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Old March 13, 2014, 12:08 AM   #1
obiwannabe
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Differences in OAL measurement methods

I know I'm not breaking any new ground here, but only just loaded my very first round about a month or so ago and wanted to share something with the other newbs around here.

I bought a 700 Varmint in 308 on Black Friday sale last year at the evil Dick's. I've been gradually upgrading things intending to use the rifle for 600yd competition at my club. We do 1000yd matches twice a year so...who knows.

I've been obsessing about finding my optimum seating depth for the next step in my load development. Last night I think I finally figured it out but I wanted to share what I found when I used the common techniques I found here. I pretty much used them all and got different results from each one.

I started with the cleaning rod method. Quick and easy no special tools other than calipers. Got repeatable max COAL measurements of 2.923" with the 175gr SMK. Based on that I set my seating depth for a COAL of 2.903", loaded up a ladder of 8208xbr and hit the range. Got good results, came home happy.

Then I saw the Hornady gauge and figured I could make one to measure that way. So I made this: http://thefiringline.com/forums/showthread.php?t=541854
With the homemade gauge I got consistent max COAL measurements of 2.968". Hmmm...big difference.

Next I tried the seat-the-bullet-with-your-bolt method. Used a normally prepped case, set the bullet in the case mouth, crammed the bolt home and got...2.985".

Then I noticed something. Rifling marks. It suddenly dawned on me what these scratches on the bullet were. The dummy cartridge I made with the bolt seating method had significant rifling marks on the bullet indicating to me that my neck tension was probably too tight allowing the bullet to jam further than what I was doing with the gauge. Since I can't do anything about neck tension with the equipment I have, I wrote that one off as erroneously long. I made a dummy round using the max measurement from the gauge and also saw rifling marks when I chambered it, though much shorter than with the bolt seating method. Clearly both these methods were erring on the long side for whatever reason. A dummy round made from the cleaning rod method showed no marks.

My comparator arrived about this time so I re-measured each method's CBTO:

Rod = 2.348"
Gauge = 2.390"
Bolt-seated = 2.409"

The method I finally settled on involved making several dummy rounds. I started with a CBTO of 2.389" (0.001" shorter than the gauge measurement) and chambered the round looking at the rifling marks. I progressively shortened the CBTO by increments of 0.001" using a fresh bullet each time until I found the point where the marks went from being freakishly hard to see to being gone. I had the barest hint of a couple of marks at 2.385", gone at 2.384". Now I'm finally pretty confident about where my lands are.

So the bolt-seating method seems very dependent on the neck tension being just right--tight enough to pull the bullet from the lands but loose enough not to jam it in too far. The cleaning rod method is pretty imprecise. I guess I just suck at measuring pencil lines on a piece of tape to 0.001" accuracy. The gauge was very close but still required some fine tuning. Maybe it was something with my homemade gauge.

I took the two nodes I found in my ladder and loaded up a couple of CBTO testing ladders with 5 rounds each at jumps of 0.030", 0.020", 0.010", 0.000", and for the low velocity node I made a rung with a 0.010" jam.

We'll see if all this even matters. It's fun though.

kendall

Last edited by obiwannabe; March 13, 2014 at 12:31 AM.
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Old March 13, 2014, 10:57 AM   #2
F. Guffey
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"I've seen several threads on OAL gauges but I don't recall seeing one done this way."

I have, it has been at least 10 years ago on a forum that crashed and all of the archives were deleted, most of the archives were an embarrassment to the shooting public. The original copy of the Hornady/Sinclair tool was much shorter and did not use a push cable that looked like a speed odometer cable.

I have made one forever, difference, I am the fan of bullet hold, I want all the bullet hold I can get. When I push the bullet out of the case I want the bullet to stay/held in the neck.

The tool I make does not require all the ship skills, a reloader simply drills out the primer pocket to a diameter that will allow the bullet to be pushed out until it contacts the lands.

Bullet hold, back to all the bullet hold I can get, I use the case with the bullet as a transfer, I transfer the dimensions of the chamber to the seating die. I do not measure the length from the ojive to the case head. I am the fan of transfers.

And now we wait for someone to act like Venny, "I am so confused".

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Old March 13, 2014, 11:54 AM   #3
Brian Pfleuger
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Quote:
My comparator arrived about this time so I re-measured each method's CBTO:

Rod = 2.348"
Gauge = 2.390"
Bolt-seated = 2.409"
In several guns of difference calibers and with different bullets, I have never been able to get consistent measurements using your 1st and 3rd methods. I even used a Lee collet neck die, with which I can very carefully adjust the neck tension on the case and still never got good results. The tension is either so light that I either can't hold it in place long enough to get a measurement or it sticks just enough in the rifling so it pulls out slightly or it's so strong that it jams deep into the rifling and is much longer than it should be.

The gauge method has been very consistent for me but you have to watch two things.

1)The modified case is not the same as your cases. Hornady uses a case sized to SAAMI minimum headspace. Real guns almost never have SAAMI minimum headspace and you should not be sizing your cases so they do either. Note that the gauge is actually measuring the distance from the case SHOULDER to the bullet ogive. In order to match that reading, you'd have to adjust for the difference between your case headspace and the gauge headspace. This would be done by using the Hornady headspace gauge, or just the appropriate deep well socket that touches near the center of your case shouler if you don't have the gauge, to measure the shoulder of your cases, zero your caliper, measure the gauge case shoulder and add that difference to the measurement you get for OAL with the gauge.

2)You must press the bullet into the rifling GENTLY. Very, very gently. It doesn't take much force to push the bullet a couple/few thousandths into the rifling.
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Old March 13, 2014, 12:11 PM   #4
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Both my 308 bolt guns have detachable box mags. So i load to max length for the mag. Im sure that leaves me with a significant jump to the lands

What kind of difference are you guys seeing by seating to the optimum length vs. Having some jump.
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Old March 13, 2014, 12:30 PM   #5
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Quote:
What kind of difference are you guys seeing by seating to the optimum length vs. Having some jump.
Sometimes, "optimal length" and "some jump" are the same thing.

Personally, I've not bothered experimenting. I normally load 0.05 off the rifling and it's so accurate and I have so little time that I never bother to do anything different.
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Old March 13, 2014, 01:32 PM   #6
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I don't think it makes much difference in how far bullets jump from the case mouth to the rifling as long as the case neck's reasonably straight and aligned with the case body. Depending on the bullet's actual shape and the angle of the rifling at its origin, different amounts of jump from zero to several thousandths can make a difference. But you'll only get valid data with each with ammo and rifle and shooter capable of shooting no worse than 1/4 MOA at 100 yards. For most folks, there's other things to do making really accurate ammo. Especially when several thousandths (a hundred in many instances; that's 1/10th inch) bullet jump to rifling with bullets having some runout on the case axis will shoot 1/4 MOA all day long in well built rifles with match grade barrels all fit to the nth degree. Good example is Federal, Hornady or Black Hills .308 Win. match ammo; it's gonna have a lot of bullet jump to rifling and not many folks load ammo accurate enough to equal it.

You can measure fairly accurate by putting a wood dowel down the rifle barrel onto a bullet that's been gently pushed into the lands then measure the distance from the muzzle to the dowel tip with a caliper's depth gauge. Then remove the bullet and measure the dowel's tip depth with the other end against the bolt face. The difference is about what the maximum over all length should be.

Besides, with rimless bottleneck cases headspacing on their shoulder, with bullets seated to maximum cartridge overall length to be a perfect fit in the chamber, they'll still jam into the rifling a few thousandths when the case shoulder's set back from firing pin impact on the primer setting the case shoulder back a few thousandths.

And whatever spread there is between the case head and shoulder dimension is will also be transferred to the point on the bullet jacket that touches the rifling.
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Old March 13, 2014, 07:46 PM   #7
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The cleaning rod/dowel method was the least accurate of the three methods I tried. My gauge is made from a case that was fire formed in my rifle. I think my gauge measurement was long by a few thousands because I wasn't cramming the gauge into the chamber as hard as the bolt did with the dummy round. But the headspace in both instances should be pretty close to zero.

The rifle shot pretty well with my first load ladder which ended up having a jump of about 0.060" because my cleaning rod measurement was so far off. I don't know if the shorter jump will matter or not. I just like to play.
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Old March 13, 2014, 08:08 PM   #8
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obiewannabe, I think you mean head clearance. Head space is the distance from the bolt face to the chamber shoulder. Head clearance is the space between bolt face and case head when the case is fully into the chamber.

I used to think that, too. It is confusing.

A .308 Win standard head space gauge is 1.630" long from its head to its shoulder reference. If it just lets the bolt close on it, head space in that chamber is 1.630".
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Old March 13, 2014, 09:09 PM   #9
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Thank you for the clarification. Yes, head clearance then.
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Old March 14, 2014, 12:49 PM   #10
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"Head clearance"?
Huh.
I could use that term in my posts.

With cartridges with more head clearance, the real trim length requirement reduces.
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Old March 14, 2014, 06:14 PM   #11
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Quote:
Originally Posted by obiwannabe
I think my gauge measurement was long by a few thousands because I wasn't cramming the gauge into the chamber as hard as the bolt did with the dummy round.
If you're "cramming" rounds into the chamber you have a serious sizing issue. Even neck sized only cases are normally fully sized when the beginnings of resistance are felt.

Even so, a case that is more than a couple thousands long on headspace will be VERY difficult to chamber. In other words, differences between what ever the bolt does and whatever you did with the gauge can not possibly account for more than a couple thou, unless your cases are literally being crushed into the chamber by the bolt, in which case you'd probably have to be closing the bolt with a hammer.
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Old March 14, 2014, 09:51 PM   #12
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I take it that you're measuring the leade in the rifle. Are you using a comparator when taking your measurements? The length of bullets can vary slightly unless you trim the meplat uniformly. I use the Hornady tool and it has worked very well for me. If the standard brass does not fit your chamber to your liking, you can use one of your pieces fit to your chamber and tap the flash hole to fit on the tool.
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Old March 15, 2014, 07:29 AM   #13
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Quote:
Even so, a case that is more than a couple thousands long on headspace will be VERY difficult to chamber.
Quote:
"Head clearance"?
Huh.
I could use that term in my posts.

I have the length of the chamber from the shoulder to the bolt face and I have the length of the case from the shoulder to the head of the case.

I HAVE ONE HEAD SPACE, ALL OF MY HEAD SPACES are measured from the datum/shoulder of the chamber to the bolt face.

I do not have a head space for everything like the chamber and case and the die.

Then there is the gage, there is the case gage, and there are the home made gages that are standards and transfers, and there is the shop practice, a good practice is verify. If I had head space everything I would it would be impossible to verify.

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Old March 15, 2014, 08:10 AM   #14
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Best method and or technique. then there is the rational/reason.

For a long time there was the sheading of the neck, I referred to the cases with shredded necks as 'squid neck cases', shredding the neck reduced bullet hold, called neck tension by some. Being fair and objective I found all kind of problems with loose neck cases. First ?, a measurement must be made from the ojive to the case head, then the reloader has to start seating bullets in in cases a little at a time then remove and measure and repeat the process until they give up and then decide 'that is good enough'.

the next process starts the same way, 'GET RID OF THE BULLET HOLD. called neck tension by most'. Then all the qualifying statements when working with loose necks like 'you gotta be careful', 'don't stick your bullet into rifling because the rifling may decide to keep the bullet' etc., etc..

I want all the bullet hold I can get, I am not a fan of making a transfer (called dummy by other reloaders for obvious reasons, they do not understand transfers). I transfer the chamber dimension straight to the seating die, I do not measure the transfer and then move the case with the seated bullet in and out of the press to measure the length from the ojive to the head of the case.

If pushing the bullet out of the case and into the lands causes a strain figure a way to hold the case and thread a sleeve that is threaded into the primer pocket/flash hole, take advantage of the incline plain.

http://www.ohio.edu/people/williar4/...ch/Inplane.htm

I have no trouble pushing a bullet out of the case and into the lands because I have shop skills, even if I had to put effort into the project, I would not complain.

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Old March 15, 2014, 08:14 AM   #15
obiwannabe
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Quote:
Originally Posted by Brian Pfleuger
If you're "cramming" rounds into the chamber you have a serious sizing issue. Even neck sized only cases are normally fully sized when the beginnings of resistance are felt.
I get some resistance as I lower the bolt on a fire-formed case. It's not a lot but it's more than with a factory round.

I ordered Hornady's headspace gauge inserts for my comparator and they arrived yesterday. The headspace measurement on my fired cases is 1.623". I thought I was setting my FL sizing die so it would not bump the shoulder and the gauge confirms that. Headspace on my sized rounds is also 1.623".
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Old March 15, 2014, 08:20 AM   #16
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Then there is seating off the lands, I am the fan of the running start, I want my bullets to have 'the jump'.

Pushing bullet, I have pushed bullets out of the case before the bullet hit the lands, some have traveled .225" before hitting the lands, what was the smith thinking? I do not know, I did not have cases I could form that would increase the length of the case with a long neck, remember, the bullet came out of the case and then traveled .225" before contacting the rifling, and the bullets were long bullets, and heavy, short bullets like 150 grain bullets traveled further.

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Old March 15, 2014, 04:04 PM   #17
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Despite Mr Guffey's adamant and frequent denial, there are in fact two headspace measurements. There is chamber headspace, from the bolt face to the chamber shoulder datum and there is case headspace, from the case head to the case shoulder datum.

These are both industry standard language, reference and measurement and have been for many decades.

In fact, virtually every reference you will find to the term "headspace" is referring to case measurements, not chamber measurements.

SAAMI calls the difference in these two measurements "head clearance" and references the gap between case head and bolt face when the bolt is fully closed.

Why does SAAMI talk of a gap between the case head and the breech face, considering that the breech face is in contact with the case head when the bolt is closed?

That's another of Mr Guffey's frequent and adamant denials. You see, in the micro seconds before primer/powder ignition and the rising pressure lock the case to the chamber walls, the force of the firing pin drives the cartridge forward until the case shoulder contacts the chamber shoulder. This results in the gap being not between the two shoulders but between the breech and case head.

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Old March 15, 2014, 04:45 PM   #18
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You see, in the micro seconds before primer/powder ignition and the rising pressure locking the case to the chamber walls, the force of the firing pin drives the cartridge forward until the case shoulder contacts the chamber shoulder. This results in the gap being not between the two shoulders but between the breech and case head.
Illustrated here:

http://www.hornady.com/ballistics-resource/internal
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Old March 16, 2014, 12:21 AM   #19
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http://www.saami.org/glossary/display.cfm?letter=H
Quote:
HEAD CLEARANCE
The distance between the head of a fully seated cartridge or shell and the face of the breech bolt when the action is in the closed position. Commonly confused with headspace.

...

HEADSPACE
The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats.
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Old March 16, 2014, 04:55 PM   #20
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Too bad Hornady mis-termed head clearance as headspace in the link tobnpr posted earlier. In spite of agreeing to use SAAMI's terminology by becoming a contributing member. But they're not the only company doing that. No wonder there's so many meanings conjured up by folks talking about cartridge and chamber dimension's differences and how they interact in fire arms operation.

They also didn't fully explain how shoulders stop against chamber shoulders by firing pin impact.

Regarding Mr. Guffey's issues, read post 20 in the following thread; he's never commented on that one, because, I think he's finally accepted it as head space facts:

http://thefiringline.com/forums/showthread.php?t=540898
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Old March 16, 2014, 05:15 PM   #21
F. Guffey
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Quote:
Despite Mr Guffey's adamant and frequent denial, there are in fact two headspace measurements.
I have posted links to SAAMI asking for someone/anyone to show me where SAAMI made a reference to head space on the case. SAAMI list case length from the shoulder to the head of the case with no reference to head space.

And still we have Bart B. and the firing pin requiring the shoulder of the chamber to slow the case down to prevent the case from outrunning the firing pin.

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Old March 16, 2014, 05:21 PM   #22
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Bart B. are you that desperate for attention? You are assuming I read your post.


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Old March 16, 2014, 05:40 PM   #23
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Brian, the third headspace measurement is the one from the head to the reference point on a headspace gauge.
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Old March 16, 2014, 06:52 PM   #24
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Well I know I am not in the august company of the above.

I tend to simplify things as my head hurts if I don't (and reading the above I have a horrible headache).

I have the Horandy tool. I get as good a measurement as I can.

I then load a bullet into a cartridge that has power and primers (no sense in wasting the effort)

I am more or less and shooting for something with about .005 to .0010 clearance.

If I chamber the shell and it is "sticky" on close, I know I am too long.

I shorten it up a bit and try again.

I note the measurement of when it just quits being sticking (lands) and take -.10 from that.

I then run a group of bullets through, as being a rank amateur I cant seem to get the danged things to be exact same length each time (yes I know I am doing something worn but what the heck they have not charge me in the Haugue with high crimes against humanity yet)

If none of those 20 cause me any issue when I cycle them, I load a bunch up and I take them to the range and start shooting away.

Occasionally I find one that is sticky in chambering . I set it aside and when I am back at my crude but more or less working re-loading setup, I measure it with my not so crude micrometers (yep, I do have some good steel digital and dial ones. I then look at it, my data, the variations and if its an outlier I push it down a tad and use it.

If my variation is too wide and some show up in that range, I move it a tad deeper to avoid it and go on with life.

I feel so much better when I am not confused.

Last edited by RC20; March 16, 2014 at 10:21 PM.
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Old March 17, 2014, 10:32 AM   #25
F. Guffey
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Brian, the third headspace measurement is the one from the head to the reference point on a headspace gauge.
Bart B., the one and only head space measurement is from the shoulder/datum to the bolt face. Problem with the head space gage, it has a fixed length for all with an exemption, me, I am not so hard headed as to insist on just a go-gage or a no go-gage or a field reject gage. I make gages from .012" shorter than minimum length to .018" longer than a minimum length case. That is 30 gages separated by .001".

Then there is verifying, verify the head space gage, verify the Wilson case gage, verify the length of the case from the shoulder/datum to the head of the case, verify the sizing die.

Then there is the chamber. I measure the length of the chamber in thousandths from the shoulder to the bolt face. I know there are rims, there are belts, when it comes to reducing all that case travel, I measure from the shoulder to the bolt face.

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