I used the word headspace last time , rather then oal

[F. Guffey]

For those reloaders infatuated with 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.

I believe that is progress because when I started the total amount of known information about head space on reloading forums was: There was a line, the line was identified as 'the datum line' and was found by finding the arrow. Again the arrow was pointing at the line.

Reloaders had no clue the line was a round hole/circle; and there was an explanation; the explication read "and that is how they do it". A few forums continue to use the drawing and they have no clue what it means.

And I said I collect datums, I make datums and on rare occasions I purchase datums and then there was that day at the gun show when I asked about a box of datums; the dealer claimed he did not have any datums, the happy shopper ahead of me asked; "What does a datum look like and another shopper said; "I don't see any datums" and there was the third shopper, he asked "Where?".

F. Guffey

 

[Reloadron]

FGuffy:

I believe that is progress because when I started the total amount of known information about head space on reloading forums was: There was a line, the line was identified as 'the datum line' and was found by finding the arrow. Again the arrow was pointing at the line.

Reloaders had no clue the line was a round hole/circle; and there was an explanation; the explication read "and that is how they do it". A few forums continue to use the drawing and they have no clue what it means.

And I said I collect datums, I make datums and on rare occasions I purchase datums and then there was that day at the gun show when I asked about a box of datums; the dealer claimed he did not have any datums, the happy shopper ahead of me asked; "What does a datum look like and another shopper said; "I don't see any datums" and there was the third shopper, he asked "Where?".

All of that and a great deal more will be defined and covered in the support group. The group will be open to all reloaders, those with experience and those wanting to learn and understand things better. I estimate we will devote at least two complete meetings to the elusive datum. We will also cover the once found datum, proper care, feeding and of course storage.

Ron

 

[Unclenick]

Bytesniffer,

You answered your own question with the photos. The difference in the two measurements was 0.587". Add that number to any ogive measurement you make with that same comparator insert and you will have COL ± any variation in bullet length that lot of bullets has.

The Hornady gauge does not produce exact absolute measurements due to the hole radius. Neither does my copy of the RCBS Precision Mic, which measures a good quality (Dave Manson) headspace GO gauge off by about 0.002". The Hornady comparator is usually off around -0.004 to -0.008, with a few units getting outside that.

But it doesn't matter that the Hornady comparator is off of absolute. All you care about is the difference between the maximums in your chamber and what you set on the gauges. It's the jump and not its distance from the case head that matters. Also, the comparator and the RCBS PM both touch the bullet close to where your seating die presses down on it, so it's at the right place for making seating die adjustments that correspond well.

I think your use of 0.003" from the lands does not make sense for a couple of reasons. First, 0.003" doesn't make much difference to pressure, as the plot below reveals. It'll still be very close to the pressure you get from full contact with the lands, so it doesn't seem to be getting you any advantage in pressure. Second, bullets are less exact than that, owing to the fact they come off different sets of tooling that aren't perfectly identical and aren't set up and running on the same machine. As a result, the exact location of the part of the ogive that touches the lands can be off by 0.003". In the table below are the results of my measurements of 15 Sierra 150 grain MatchKings. You can see ogives differ by up to about 0.008" with respect to the base. I think you want to back off at least 0.020" to make your first tests of difference from contact with the lands.

The second reason for that bigger step is that the cartridge case moves forward during firing to contact the headspace surface. If you want to achieve exact positioning of the bullet distance off the lands, you want a consistent difference (again, not absolute values from the head; just the difference) between the case headspacing surface (in this case, the shoulder) and the bullet ogive position. Just make separate case and ogive position measurements and look at the difference between them for consistency and for adjusting the seating die. You'll get a couple thousandths difference just because the seating die works with respect to the head instead of the headspacing surface.

The terminology difference just has to do with English usage changing over time. If I look in my Webster's 2nd Edition (printed in 1948), there are two spellings:

Overall (initial "O")

and,

Over-all (initials "OA")

At that time, the former was used to mean "taken altogether" as in, "overall, it was a nice house" (this usage goes all the way back to Chaucer). The second applied strictly to the length of a thing, as in, "the boat was 13 feet over-all". As a result, prior to 1950, Cartridge Over-all Length, or COAL would have been a standard set of initials.

But when I look in my Webster's 3rd Edition (printed in 1962), the hyphenated form is completely eliminated and 'overall' is the only spelling for both meanings. As a result, COL and OL, would be current usage and COAL and OAL are initials for the obsolete usage, though many load manuals still have them out of what I suppose to be habit.

 

[ShootistPRS]

We have cartridge base to neck; case length
cartridge base to ogive (where the bullet reaches caliber diameter)
we have overall length or cartridge overall length which is the distance from the cartridge base to tip of the bullet.
For me, as a hunter, I need to know that my round will fit in the magazine and chamber in my gun. That means the cartridge overall length is important because it has to fit and that the case length allows the round to chamber. The only time I need worry about cartridge base to ogive dimension is if I need to know the bullet jump. It is unlikely that a round that fits the magazine will come close to the lands - especially with a pointed bullet.