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Old March 23, 2015, 12:13 PM   #1
06shooter
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Bullet ogive ?

What is the best way to measure bullet jump ?

I have Hornady's bullet comparator , but something tells me it's not right like the head space comparator .

Yes I have their oal gauge.

Last edited by 06shooter; March 23, 2015 at 12:48 PM.
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Old March 23, 2015, 12:47 PM   #2
mehavey
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The *Bullet* comparator will tell/measure where that particular bullet/design
will contact the lands in a rifle.

You can then use that comparator measurement (which by definition references a specific
point on the bullet curvature) to set yourself a specific distance off the lands.
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Old March 23, 2015, 01:41 PM   #3
06shooter
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Thanks , I figured it would be out of spec
like the head gauge is !
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Old March 23, 2015, 01:59 PM   #4
T. O'Heir
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Head space is a firearm manufacturing tolerance only. Cartridges do not have headspace.
Bullet jump isn't measured. Every rifle has a slightly different chamber. Even two, identical, consecutively numbered, rifles have slightly different chambers. The distance from the lands will, therefore, be different. Really isn't something to worry about anyway.
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Old March 23, 2015, 02:11 PM   #5
mehavey
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Chambers have a headspace dimension ...and spec.
Ditto, cartridge cases have a headspace dimension ...and spec.
(Check the SAAMI drawings sometime.)

While Hornady's Headspace Comparator is a few thou off, it's superb for
precisely & repeatedly setting up sizing dies to fit specific chambers.

Ditto the Hornady OAL Gauge. It is equally precise on setting up total cartridge
lengths for specifc bullet in specific rifles.
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Old March 23, 2015, 03:41 PM   #6
NoSecondBest
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Quote:
Chambers have a headspace dimension ...and spec.
Ditto, cartridge cases have a headspace dimension ...and spec.
(Check the SAAMI drawings sometime.)
Cartridges do not have a headspace dimension....even per SAAMI. You're referring to the cartridge case length in a pistol caliber that headspaces on the case mouth. This isn't true headspace, it's the spec on the case length.

Here's a good article explaining what it's all about: http://www.gundigest.com/headspace-1...rifles-chamber
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Old March 23, 2015, 04:11 PM   #7
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Quote:
Cartridges do not have a headspace dimension....even per SAAMI.
Please look at the two drawings and their specs in this SAAMI page:


SAAMI .30-06 drawing.


While a chamber may be the entity that has "headspace," per se (a noun)
BOTH chamber and case have headspace (an adjective) dimensions.

Last edited by Unclenick; March 24, 2015 at 02:38 PM.
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Old March 23, 2015, 04:19 PM   #8
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I'm missing something I think.
I have the same unit, but made earlier under the stoney point brand name.
What do you think is out of spec?
Can you measure the diameter of the hole in the face?
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Old March 23, 2015, 04:30 PM   #9
mehavey
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Are you speaking about the Hornady (Stoney Point) OAL Guage Set, or their Headspace Comparator set ?
Each comes with 5-6 inserts with different diameter holes to match either

- Bullet caliber (224, 243, 257, 277, 284 and 308)
- Headspace case datum diameter (See http://www.hornady.com/store/Headspace-Gauge-Bushings/)

All inserts/bushing are marked.
(What are you try to measure ?)
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Old March 23, 2015, 05:48 PM   #10
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Bullet ogive & headspace are two different measurements. OAL to ogive will give you the amount of jump to the rifleing,Hornady Lock-N-Load Bullet Comparator. Headspace is the brass measurment. A fired round in your chamber will give you a perfect shape of your chamber,, when sizeing that brass you want to keep it as close to that fired size as possible. For bolt guns .001 - .002 with F/L sizing. Measures from bolt face (case head)to datum line on the case shoulder, Hornady Lock-N Load Headspace gauge. Hope I helped
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Old March 23, 2015, 06:27 PM   #11
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Hey guys , I had a post recently about using Hornady head space body comparator, the diameter of it is .373 not .375 like what is stamped on the side of it.

I was thinking that the ogive comparator might be off too , that's why I'm asking about the most effective way to measure bullet jump.
The bullet comparator is .296"for .308" cal., if that's what it is supposed to be ,I guess I'm golden to rely on it.
I don't want to be going through the processes miss guided.

I want to be consistent with measuring ogive to rim , not tip to rim since that is inconsistent.

Thanks.....

Last edited by 06shooter; March 23, 2015 at 06:37 PM.
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Old March 23, 2015, 08:07 PM   #12
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06Man listen, everybody's comparator gets a different reading. When you use the tool correctly and your caliper is a decently calibrated tool, you can accurately measure your BTO, Whats BTO thats base to ojive, base to ojive, I said it twice so you won't forget. When measuring BTO with a certain bullet, in a certain chamber, the measurement will be certain, ( Bart B. Says every 30rounds or so it will increase by .001.

Take the measurement with confidence! And use the measurement to acertain the the "jump" or "jam" your rifle bullet combination likes, and for god sakes pick up more literature about precision handloading, not that your asking too many questions, but some that are befuuzzlin you are totally academic.
Just study dude.
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Old March 23, 2015, 08:42 PM   #13
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Dimensions don't equate to meaning headspace. Headspace is the dimensions of a rifles chamber. The dimensions of the cartridge are designed to fit into the rifle headspace. I can read the drawings and understand what the numbers are. However, headspace isn't a dimension on a cartridge. It's simple to find out what headspace is by definition. You can go to any number of knowledgeable places and get this meaning/definition. Cartridges are given critical dimensions so that they can fit into the spec/dim. of the rifles headspace. It's really not all that complicated. Don't try to interpret it differently.

From Wikpedia:
In firearms, headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge (the datum reference) to the face of the bolt.
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Old March 24, 2015, 07:23 AM   #14
Bart B.
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Don't forget, folks, in ammo plants making bottleneck cases, a quality control guy measures finished cases with a gauge to measure their head to shoulder reference distance to ensure it met specs; SAAMI and military specs establish them. That's been dubbed "case headspace" for decades and SAAMI acknowledges it. So does 98% of everyone else making or reloading bottleneck cases as well as making tools and dies related to those processes.

1% that populace demand that headspace be the space between the breech/bolt face and case head when its full forward in the chamber. They either ignore, or are not familiar with SAAMI's glossary stating that's correctly called "head clearance."

The other 1% insist that headspace is only a dimension of the chamber from back end to some reference point on the chamber shoulder.

Have any of you "case headspace" naysayers ever considered championing your thoughts on the differences between "throat," "freebore" and "leade" as it relates to dimensions from chamber mouth to bore diameter?
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Old March 24, 2015, 08:00 AM   #15
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-- deleted --
(Bart B said it)
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Old March 24, 2015, 08:11 AM   #16
NoSecondBest
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Quote:
Don't forget, folks, in ammo plants making bottleneck cases, a quality control guy measures finished cases with a gauge to measure their head to shoulder reference distance to ensure it met specs; SAAMI and military specs establish them. That's been dubbed "case headspace" for decades and SAAMI acknowledges it. So does 98% of everyone else making or reloading bottleneck cases as well as making tools and dies related to those processes.
My point exactly. It's a reference dimension. It references the SAAMI spec interface for the rifles chamber. It may loosely be called "headspace" on the cartridge but it's simply referencing the rifle chamber interface dimension. The definition of headspace still stands as the true term for a rifles chamber. The cartridge must fit in to it. I've read thousands and thousands of drawings over almost forty-five years working as an engineer. I also taught a training class on reading blueprints. I'm not confused by this terminology. I've struggled with other engineers over the years on how to interpret drawings so it's not surprising that this is confusing to some shooters also.
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Old March 24, 2015, 08:18 AM   #17
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Quote:
Originally Posted by 06shooter
I want to be consistent with measuring ogive to rim , not tip to rim since that is inconsistent.
Bullet ogive is not always that consistent either, especially on hunting bullets. Try it yourself. Use your comparator to check base of bullet to ogive reference.

...bug
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Old March 24, 2015, 08:40 AM   #18
mehavey
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Quote:
almost forty-five years working as an engineer.
IR12....
Which is why I simply tell folks that:

- Chamber headspace (dimension) is from the bolt face to the shoulder datum reference (diameer)
- Case headspace (dimension) is from case base to the same shoulder datum reference (diameter) -- and should be 1-2 thou less than the chamber dimension

Somehow that makes sense to them, ...and the bumblebee flies away happy.
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Old March 24, 2015, 08:40 AM   #19
06shooter
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I didn't mean to start again about head space.
I just wanted to know about the bullet comparator accuracy compared to the case gauge from Hornady.
I was told and realized the case gauge wasn't exact.
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Old March 24, 2015, 08:44 AM   #20
mehavey
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Bullet comparator and case gauge are two totally separate animals.

What are you trying to actually get at for your reloading -- case headspace [dimension] or cartridge OAL ?
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Old March 24, 2015, 08:46 AM   #21
Bart B.
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06shooter, you want to be consistent with measuring ogive to rim?

I think you're overly concerned about a cartridge dimension that in reality, is not super critical for even the most accurate shooting known to man. While it's easily measured and the numbers may seem big, in reality they're way too small to make any measureable difference in accuracy.

If you shoot a single few-shot group with one dimension set up, then change it .005" and shoot another few-shot group, the odds of that .005" change making a difference are very small. You'll see the same difference in group sizes across 10 other few-shot groups with the same setting on your seater die.

The only way to get the same bullet jump distance to the origin of the rifling for every shot is to size cases with minimal neck tension on seated bullets, then seat bullets out far enough that they're set back into the case neck as they are pressed into the origin of the rifling. Jump distance is always "zero" when that happens.

Anytime there's a space between the bullet and where it first touches the rifling, it won't be exactly the same for every shot fired. One other variable with bottleneck cases headspacing on their shoulder is shoulder setback. That happens when the firing pin drives the case shoulder hard into the chamber shoulder. What happens is exactly the same when sizing a fired case in a full length sizing die. The case shoulder gets set back and that moves the bullet a few thousandths inch closer to the rifling before the round fires. The slight shoulder angle and smaller shoulder area of the .30-06 case compared to others lets it often set back .005" from firing pin impact.

You can easily measure that, Disable a primer with a drop of oil, seating it in an empty case, then measure the case headspace with a suitable gauge. Then chamber that primed case in your rifle and shoot it; the primer will not fire but the force of the firing pin slams the case hard into the chamber shoulder. Extract that case then measure its headspace. Note the difference from before it was fired. Example; 2.049" before then 2.045" after which is a .004" difference. That difference is how far the bullet gets moved forward in the chamber before its fired.

I've never been concerned about that .004" spread in such stuff with my .308 Win. match rifles. Therefore, I don't think it matters with someone's .30-06.
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Old March 24, 2015, 08:52 AM   #22
mehavey
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^^^^^^^
I like it, Bart

Corollary offshoot from a live primer....

Everyone talks about primer backout as a measure of headspace, but that's
the first time I've heard someone think about how it might not be indicative.

Does the firing pin strike actually set the shoulder back that significant amount...
and therefore primer backout is not a measure of excess headspace (?)

Last edited by mehavey; March 24, 2015 at 09:28 AM.
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Old March 24, 2015, 09:50 AM   #23
06shooter
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Quote:
Ditto the Hornady OAL Gauge. It is equally precise on setting up total cartridge
lengths for specifc bullet in specific rifles.

-CENSORED--CENSORED-
I was wanting to know if the bullet comparator bushing was reliable for .308.

Mr. Bart , from your last post , I learned something new. Thanks !

What is the diameter for .308 in a barrel ( lands across one another) ? Forgive me for not knowing how to ask correctly!
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Old March 24, 2015, 10:47 AM   #24
mehavey
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The 308 bushing is fine for measuring to a standard reference/diameter point on the ogive --
after establishing your desired standoff using the OAL tool. (or any other method you choose)

The bushing's diameter doesn't have to be "exact" (though it's pretty much so), since all you're
doing is measuring to the same reference diameter every time.

As to bore diameter, see here:
http://www.milsurps.com/showthread.p...l=1#post158154
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Old March 24, 2015, 10:56 AM   #25
Bart B.
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mehavey:
Quote:
Does the firing pin strike actually set the shoulder back that significant amount...

and therefore primer backout is not a measure of excess headspace?
Yes, the firing pin does that. Do you own tests as I described.

Primer back out happens virtually all the time. It gets pressed back in when peak pressure pushes the case back against the bolt face. I've shot reduced loads in 308 Win cases more than 10% below max and extracted cases have their primers backed out several thousandths as well as case headspace being shortened a few thousandths more than normal. Further reduction of loads ends up with the primer sticking out even more. Increase the loads back to maximum and the primer gets reseated deeper. Interesting. Fascinating. I did 1 grain reduction increments in .308 Win. charge weights from 44 grains of IMR4064 under Sierra 168's all the way down to 35 grains. At 39 grains, ejected cases had primers protruding past the case head and fired case headspace was less than before firing. At 35 grains, primers stuck out almost .007." This was with Federal nickel plated cases; very slippery compared to bare brass ones whose shoulders didn't set back quite as far; .002" to .003" per pin smack.

06shooter:
Quote:
What is the diameter for .308 in a barrel ( lands across one another) ?
That diameter is the bore diameter shown in the SAAMI picture above for the .30-06 case. .300" is normal.

One other thing I forgot to mention that adds another variable to the actual bullet's jump distance to the lands. With the full length die sizing down a fired case, the distance from the case head to the shoulder will vary a few thousandths. That's a normal spread caused by a combination of the lube on the case and how much spring the press has. So, even though a bunch of cases have their bullets seated to an exact distance from their ogive back to the case head, that spread in head to shoulder dimension (case headspace) will have a few thousandths spread. The distance from the case shoulder to that point on the bullet's ogive will have that same few thousandths spread. Measure a bunch of sized cases and see what the spread is.

When the round fires, it's hard against the chamber shoulder and that spread changes the bullet's jump to the rifling whatever its amount is. The end result is, with all the variables at hand, an exact distance from head to bullet point ends up with a few to several thousandths spread in the bullet's actual jump to the rifling.

The reality of all of this is, if you want the most consistent bullet jump to the rifling, the reference point on the bullet must be equidistant from a reference point on the rimless bottleneck case shoulder. How far away the case head doesn't matter; it'll vary a small amount.

Last edited by Bart B.; March 24, 2015 at 02:22 PM. Reason: Updated for cases uses in test.
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