Measuring CBTO - What is wrong with my approach?

[Mark37082]

There are some very interesting views on this topic. In the end, I've learned something. That is what was looking for in the beginning. When I first started I just did everything to Saami specs. At least I have progressed to the point where I can be more confident in my processes. I'm going to stop chasing the lands, measure BTO, measure the maximum distance to the lands for safety reasons, and seat each particular bullet to a consistent depth. Shoot out the barrel and start over.

 

[asd9055]

There are some very interesting views on this topic. In the end, I've learned something. That is what was looking for in the beginning. When I first started I just did everything to Saami specs. At least I have progressed to the point where I can be more confident in my processes. I'm going to stop chasing the lands, measure BTO, measure the maximum distance to the lands for safety reasons, and seat each particular bullet to a consistent depth. Shoot out the barrel and start over.

Each rifle/bullet likes a different setting (distance), but as long as you are happy with what you get, that is all that counts....none of what we say here counts...

 

[MagnumManiac]

So I may be missing something or maybe I am too cheap and lazy to buy another tool, but I am sacrificing one piece of brass for each of my rifles in order to measure CBTO. I use a Dremel to cut two slots in the neck of the brass and can easily adjust the tension on the bullets. Simply place the bullet in the brass with a shallow seat and carefully load into the action. I close the bolt and even more carefully remove the cartridge. Measure the CBTO with a Hornady Lock N Load bullets comparator several times after repeating the preceding steps to come up with a consistent measurement to the lands. I then seat that particular bullet .002 shorter than the distance measured to the lands.
When I am reaching the point of load development where I want to tweak the bullet seating depth, I just increase the jump .005 at a time and shoot a 3 shot group to determine optimum seating depth.
Thoughts/Suggestions?

Exactly how it should be done.
If you buy 50 cases of Remington brass, there is always 2 extra pieces in every bag.
So……your split cases end up free!
The only case I have I am not willing to split cases on is my 6.5x47 Lapua brass, just too hecken expensive for that.
For that, I use a modified expander that allows just enough drag to allow the case to slide and stay put after touch.

Cheers.

 

[jrstraw]

You took the first step, That is just the begining, Like Mike said...and QT...

There are a couple of competing camps. I suggest you do some searching and reading.
Here is something worth reading

Bullet Jump Research: Executive Summary & Load Development Tips

This is the last post in a series of articles focused on bullet jump research that has been conduct...

precisionrifleblog.com

Berger has some guidlines as well...

Good article— I have the Hornady tool for all my calibers but have trouble getting repeat numbers from it— a friend suggested the Sinclair Gauge — I have to read the instructions every time I use it but it is repeatable consistently— It looks like it is doing the same thing the video showed in the article with out removing the barrel—https://www.sinclairintl.com/reload...lair-bullet-seating-depth-tool-prod35491.aspx

 

[P7M13]

All the hooplah about touching and this and that and the other doesn't matter. Every measurement in this process is relative to some other dimension - even if you cast the chamber you're still at the mercy of the accuracy of your tools to measure the actual caliber diameter ogive and the actual datum line on the shoulder and so on and so on, where do you pick where to stop and be reasonable? A few thousandths here or there won't add up to anything that makes a difference if you're shooting for a wide seating depth node for practical use.

I feel a Zen moment coming upon me.... "Use the Force, Luke."

The thing I actually take the most issue with in your method is shooting 3-shot groups. Any string less than 10 shots (but preferably 20 or more) doesn't provide inferential results. It provides negative confirmation that the group won't get any smaller than what the shots did, but can very certainly get larger. So if the first two shots are further apart than you want, stop wasting the third shot and pull the bullet when you get home. Or...

Good point.

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.

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....

Push the cartridge in to maintain the headspace only applies if the cartridge was already fired in your gun.
The ejector spring provides anywhere between 5 and 13lbs of force (estimate). The extractor needs to fit over the rim, and on some rifles, that can take a fair amount of force to snap it over the rim with the cartridge in the chamber. Either one can cause grief if your cartridge is not the right size. If your case is shorter than your chamber, that difference is your error. A problem may arise when your case is shorter than the chamber, because the majority of the stretch is at the case head, and can lead to premature rupture.

Same goes for fire forming cases.

The Hornady Modified Case gauge provides a case that is within SAAMI spec.
I have had rifles that have chambers longer than spec, and others shorter than, or at minimum SAAMI spec.

I have never found a bullet loaded to the lands to be the most accurate.
Disclaimer - I don't compete and don't consider myself a marksman.

 

[SSgt G Cody]

On most non-magnums, start from bullet jammed lightly into lands, then reduce OAL .020". On magnums reduce OAL .030". Then fine tune for best accuracy load, but final solution is usually pretty close to this! Never go less than .010", for this can cause extremely high pressures!

 

[MagnumManiac]

Not a single comp rifle of mine have an ejector fitted….it just simply isn't conducive to accuracy and consistency and isn't required.
If I put them back in, accuracy is not the same across all brass that it was with previously.
I'm not going to get into the argument over how a cone self centres and this overcomes the sideways push of the ejector…..not it doesn't.
The only way to have a case centred with an ejector is to have 2 opposite each other, which is being done by some rifle makers as we speak.
I prefer not to have them installed, the targets speak volumes.

Cheers.

 

[nksmfamjp]

Never go less than .010", for this can cause extremely high pressures!

Yes, but no. This statement is correct. Using Quick Load, I see that start pressure touching the lands increases from 3600 psi to about 11000 psi. This change should cause you reduced powder charges to keep pressures safe. Looking at the chart below, I manipulate the start pressure some jammed through 0.050".

https://www.shootersforum.com/attachments/seating-depth-v-pressure-gif.90780/

 

[QuietTexan]

You can jam the lands hard if you start at a reasonable load and work up.

It's also the basis for a fireforming method - pull ejector spring, seat the bullet hard jammed, let it rip with an almost full load - seemed like it used to be pretty common when 6 Dasher brass wasn't commercially available. I'd probably use COW method for a completely moved shoulder like that, but if you get an AI chamber set up to slightly crush parent cartridge brass you can fireform with standard loads just fine.

Not a single comp rifle of mine have an ejector fitted….

I agree. I got the Sinclair tool and it's maybe a five minute process to remove or reinstall ejector springs on any given rifle. I don't see a reason to leave it in for a comp rifle - even if the only thing it did was keep my brass from rolling around all over the world it's worth it just for that.

I'll admit though, I really have no idea if it doesn't anything else. It not being there for sure isn't the reason I'm missing

 

[SealT5]

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.

@Alex Wheeler

 

[philipbrousseau]

I have started taking a fully fire formed case that will hold a bullet but will let it slip easily and put a small bead of super glue half way down the inside of the neck and push the case and bullet into the chamber keeping the barrel pointed up to eliminate the possibility of getting glue in the chamber. When I remove the case I have a solid dummy to measure with my comparator.

 

[Alex Wheeler]

I have a video on my website for how I find touch, its a very common method used in the competition world. Still to this day I have not found a more accurate or repeatable method. You have to remove the ejector to get the pressure off the case which will make it hard to feel what your trying to feel. You have to leave the extractor in. But make sure the extractor does not drag on the case rim. Some do and you have to fix that. Defiance extractors need a layer or 2 of tape under them to get then to stop dragging. You also need a straight round, one with a lot of runout will drag in the freebore and you will get false readings. A little effort on the reloaders part and you will get touch numbers to repeat better than .001". And for my customers I especially recommend this, because when I say to start .020 off the lands you will actually be .020 off the lands. With other methods like the Hornady tool your going to be .005-.010 closer to the lands because your pushing the bullet into the rifling more than you think.