CBTO vs COAL question

[ReloadKy]

Since I am starting to reload a new cartridge in 7mm-08 I am trying to do some things a lot different and hopefully learn some things along the way. I have never fired this rifle and decided that before I loaded any ammo I would try to compare CBTO vs. COAL. I used the Hornady LNL gauge / modified case and the hornady comparator. I am starting out with 2 different bullets the 120 and the 140 Nosler ballistic tips. The CBTO for both was either 2.285 / 2.286. I used a second set of calipers to measure the COAL of the exact same trials and the OAL was 2.873. This measurement is 0.073 longer than SAAMI specs.

Would it be okay for me to attempt to load at this length?? My magazine is 3.029 inches so the cartridge would fit. I am new to this technique and concept so any help would be appreciated.

[USSR]

I used this tool before Hornady bought it and it was called Stoney Point. Here is what I found in developing precision long range loads using it. Never load to a COAL that corresponds to the CBTO measurement you took. Reason? Bullet ogives vary on even good match bullets such as the Sierra Matchking, due to them coming off different machines at the factory. If you load to the corresponding COAL, you will undoubtedly have some of your bullets seating into the lands and some of them slightly off the lands. Not exactly conducive to accuracy. If I was having them soft seat when I chambered the cartridge, I would seat them 0.020" longer then the aforementioned COAL, otherwise, 0.010" short to account for ogive differences. Just what I found and did in competitive long range shooting.

Don

[big al hunter]

Using the comparator and modified case tells you where the lands are v.s. the ogive. Now you load some and adjust the CBTO to see where the gun likes it. If it fits the magazine, and feeds properly, and the bullet is not jammed into the lands, your good to go. Load to the best accuracy. Every gun is different. Some like close to the lands, some like a bit more jump.

[Unclenick]

USSR is correct that bullets have variation in bullet-base-to-bullet-tip measurements, though I think the bullets with synthetic tips are usually pretty consistent, where hollow point match bullets can vary 15 thousandths or so. The base-to-ogive measurement where the gauge touches it is also a variable. However, the tip is in the bore, so if the cartridge fits the magazine, that part isn't a worry. Seating dies generally seat by pressing on the ogive, so the variation you worry about in the bullet is the variation between where the measuring tool contacts the ogive and where the seating die contacts it. The distance from the case head will only vary that much, and usually it isn't a lot.

Of bigger concern is if your resized cases are a different length from the head to the shoulder than the gauge's adapter case is, because the seating die is seating a certain distance off the base, but when the cartridge is fired, the shoulder moves forward into contact with the chamber shoulder and so it is the case shoulder that determines how far the bullet sticks into the chamber and not the case head. So if your cases are, say, 0.006" longer from head to shoulder than the adapter is, you want bullets seated in your cases to be 0.006" longer from head to ogive in order for the ogive to be the same distance from the shoulder that it was in the adapter case.

To calibrate to this, you want to get the case comparator insert for your Hornady caliper comparator head (they fit the same base as the bullet ogive inserts), and compare your resized and ready to load cases to the gauge adapter case so you can add the difference to your ogive position.

[44 AMP]

Just out of curiosity, what rifle are you going to be loading for??

There are a number of 7mm08s that are built as big game hunting rifles and may not show significant improvements from the "chasing the lands" technique used by bench rest and long range shooters.

[ReloadKy]

I am shooting a savage 110 hunter.

[Bart B.]

Originally Posted by USSR

I used this tool before Hornady bought it and it was called Stoney Point. Here is what I found in developing precision long range loads using it. Never load to a COAL that corresponds to the CBTO measurement you took. Reason? Bullet ogives vary on even good match bullets such as the Sierra Matchking, due to them coming off different machines at the factory. If you load to the corresponding COAL, you will undoubtedly have some of your bullets seating into the lands and some of them slightly off the lands. Not exactly conducive to accuracy. If I was having them soft seat when I chambered the cartridge, I would seat them 0.020" longer then the aforementioned COAL, otherwise, 0.010" short to account for ogive differences. Just what I found and did in competitive long range shooting.

Sierra has never mixed different machine made bullets into a single lot into their green boxes for retail sales. They used to not start a new lot after adjusting a forming die. Match bullets still tested one fourth MOA average. After Sierra moved to Missouri in the 1990's, they started a new lot after any change in the forming dies.

The forming die's dimensions and setting in the machines are constant but the copper sheets fed into them are not equal in metal all over. A few thousand coin shaped disks punched out of a copper sheet are not equal in metal properties.

Like brass ones shaped into cartridge cases, their finished dimensions are not exactly the same across all. How many people think all those dimensional differences across all cases in a given lot come from different machines?

[Unclenick]

Cases come off mixed tooling, for sure. I found this by weighing them. If you sort by weight into a histogram you get multiple peaks that are the bell curve distribution of the tooling. I got four peaks out of the last Winchester bulk brass I got. I got two peaks with zero overlaps in the last Lapua .308 I got, though the sample was small. Still, indicative of quality control that is pretty tight. My assumption is the difference is due to slight differences in the die setup.

The Winchester was just stacked over the individual case weight on a line I set up on a large sheet of graph paper. For the Lapua I just recorded the weights on my laptop into Excel and made the histograpm there.

[Bart B.]

Mid Tompkins and one of Sierra's tool and die machinists used a 50X optical comparator to see shape differences within a lot of Lapua 30 caliber match bullets. Four distinct shapes were observed. Good evidence four different sets of dies were used. Tiny variables in each one were normal and were those for a given lot of Sierra's match bullets.

Lake City arsenal match ammo lots had FMJBT bullets from 3 or 4 different sets of dies. Easy to see the 3 or 4 different die prints on bullet bases from a given ammo lot.

Unclenick, I've used a 2 x 3 foot lamp baffle sheet with half inch squares to histogram 308 Winchester cases in tenth grain weight increments. Three peaks was the most my "mountain ranges" ever had.

[Unclenick]

Yes. The same with their ball ammo, too, of course. I took this photo of bullets I pulled from the same can of M2 Ball.

[Bart B.]

Is it possible that 3 or 4 different lots of cartridge case brass run through the same set of dies would produce 3 or 4 peaks in a weight histogram?

[USSR]

Quote:
Originally Posted by USSR
I used this tool before Hornady bought it and it was called Stoney Point. Here is what I found in developing precision long range loads using it. Never load to a COAL that corresponds to the CBTO measurement you took. Reason? Bullet ogives vary on even good match bullets such as the Sierra Matchking, due to them coming off different machines at the factory. If you load to the corresponding COAL, you will undoubtedly have some of your bullets seating into the lands and some of them slightly off the lands. Not exactly conducive to accuracy. If I was having them soft seat when I chambered the cartridge, I would seat them 0.020" longer then the aforementioned COAL, otherwise, 0.010" short to account for ogive differences. Just what I found and did in competitive long range shooting.

Sierra has never mixed different machine made bullets into a single lot into their green boxes for retail sales. They used to not start a new lot after adjusting a forming die. Match bullets still tested one fourth MOA average. After Sierra moved to Missouri in the 1990's, they started a new lot after any change in the forming dies.

The forming die's dimensions and setting in the machines are constant but the copper sheets fed into them are not equal in metal all over. A few thousand coin shaped disks punched out of a copper sheet are not equal in metal properties.

Like brass ones shaped into cartridge cases, their finished dimensions are not exactly the same across all. How many people think all those dimensional differences across all cases in a given lot come from different machines?

Bart,

I expected a contrary view from you due to experience with you in the past on various sites, and you have not disappointed me. First, your assumption is that someone has bullets from a single lot. Definitely not the case, at least for me. Second, I have found that Sierra MatchKings vary in ogive dimensions by as much as 0.010", hence my statement about never seating a bullet less than 0.010" from the lands. This is just my personal experience from long range rifle competition for 6 years. So, I could care less how Sierra makes their bullets, I can only state that this is how the finished product arrives to the consumer and to be aware of ogive variations. Thanks for listening.

Don

[Bart B.]

Originally Posted by USSR

I expected a contrary view from you due to experience with you in the past on various sites, and you have not disappointed me. First, your assumption is that someone has bullets from a single lot. Definitely not the case, at least for me. Second, I have found that Sierra MatchKings vary in ogive dimensions by as much as 0.010", hence my statement about never seating a bullet less than 0.010" from the lands. This is just my personal experience from long range rifle competition for 6 years. So, I could care less how Sierra makes their bullets, I can only state that this is how the finished product arrives to the consumer and to be aware of ogive variations. Thanks for listening.

Don

I, too, have measured up to several thousandths spread in Sierra match bullet ogive reference diameter to heel, tip meplat or base. I grabbed them as they dropped out of the one machine's die sets making them. They were then seated in primed & charged cases for accuracy tests.

I and others have shot such bullets from zero to many thousandths jump to the lands. Sub two thirds MOA test groups at long range seated into the lands or some thousandths off.

There's no universal standard for ogive measuring. No lot of bullets will produce the same value across all methods. This is another "rubber ruler" syndrome.

What reference diameter did you use on bullets ogive? What did you measure it to? Were those bullets from the Sedalia, Missouri plant?

Thanks for learning.

[Unclenick]

Originally Posted by BartB.

Is it possible that 3 or 4 different lots of cartridge case brass run through the same set of dies would produce 3 or 4 peaks in a weight histogram?

If they reset all the die adjustments between runs, it certainly is. Perhaps even leaving the dies alone, but changing brass supply (the roll of wire being cut for the slug forming process or the roll of sheet brass for the disc forming process). If it were random variation, as you are describing for bullets off the same tooling, you expect a single bell curve distribution with a single peak.

I think it was Humpy who told me Lake City mixed bullets off different lots until, in the '60s, for a time, they had someone in charge who made them segregate machine output of the of the M1 Type match bullets. Apparently he wasn't very popular for doing that, but it would have made for more consistent bullets.