Cartridge Overall Length Measurement

[chkunz]

If you are new to the subject of loading for accuracy one of the first things you will see talked about is the importance of controlling the location of the bullet on the loaded round relative to the rifling; this is normally referred to as cartridge overall length or OAL. If you are experienced in loading for accuracy you are most likely already paying attention to this. There are tools like the Hornady and Sinclair bullet seating depth tools that are made to help with this and you will also find descriptions of using a dummy round with light bullet tension to use to determine this dimension. The problem with all of these methods is the variability in the measurement data. Many folks make one measurement and assume it is exact and proceed to set bullet seating depth with this measurement and believe they are able to do this with great precision. After all that is what is reported by many other folks so it must be so. If you have not done so already, I would recommend that you take ten measurements with what ever tool your are using, Hornady, Sinclair, etc. and take a look at the variability of the data. This variability is real and you must take it into consideration when making decisions on setting up you bullet seating die to produce a desired amount of bullet jump. Do not be surprised if the variability in your data is larger than what you expected and larger that what it appears other are getting based on what they are saying on this subject. You can not expect to set jump any more accurately than the variability you are seeing in your data.

 

[Lapua40X]

Good point, Rex. I lean towards using the mean average of ten measurements but I'm still not convinced that method produces a solid gold conclusion. Got any better mathematical processes in mind that might work better?

 

[chkunz]

Good point, Rex. I lean towards using the mean average of ten measurements but I'm still not convinced that method produces a solid gold conclusion. Got any better mathematical processes in mind that might work better?

I usually throw out the high and low values and use the mean of the remaining eight measurements. I think there are better ways of throwing out bad data points and I did this in another life when I dealt with a lot of data analysis. Probably throwing out points that are one or more standard deviations from the mean would be a consideration but only ten data points makes this problematic.

 

[mikecr]

What matters is not OAL, nor land contact distance from cartridge base.
All that matters is the local CBTO that correct testing reveals as best.

And there is no averaging of best CBTO. You set every round exactly to it.

 

[chkunz]

What matters is not OAL, nor land contact distance from cartridge base.
All that matters is the local CBTO that correct testing reveals as best.

And there is no averaging of best CBTO. You set every round exactly to it.

I think you make a good point. I think you are saying to fine tune based on the cartridge base to ogive dimension which can be controlled very accurately. What I see is folks that talk about bullet jump to be a specific number that is derived from data that has variability that is significant relative to the amount of jump they are quoting.

 

[Erik Cortina]

T-Rex, if you have to average 10 measurements it is an indication that you are either using the incorrect tool/method or using the tool incorrectly. Find a method/tool that gives you accurate repeatable measurements and quit guessing.

 

[noload]

Throw that tool away, Do this , use your vernier caliper to measure from the back of the action to the head of the case. Write it down. Make sure the case fits the chamber well and touches on the shoulder.
NEXT .. Load a round long with out any powder and measure again, if the dimension is .010" smaller than your written number than it's a .010" jam. You can figure anything from there.
I do this on all my calibers and actions.... I thru-out that other pos yrs ago.
When you figure out how to do this ....it will be the only way to check bullet seating depth..

 

[chkunz]

T-Rex, if you have to average 10 measurements it is an indication that you are either using the incorrect tool/method or using the tool incorrectly. Find a method/tool that gives you accurate repeatable measurements and quit guessing.

Erik, than you for your thoughtful response. However you missed the point of my objective of this thread. It is not about me, I understand the variability of the data that I obtain from the various tools and methods for making this measurement and how to apply the data that I obtain to the reloading process to achieve my accuracy objectives. The objective of this thread is to make the folks that are new to accuracy loading aware of the variability that they are dealing with and how that applies to their loading practices. Anything you have to add to support the objective of this thread would be appreciated.

 

[Erik Cortina]

T-Rex, if you have to average 10 measurements it is an indication that you are either using the incorrect tool/method or using the tool incorrectly. Find a method/tool that gives you accurate repeatable measurements and quit guessing.

Erik, than you for your thoughtful response. However you missed the point of my objective of this thread. It is not about me, I understand the variability of the data that I obtain from the various tools and methods for making this measurement and how to apply the data that I obtain to the reloading process to achieve my accuracy objectives. The objective of this thread is to make the folks that are new to accuracy loading aware of the variability that they are dealing with and how that applies to their loading practices. Anything you have to add to support the objective of this thread would be appreciated.

My point was that if measurement varies, a better tool/method must be used.

What I found with the Hornady tool is that if you push the bullet in firmly and lock the plastic rod, the measurements will be much more consistent. Bullet will have to be pushed back out because it will stick in the rifling. Try it.

 

[chkunz]

T-Rex, if you have to average 10 measurements it is an indication that you are either using the incorrect tool/method or using the tool incorrectly. Find a method/tool that gives you accurate repeatable measurements and quit guessing.

Erik, than you for your thoughtful response. However you missed the point of my objective of this thread. It is not about me, I understand the variability of the data that I obtain from the various tools and methods for making this measurement and how to apply the data that I obtain to the reloading process to achieve my accuracy objectives. The objective of this thread is to make the folks that are new to accuracy loading aware of the variability that they are dealing with and how that applies to their loading practices. Anything you have to add to support the objective of this thread would be appreciated.

My point was that if measurement varies, a better tool/method must be used.

What I found with the Hornady tool is that if you push the bullet in firmly and lock the plastic rod, the measurements will be much more consistent. Bullet will have to be pushed back out because it will stick in the rifling. Try it.

My first objective is to help the new folks understand that there is variability in their measurements and it is important for them to quantify that variability. Once they have established the variability that they are getting with the tools and methods that they are using they will be ready to learn ways to reduce the variability and they will have a base line to compare to so they will know when they have done something that helps or hurts.

 

[Brians356]

This feels like two "Ships that pass in the night".

 

[mikecr]

What matters is not OAL, nor land contact distance from cartridge base.
All that matters is the local CBTO that correct testing reveals as best.

And there is no averaging of best CBTO. You set every round exactly to it.

I think you make a good point. I think you are saying to fine tune based on the cartridge base to ogive dimension which can be controlled very accurately. What I see is folks that talk about bullet jump to be a specific number that is derived from data that has variability that is significant relative to the amount of jump they are quoting.

About seating depth, you got what I'm saying exactly.

About land distance (DTL, TL, ITL), This will always be relative to local measure, and always changing.
So I guess we're lucky it doesn't matter.
But if you want to consistently identify land contact/movement, the best method I know of is demonstrated with the R-P Tool. Woods post here: http://www.reloadersnest.com/forum/topic.asp?TOPIC_ID=9027
Basically, a nice cleaning rod system.
This takes shoulder contact variance out of measure & averaging is not needed. You measure from boltface and to a bullet tip against the lands. You make a dummy round matching in COAL. This is your baseline -with that bullet, and you get your TL CBTO from this dummy round.

Comparing best CBTO to this baseline CBTO gives you a seating/land relationship to tell everyone about. But 100rnds later, it'll probably be different.

 

[dcali]

I don't worry about measuring jump, because it is hard to measure (or even define) where the lands start, and it's easy to measure the CBTO very precisely.

Load up a dummy to find *approximately* where the lands are, and then forget about it. Measure and track CBTO. The lands are always going to be in the same place (ignoring erosion for now). Who cares if you're jumping 10 or 15 or 25 thous? As long as you keep track of CBTO and how it impacts performance, it doesn't matter.

 

[AckleymanII]

T-Rex, if you have to average 10 measurements it is an indication that you are either using the incorrect tool/method or using the tool incorrectly. Find a method/tool that gives you accurate repeatable measurements and quit guessing.

+1

I get the same measurements over and over and over after an initial "get close" measurement. I use the split neck system, been giving me very accurate, repeatable numbers for over 25 years, then the Davidson ogive tool on the fixed blade of my caliper.

I have never got another popular measurement system to be repeatable, owned two of them...sold them. This NON repeatable measurement system is probably the subject of this entire discussion because it is a POS!

Sticking a cleaning rod down the barrel and measuring off the tip of the bullet just gives a guy a good guess on what is going on.

Learning to accurately measure the OAL from the base of the case to where that particular bullet contacts the lands is just the start. It is a good idea to save that particular bullet and use it to measure leade growth with. Various bullet shapes can vary in their OAL to contact the lands.

Very often, a barrel may like a .005-.030 jam into the lands, or a jump of 0.050. You have to be able to duplicate this as the leade grows.

Various shooting disciplines require various levels of precision in their reloading, and a lot of people just do not like digging into a lot of detailed work in their reloads, especially people that are non group shooters. You have to make this hobby fun for what ever level you want to take it to. I just have one suggestion, if you want to accurately measure your OAL to the 0.001 and your particular tool does not give you repeatable results, throw that tool in the trash.

In my custom and factory rifles, none of them are very forgiving in their "sweet spot OAL" that gives best accuracy.

 

[mikecr]

Very often, a barrel may like a .005-.030 jam into the lands, or a jump of 0.050. You have to be able to duplicate this as the leade grows.

There never was a reason to chase lands when your best seated is jumping. Your notion here is a myth similar to 'VLDs like jammed' always was..

If you need jam to keep pressure, then you need to stay jammed of course. And this is tweaked to shape grouping.
But otherwise, best seating depth never changes. That's the advantage in NOT NEEDING jam for best results.

Also, your split neck method cannot find touching (TL). You're jamming with this -to some degree provided by the split neck tension.
The cleaning rod method finds touching.

 

[Erik Cortina]

Very often, a barrel may like a .005-.030 jam into the lands, or a jump of 0.050. You have to be able to duplicate this as the leade grows.

There never was a reason to chase lands when your best seated is jumping. Your notion here is a myth similar to 'VLDs like jammed' always was..

If you need jam to keep pressure, then you need to stay jammed of course. And this is tweaked to shape grouping.
But otherwise, best seating depth never changes. That's the advantage in NOT NEEDING jam for best results.

Also, your split neck method cannot find touching (TL). You're jamming with this -to some degree provided by the split neck tension.
The cleaning rod method finds touching.

Oh no, last year I did not keep up with my seating depth and traveled 8 hours to a match just so I could scratch my head the entire weekend because my gun was shooting so bad. I came back and measured the lands and did another seating test and sure enough, once I seated bullets .006" further out, my gun was back in business. Shot over 99% the next weekend. I jump bullets, and accuracy does change with throat erosion.

 

[Ackman]

Very often, a barrel may like a .005-.030 jam into the lands, or a jump of 0.050. You have to be able to duplicate this as the leade grows.

1) There never was a reason to chase lands when your best seated is jumping. Your notion here is a myth similar to 'VLDs like jammed' always was..

2) If you need jam to keep pressure, then you need to stay jammed of course. And this is tweaked to shape grouping.
But otherwise, best seating depth never changes. That's the advantage in NOT NEEDING jam for best results.

3)Also, your split neck method cannot find touching (TL). You're jamming with this -to some degree provided by the split neck tension.
The cleaning rod method finds touching.

1) Wrong. If you're seating .005, .010", or whatever seat depth is best....the leade will grow and you'll be following it

2) WTF? I don't shoot anything jammed. But NOT NEEDING or NEEDING just is, whether it's an "advantage" or not has nothing to do with it.

3) Wrong. Split neck case is very accurate and how hard a bullet pushes the lands - whether it jams or kisses - is easily controlled by neck tension.

 

[AckleymanII]

Very often, a barrel may like a .005-.030 jam into the lands, or a jump of 0.050. You have to be able to duplicate this as the leade grows.

There never was a reason to chase lands when your best seated is jumping. Your notion here is a myth similar to 'VLDs like jammed' always was..

If you need jam to keep pressure, then you need to stay jammed of course. And this is tweaked to shape grouping.
But otherwise, best seating depth never changes. That's the advantage in NOT NEEDING jam for best results.

Also, your split neck method cannot find touching (TL). You're jamming with this -to some degree provided by the split neck tension.
The cleaning rod method finds touching.

Mike, some rifles like a very exact jump, Trial and error will tell. Very often with Berger target bullets, they will like a .030 jump or a .050 jump, but you do have to play with various lengths, not all "jumps" produce the same amount of accuracy.

The split neck method is very repeatable, and it is a jam length. So, by shooting the rifle, you will play with this jam length, back off so much, go in so much, what ever that particular barrel and chamber likes. The split neck method will produce very accurate, repeatable results again and again.

If you are using a rod down the barrel, mark the barrel with a razor blade that leaves a very sharp line to measure to, that line still has width that is a hope and prayer in measuring the thickness. I use a visor with a magnifying lens, and I also have other optical measuring devises that magnify up to 25x. Measuring a line on a rod gets you close, and with a lot of really good shooting barrels, that may be all you need. How you mark that line on the rod, tool you use to mark the line is a science in and of it's self...been there, done that!

When it comes to jamming the lands, I have seen a lot of barrels where the groups suck down to a single bullet hole in the area of a .020-.030 jam, and there is a difference in group size along with Extreme Spread from a .020-.030 jam, but you still have to measure the OAL to know how to adjust for leade growth to keep the jam consistant.

Eric C. has obviously mastered the Stony point tool...good for you!

One thing for sure, in order for a guy to get the most in accuracy out of his rifle no matter what the caliber or make, he has to somehow master how to accurately measure the OAL as the lands grows. If a guy can not measure the OAL accurately, he can not duplicate pressures from one set of reloads to the next, all other variables being equal. This OAL issue is one of the basics in reloading for extreme accuracy, and shooters that take their accuracy serious should master it. A measurement system that give you +/- 0.015 is not worth a darn...take that to the bank.

 

[Erik Cortina]

[quote author=ackleyman II]
Erik C. has obviously mastered the Stony point tool...good for you!
[/quote]

Oh no, I do not use the stoney point tool as I find it a pain the the rear to use. I have better methods of obtaining accurate distance to the lands.

 

[ericbc7]

I am one of those guys that can't seem to master the "art" part of using a caliper and headspace tool like the Sinclair or hornady. I got an "innovative tech." Tool and haven't looked back. It's digital and repeatable and thus I am happy. Kudos to those that can master calipers- I envy you!