wear.

Now if the magazine length is short and the bullet cannot be seated as long as required to get it close to the lands to find the node, then a person just has to seat them to max length that will fit in the magazine and shorten them up .003" at a time until they find an accuracy node that is two bullet seating depths wide. BUT the reloader still has to measure from the base to the ogive, not the tip because the measurement from the ogive to the tip isn't consistent.

At least that's the way I see it. Please convince me otherwise if I am wrong.

Basically you are listening to what your rifle barrel is telling you by looking at the group size. If the group does not open up, then nothing needs to be done. If your group opens up, then the rifle is telling you something and you need to listen.

 

[asd9055]

I am sure I will get a lot of hate here....but the truth of the matter...not too long ago...a lot of people now saying chasing the lands is stupid....they were actually chasing the lands...if you research it, you will find articles by them.
The reality is, back around the end of WWII, a wildcutter named Roy Weatherby developed his rifles with a lot of freebore...(bullet jump" in todays lingo. So many people "diss" it....but that is what they are doing now. Freebore, only they call it bullet hump...
Now Mark Gordon (Great guy) might have been the first in recent history to walk away form chasing the lands...if I am wrong, please correct me, ...If I understood him correctly...he chases the "most forgiving" jump, because while you are shooting in a competition, your rifle's throat erodes, and the "actual" bullet jump changes, so you need to be in the "most forgiving" section, not the most accurate, because it will change.
Now even bullet manufurers like Berger recomend a bullet jump...
Quick summary...first came OCW, then OBT, then the BOSS system, we call them tunerswith brake now, then the various evolutions of ladder test...I am not sure when chasing the lands came...but that was back there somewhere...
Now it is seating depth...it works, I am not nocking it down....just one comment....when you seat the bullet deeper, you changing pressure and jump....not just jump...
I play with all...BOSS, tuners, ocw, obt, jump, ladders....I use what works for me...
There is a lot of information out there....you decided what works for YOU!

 

[dragon798]

To get the most accurate measurement in your rifle, you need to remove the extractor and ejector off your bolt, or your measurement will be "off" by whatever the base to shoulder difference is between your chamber and the cartridge you are using. The ejector will push it in all the way otherwise.
When I first started down this rabbit hole, I left the extractor and ejector installed. Someone learned me on the extractor/ejector. I then used a dowel to push the bullet out (pushing the bullet into the case). Now I have a modified "hook" to extract the case once the bolt is pulled back. A paint can opener can work for this if you're careful.

Or, just use cerrosafe and make a chamber cast. It melts at about 175°F and holds exact shape for about 20 minutes (or longer) after you pull it out. You can search for instructions on that process.

So I'm curious as to how removing the ejector and extractor would help to get a more accurate measurement? Unless that's the way you fire the rifle all the time. Seems to me you want the ejector to to push the cartridge in and maintain the headspace, thereby giving you a more accurate CBTO reading. It also seems to me that, removing the ejector could cause serious issues if you want to load into the lands, as replacing the ejector would then cause the cartridge to be forced 3 or 4 thousandths further forward. Thus moving the bullet the same amount further into the lands. I might be completely off base here and if so, please explain how. Ultimately I would like to say, I use the OP's method, basically, and have realized that this is a reference measurement. It is not a number that can be used from bullet to bullet, or gun to gun or even from one person to the next. It is a reference you use for a particular bullet in a particular load. If any part of the process, tools or components changes, so does the number. Therefore, do it the way you find best and stick to it.

 

[asd9055]

So I'm curious as to how removing the ejector and extractor would help to get a more accurate measurement? Unless that's the way you fire the rifle all the time. Seems to me you want the ejector to to push the cartridge in and maintain the headspace, thereby giving you a more accurate CBTO reading. It also seems to me that, removing the ejector could cause serious issues if you want to load into the lands, as replacing the ejector would then cause the cartridge to be forced 3 or 4 thousandths further forward. Thus moving the bullet the same amount further into the lands. I might be completely off base here and if so, please explain how. Ultimately I would like to say, I use the OP's method, basically, and have realized that this is a reference measurement. It is not a number that can be used from bullet to bullet, or gun to gun or even from one person to the next. It is a reference you use for a particular bullet in a particular load. If any part of the process, tools or components changes, so does the number. Therefore, do it the way you find best and stick to it.

The theory is that that force applied by the ejector is enough to be sitting the bullet into the lands...you want it to be just touching them. And want to be able to feel the bolt "just dropping".
I personally have never removed the firing pin and ejector...I did it either with the cleaning rod and Frakford Arsenal tools, the modified case tool or the marker, bullet and fired case....those are the three ways I used....

 

[esshup]

After thinking about this for a little bit, the reason why the Hornady tool and the modified case works for me is because I size my brass to each individual gun, and I size my brass so the last 1/4 (or less) of bolt "turn" has a slight resistance to it, meaning that the shoulder of the case is touching the front part of the chamber. I can see where if the case is fully sized and there is a little bit of slop in the chamber the ejector will push the case forward and when the powder ignites; then the case moves backwards against the bolt face before the brass expands, grabbing onto the cylinder wall and holding it in place, therefore actually moving the bullet back off the lands/grooves more than what the measurement was when using the bolt to hold the shell in place to get the measurement. If the ejector was left in the bolt.

If the case is sized the way I do it then it shouldn't matter a bit.

Same for belted cases, correct? If the Hornady tool is used, you are pushing the case forward so the headspace is on the belt. and then if you have a wee bit of resistance when closing the bolt the belt is fully forward and all the slack is taken out of the equation.

 

[asd9055]

After thinking about this for a little bit, the reason why the Hornady tool and the modified case works for me is because I size my brass to each individual gun, and I size my brass so the last 1/4 (or less) of bolt "turn" has a slight resistance to it, meaning that the shoulder of the case is touching the front part of the chamber. I can see where if the case is fully sized and there is a little bit of slop in the chamber the ejector will push the case forward and when the powder ignites; then the case moves backwards against the bolt face before the brass expands, grabbing onto the cylinder wall and holding it in place, therefore actually moving the bullet back off the lands/grooves more than what the measurement was when using the bolt to hold the shell in place to get the measurement. If the ejector was left in the bolt.

If the case is sized the way I do it then it shouldn't matter a bit.

Same for belted cases, correct? If the Hornady tool is used, you are pushing the case forward so the headspace is on the belt. and then if you have a wee bit of resistance when closing the bolt the belt is fully forward and all the slack is taken out of the equation.

Since I have never done the bolt method, I could no comment on belted cases...which I use a lot...But I understand what you are saying.

 

[QuietTexan]

It is this: perhaps we don't need to measure the CBTO precisely to start.

I've been saying this for years. It doesn't matter where you start at all, some people say "oh well you have to work backwards only" or "I know where the lands are this way" but it literally doesn't matter at all because as long as the cases you're loading have consistent BTOs then you can adjust that BTO to be whatever you want. The absolute distance from the ogive to the lands is meaningless vis a vis the results on the target of a particular BTO. Trying to work in one direction or starting long and working short are process decisions that help people be more systemic in approaching how they test changes in BTO, but the order you test or load in makes no impact on the results - the actual node location for seating depth. If a process helps someone get their depth and confirm faster, that's great, but there is nothing about the process itself that actually influences the results.

Where to start? Why not start with published COAL and seat deeper and deeper from that point? Do you really need to know where the lands start? Really? Especially if they erode over time, in which case all your carefully measured data from when the barrel was new goes out the window?

There are a couple of other concepts here though - when you seat the bullet you have to consider the location of the base relative to the bottom of the neck, and the associated impacts on case volume and neck grip (and donuts). You have to balance multiple variables, jump is just one of them. Starting at book or SAAMI COAL might work fine and is ultimately just as arbitrary as starting 0.020" off the lands, but starting that short will likely get you to the point you seat the bullet too deep a lot sooner than if you start longer. If you're starting there you'd be better of working longer and not shorter IMO. You can seat a long bullet to where the ogive is behind the mouth sticking to SAAMI specs - try loading a 245 Berger into a 300 RUM and keep it to 3.600" inches and see what it looks like. The base of the bullet is more past the neck than above it, and you're loosing a lot of case capacity for slower, larger kernel powders. More modern designed cartridges are built to seat bullets longer so won't be as impacted, but there are a lot of older cartridges that are still running very well (300 WM and 243 WIN) with longer seated, longer bullets.

 

[DoneNOut]

So I'm curious as to how removing the ejector and extractor would help to get a more accurate measurement? Unless that's the way you fire the rifle all the time. Seems to me you want the ejector to to push the cartridge in and maintain the headspace, thereby giving you a more accurate CBTO reading. It also seems to me that, removing the ejector could cause serious issues if you want to load into the lands, as replacing the ejector would then cause the cartridge to be forced 3 or 4 thousandths further forward. Thus moving the bullet the same amount further into the lands. I might be completely off base here and if so, please explain how. Ultimately I would like to say, I use the OP's method, basically, and have realized that this is a reference measurement. It is not a number that can be used from bullet to bullet, or gun to gun or even from one person to the next. It is a reference you use for a particular bullet in a particular load. If any part of the process, tools or components changes, so does the number. Therefore, do it the way you find best and stick to it.

I'm right with you on this.