Micrometer is much less than min OAL?

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Mattj4867

New member
Hi all, I got a 7mm-08 that I’m reloading for. Im using Hornady 139gr SST bullets with a starting load of 40.5gr of hogden Varget. I used the RCBS precision mic and got 2.620, so I’m setting my overall length for 2.607. I loaded all of my bullets to 2.607. I looked at the min OAL in my manual and it says 2.800. Two tenths of an inch is ALOT. Is my rifle’s chamber too short? Could this be caused by the taper on the SST bullets? I’m beyond confused and would appreciate any help. Thank you
 
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Matt,
First off, you more than likely loaded 139gr. Hornady SST. The 143gr are 6.5mm, not 7mm.
Second, i'm not sure how the RCBS mic operates, as in does it measure from the ogive, or the bullet tip?
Third, the SAAMI measurement of 2.800" is actually max length, not min.

While measuring from the tip will show some minor discrepencies, you will be closer to your 2.80" measurement.
Also, take a fired casing, size the neck to where a bullet is snug in the neck, but still hand movable. Leave it long and chamber it in your rifle. Carefully remove and measure to tip.
Do this 3 times to ensure consistancy.
For SST's i take that measurement and subtract 0.020".
This will be my load to measurement.

Don't be surprised if it is longer than 2.80".
 
The RCBS PM measures to the ogive, not the tip, so it's number will be shorter than overall length, which is taken from the bottom of the case to the very tip of the bullet. The difference in measurement you get will depend on the shape of the ogive, with some having more or less curvature than other.

Let me suggest you use the bullet manufacturer recommend overall for the 139 grain SST, which is 2.755". Seat a bullet that way and then measure it with the Precision Mic. The difference in the two measurements can then be added to any PM number you get with that particular bullet to have the actual overall length (COL; Cartridge Overall Length, or the older term based on the hyphenated spelling of overall that became obsolete in the 1950's, COAL for Cartridge Over-All Length). Note that if you change bullets you will need to start this over again.

Some basic bullet and cartridge terms are given below to clarify the discussion, including overall length on the right side of the cartridge, and the measurement to the ogive made by the RCBS PM on the left, next to the case length dimension.

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Thanks it’s 139gr. I am referring to the free bore tool (shaped like a bullet) is this what your referring to Unclenick?
 
I think this is a case of too many tools in the box. Keep it simple and load a round to the suggested OAL in the manual you obtained the data from. Now see if it chambers, if it does you are done.

The RCBS Precision Mic is an indispensable tool for safe and accurate loads that help to extend the life of brass. The Precision Mic measures from the datum point on the case shoulder to the base of the cartridge
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It’s only “safe and accurate” if you understand that “datum point” is not“OAL” A Datum point is simply a reference point that you can run a taper or ogive into and have repeatable results despite differences in “tips”.

Maybe get some factory ammunition that you have shot before with the rifle and play with your measuring tools with it, might help in getting a better understanding of what it is you are actually measuring.

They don’t like folks posting others photos here but maybe a link is ok?
https://cdn.athlonoutdoors.com/wp-content/uploads/sites/6/2015/12/RCBS-Precision-Mics-6.jpg

If unclenick deletes the above because it’s not allowed, it’s in this article.

https://www.personaldefenseworld.com/2016/01/rcbs-precision-mics-measuring-headspace-oals/
 
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Datum point is simply a reference point
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Please tell me you are not an engineer! Not intended as an insult, just light hearted ribbing.

A datum point is your primary control point. Your tooling should be adjusted to control your datum to datum pitches. Base to ogive for example.

In this case, your axial datums are the net surface of the cartridge base and the net circle described as the ogive....this one is tricky because, while close, the seating die, measuring tool and lands likely do not all share the same datum point.

A reference point is another measured point, not directly controlled by the tooling. OAL is a reference dimension. It is reference because neither seating dies nor bullet making dies control tip location directly. It is the result of other datums being controlled.
 
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Datum point is simply a reference point
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For real? You have no ideal how silly reloaders get when 'the datum' is mentioned. Again, I make datums, I collect datums and on rare occasions I purchased datums. many reloaders insist 'the datum' does not exits,

The digital head space gage is not a head space gage; Starrett claims the digital head space gage is a dial indicator stand. The dial indicator stand has been listed in catalogs for 70+ years. At best the dial indicator stand can be used as a comparator but it takes a misguided and confused reloader to purchased one thinking he has purchased a head space gage. And I have had all kind of difficulty finding case head space.

I guess you guys can get together and boo-hoo to SAAMI, not me; I have no trouble accepting the fact the case does not have head space. I have no trouble off setting the length of the chamber from the datum/shoulder to the bolt face with the length of the case from the shoulder/datum to the case head.

I have no infatuation with head space, the fact head space gages come in SAAMI lengths as in go and no-go and field reject lengths does not drive me to the curb. HOW? I measure the length of a chamber from 'ZERO' , I know, no one understands what that means. And; it is nothing new, there were a few smith having it out 70 years ago, Back then there were these smiths giving this old man a difficult time; they just could not get the concept of 'zero'. To prove it they had tool boxes full of gages.

All I have is the datum and a way of verifying it.

F. Guffey
 
"...min OAL in my manual..." That's not the minimum OAL. That's the SAAMI Max OAL for your cartridge. That's measured from the point of the bullet to the flat of the case. With no ogives or datum's involved.
Your RCBS micrometer die is confusing you. It doesn't measure chambers. It's for setting the OAL, but like most stuff like it, it usually just confuses the new guys(lotta the older guys too). You can adjust the seating plug so you get the 2.800" and forget about the micrometer part altogether.
Anyway, your 2.607" is fine. The SAAMI minimum length for the 7mm-08 is 2.530". Any OAL between that and 2.800" is ok.
 
Unclenick...I don’t load rifle... but that is a beautifully descriptive diagram. I’m tempted to buy a rifle just so I can use your ammo template.

Thanks for what you do for this forum.
 
Matt4867:

I would go back to the real basics. Resize a case, put the bullet of your choice in it.
This is done so you can be sure the bullet does not get pushed in further or yanked out if it sticks in the lands the way ELD bullets tend to.

Seat it to about 2.850 COAL (in this case for that bullet)

Gently try to close the bolt. You should meet resistance.

Genlyt try to pull bolt back out. You may meet some resistance if the bullet sticks in the lands, gentle tap with a screwdriver handle.

If its stuck more than that, a brass rod or old sacrificial cleaning rod (not one of your new ball bearing types) and tap it back.

Measure COAL again to be sure no affect.

Seat it .020 deeper. Repeat until it no longer sticks in the lands.

You now are very close to the true COAL for that bullet and that rifle throat.

Seat the bullet .020 deeper (its safer as the lest jump the higher the pressure)

You now have a standard to load to and set your seater to.

Load up rounds and try them and see how it goes.

Once you find a powder load that is promising accuracy wise, you can play with searing deeper. Further out as well but if your loads are in the upper range, you can induce pressure with no jump.
 
https://thefiringline.com/forums/member.php?u=183621 said:
Thanks it’s 139gr. I am referring to the free bore tool (shaped like a bullet) is this what you're referring to Unclenick?
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Whether you are measuring that cartridge-shaped gauge or an actual cartridge, the idea is to measure it from the outside bottom of the case head to the place on the curvature of the bullet ogive (see my last illustration) to where it makes contact with the lands of the rifling in your gun. Comparing the measurement of the gauge to your actual cartridges, you subtract the cartridge measurement from the gauge measurement to learn the actual bullet jump from the cartridge case to the lands of the rifling during firing.

The only mistake you made was in thinking that length the Precision Mic measure is cartridge overall length (COL). It is not (see my drawing). It is a shorter measurement than COL because it measures from the outside bottom of the case head to a point lower on the bullet than its tip. But because you thought the gauge measured COL you were surprised to find it is shorter than 2.800 inches. 2.800 inches is actual COL to the tip, and not to that throat contact location lower down on the bullet. The measurement was shorter because it should be shorter because it doesn't measure all the way to the bullet tip.

Below is an illustration of two different bullet shapes that have the same jump to the lands but very different COLs due to one having a more blunt nose profile than the other. Both would measure the same on an RCBS PM but would have different COL's as measured with a caliper.

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Here, I think, is a point of confusion: even though your Precision Mic does not measure COL, it does control COL choice by getting the bullet jump where you want it. It's just that the actual COL includes the bullet tip.

So why not just use COL in the first place? The answer is that bullet makers don't usually run their bullets off just one machine. The box you buy is typically a mixture of bullets off several sets of tooling. With some bullet designs, like the hollow point match bullets, that means there are several slightly different lengths in each box. I've measured Sierra MatchKing 150 grain .308 bullets to have up to 0.015" spread in length in one box. So if you were trying to have 0.015" jump and measured an average bullet to get that length, but then gave them all identical COL, the jump to the lands would vary ±0.0075". The PM won't do that. If it says they are the same, they will have the same jump.
 

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Hi all. Thank you so much for the help. I used a fired case that had enough tension to hold the bullet and fed it into the chamber. I consistently recorded 2.908 inches. It is now clear to me that bullet taper plays a big part to COAL.
 
It is now clear to me that bullet taper plays a big part to COAL.
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Glad you recognize that. Except it doesn't, or at least not all by itself.

datum point
Any reference point of known or assumed coordinates from which calculation or measurements may be taken.
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yep, that's the definition. ANY POINT you choose. SO most of the confusion comes when I pick one point and you pick another..or so it seems to me.

And there are some other things to consider. COAL (Cartridge Over All Length -aka overall loaded length with bullet) is measured from the base of the case to the tip of the bullet. Not to any "datum point" on the ogive but to the tip of the bullet. That's the datum point for COAL.

And COAL as listed in the loading manuals is the maximum length by SAAMI standards. However, all that means in reality is its the max length by SAAMI standards.
Sound confusing?? it can be..

Understand that the SAAMI standards for COAL are not "you must meet this or DIE!!!" kind of things. They are agreed upon industry standards to produce ammo that is expected to work in all firearms chambered for that cartridge.

Some guns will work with rounds that are slightly longer, some will not, and some need rounds that are slightly shorter than the max standard length.

AND, we're talking about two important measurements here, the length that will chamber and NOT be in contact with the rifling, and the length that will work through the gun's magazine and feed into the chamber.

Generally they are NOT the same. The SAAMI length is intended to be off the rifling AND work through the magazine. And, to further muddy the water, there are guns where one can load to just off the lands, and be too long to work through the magazine. AND, it can change when you change bullets.


A good illustration of this is the difference in COAL between a spitzer seated to X.xxx" length and a flat point/round nose bullet of the same weight, seated in the same die with no changes to adjustment.
 
Look up the definition.

datum point
Any reference point of known or assumed coordinates from which calculation or measurements may be taken.
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I'm surprised that statement was even questioned. We use datum points all the time for aircraft weight and balance. The datum point (where ever located) was set by the designer. And it can be where ever the designer chooses. Use the correct specs, and the datum point will be the same for every one.
 
The SAAMI cartridge and chamber drawings have several locations marked with a B for "basic" meaning they are defined reference locations. We call the headspace reference location a datum (singular of the word data) when it is actually a data ring around the surface (inner for the chamber but outer on the case) of the shoulder, defined by where the surface intersects with a plane perpendicular to the cartridge axis at a defined distance from the base.

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While that can be useful information, it’s also obvious that a .3750” dimension simply cannot work as a “datum point/intersect location” for every round we reload.

So again, it’s simply a reference point, change the diameter and you change the point, by how much depends on the taper.

If you are making relative measurements you can pick any diameter you want, to be the “Datum point”. A really good one would be to take a drop off the barrel blank and use the same chamber reamer the chamber was finished with but that’s a bit beyond the topic.
 
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How about looking at it from another angle.

Let’s say one has a ring that is tight on their finger and takes it to be resized.

So the jeweler drops the ring on a tapered shaft that is also graduated for measurement.

Where it falls is his “datum point”, doesn’t matter how big or small it is, that’s what it is and he can quantitatively change it’s diameter relative to the point of reference.
 
While that can be useful information, it’s also obvious that a .3750” dimension simply cannot work as a “datum point/intersect location” for every round we reload.
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There are reloaders that are wired believing nothing is possible; not me. There is a datum/round hole that is .400", according to SAAMI the .400" round hole/datum works with the 308W family of cases. And then there are magnum/belted cases in the family of the 300 Win Mag. The round hole/datum for the 300 Win Mag is.410".

And then there are very few reloaders that do not allow themselves to be driven to the curb; all they have to have is a round hole that will allow the case to seat/imprint on the edge of the datum/round hole. Once the concept soaks in they realize they have discovered the comparator.

After that they can verify the datum/round hole; if in verifying they can manage to zero their gage they can manage to measure the length of the dase from the datum to the case head.

Or they can just sit around trying to convince each other it just can not be done because of tolerances, variations and those pesky pluses and minuses.

F. Guffey
 
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