Determining COAL: Process question

[daved20319]

So I'm getting started working up loads for my new 6.5 Creedmoor barrel on my Savage 12 FV. I use the Hornady OAL gauge with a Hornady Dummy case to determine max COAL to the lands. My question has to do with that dummy case. It's shorter than my brass fired in that chamber by about .004", so call it .002" shorter than my target length. So the question is, do I add that .002" to my max COAL, do I just ignore it, treating it as a safety cushion, or is there some middle ground I should be considering?

If it matters, I plan to start with the Barnes 145 gr. Match Burner, using new and once fired Alpha brass. Fired brass is 1.559" base to datum using a Hornady case comparator, the Hornady dummy case is 1.555". Barrel is a Criterion 28" SS bull barrel that's basically only been broken in, 30 rounds through it so far. Goal is F-class (maybe) and longer range steel. Thanks.

Dave

[CFJunkie]

COAL is measured from the base of the cartridge case to the tip of the bullet.
The COAL measurement has nothing to do with the length of the brass case, only the measurement of the cartridge base to bullet tip.
That measurement is not a measure of the chamber depth which is measured from the base to the bullet ogive touching the lands and has nothing to do with the tip that is actually sitting well into the chamber but not touching the lands.
The overall length of the round (COAL) and the base to bullet ogive would also remain the same if you keep the COAL the same regardless of the trim length of the brass.
However, if you stick the bullet in the lands and measure the COAL, the jump from the bullet ogive to the lands would be zero because the ogive is touching the lands.
That would cause an spike in the chamber pressure when fired.
I would recommend you set the COAL back 0.020 from a COAL measurement with the bullet in the lands for starters.

A slightly shorter cartridge neck with identical COAL has the same effect as trimming the case shorter by 0.004 and then setting the COAL the same as with an untrimmed case.
There is a slight, but largely insignificant, difference in the available neck to hold the bullet body, and a slight drop in velocity (about 1 fps per 0.002 reduction) and a slight drop in chamber pressure.

[257ack]

Daved, they make a special tap for the cases. I forgot the thread size but if you do an internet search you should find it. I take a case that has been fired 2x and tap it without resizing or with a slight(001) sholder bump. I then seat a bullet in an unprimed case and polish that bullet with scotchbrite or 000 steel wool. I then chamber the dummy round and look for faint land marks. I usually need to seat one about .002 longer. I start at just touching and work my way out until i have about 010 jam. If I have still not found a sweet spot, then I will back off no more than 008 jump. Hope this helps, Jerry

[daved20319]

COAL is measured from the base of the cartridge case to the tip of the bullet.
The COAL measurement has nothing to do with the length of the brass case, only the measurement of the cartridge base to bullet tip.
That measurement is not a measure of the chamber depth which is measured from the base to the bullet ogive touching the lands and has nothing to do with the tip that is actually sitting well into the chamber but not touching the lands.
The overall length of the round (COAL) and the base to bullet ogive would also remain the same if you keep the COAL the same regardless of the trim length of the brass.
However, if you stick the bullet in the lands and measure the COAL, the jump from the bullet ogive to the lands would be zero because the ogive is touching the lands.
That would cause an spike in the chamber pressure when fired.
I would recommend you set the COAL back 0.020 from a COAL measurement with the bullet in the lands for starters.

A slightly shorter cartridge neck with identical COAL has the same effect as trimming the case shorter by 0.004 and then setting the COAL the same as with an untrimmed case.
There is a slight, but largely insignificant, difference in the available neck to hold the bullet body, and a slight drop in velocity (about 1 fps per 0.002 reduction) and a slight drop in chamber pressure.

Your reply makes it clear that I didn't do a good job framing my question, something I was concerned about. So in the interest of clarity, disregard any mention of the term COAL, and see if I can make myself clearer. Instead, focus on just base to datum.

For all intents and purposes, measuring brass fired in your particular rifle, that's going to be the length of the chamber from bolt face to the datum point on the case shoulder, less spring back. So, if you have 2 different pieces of brass, one .003" longer than the other, that difference isn't going to be picked up when using the OAL gauge, as all you're measuring there is shoulder to lands. But the shorter brass will result in a shorter BTO length, and trim length has nothing to do with that. Using that measurement to set your seating depth, then seating them in the longer brass, means the bullet is .003" farther off the lands than what you measured. At least that's how it looks in my head, am I totally out to lunch? And even if I'm right, the bottom line question is, does it matter, at least in this case? Of course, if the gauge case was longer than my shooting brass, that could cause problems, couldn't it? I mean, suppose you want to just be touching the lands, if the numbers in my case were reversed, you'd actually have a .003" jam, wouldn't you?

Again, I have no idea if this is significant, although considering the number of folks I've seen recommending seating depth increments of .003", I have to think it's at least worth thinking about. Thanks for the responses so far, guys, later.

Dave

[Woopig]

Your coal gauge is sized to saami specs,I.e. will safely chamber in factory chambers. Go with cfjunkie and then tinker with powder charge and seating depth. I truly hope you come up with an impressive load and you will.

[CFJunkie]

Dave,

Sorry if I misinterpreted your question.
I think you are talking about two different elements of the reloading equation.

Pardon me if I am dense, but I still I don't understand how your concern about base to datum measurement has anything to do with measuring base to ogive using your tool.
The base to ogive length and the 'jump' it produces is determined in reloading by setting the seating depth of a particular bullet, which also has nothing to do with the base to datum measurement.
If your tool is the same as mine, the Hornady case has a slightly oversize neck that allows the bullet to slide forward in the neck of the case and touch the lands.
When you tighten the tool and measure the base to ogive, the base to datum length has nothing to do with the base to ogive length because the bullet slid along the neck to touch the lands.
The position of the shoulder datum doesn't change where the bullet seats in the lands.

Yes, the base to datum measurement may be shorter in one piece of brass than another, but most center fire brass does not seat on the shoulder.
The brass seats on the base against the bolt although the shoulder might expand to fit the chamber after several firings.

In my experience, chamber erosion doesn't effect the shoulder, only the lands depth (and I regularly measure my chamber to gauge erosion).
Most of my rifles have over 3,000 rounds down their barrels ( I shoot over 5,200 rounds a year) so chamber erosion does force adjustments in seating depth in order to maintain a specific jump to the eroding lands.

Relative to shoulder position, I have a .308 that won't allow me to neck size more than once because the shoulder has to be set back by full resizing after firing with a neck size.
If you resize your brass after every firing, you may never have your shoulder expand to fill the chamber.
If it does, you might have trouble closing your bolt.
If your rifle has a tight shoulder like that, you may have to be more concerned with the base to datum measure varying.
However, it won't have anything to do with the jump or seating depth of the bullet.

I have an identical .308 that I could neck size seemingly forever and the brass never has to be fully resized.
That probably means that the shoulder was cut pretty deep and expansion did not create a problem.

[Texas10]

I'm going to try to save you a whole lot of time, effort and frustration. What really matters is finding a reliable starting point for bullet seating, and there is an easy way to do that.

First make sure the chamber and throat are clean. Then take one of your shot cases and neck size it. Back off your die if you need to so you don't bump the shoulder back. Then seat a bullet long. I prefer to seat and pull the bullet a few times to get a reliable but not too tight seating force. You can also rub a little die wax on the bullet if you find that works better.

It'll help to remove your extractor too, but it's not necessary.

Now chamber the round with a firm push on the bolt. If you've removed the extractor as suggested, open the bolt and tap the case out with a cleaning rod inserted from the muzzle end. Measure base to ogive and record that dimension. Repeat a few times to get a good average.

This is your hard jam measurement and a reliable starting point THAT WILL CHANGE AS THE BARREL WEARS. Chasing the lands is what we say.

Back off that dimension by say, .020 and start your seating depth study using a moderate charge and working your way deeper into the case until your find paydirt. Berger has some good suggestions as to how far to move the bullet at each stage.

I'm shooting a 140 gr SMK at .020 off and it's extremely accurate.

[257ack]

The Hornady modified case is not often the size of your chamber. You can make you own custom modified case with a piece of twice fired brass. There is a special order tap that you can buy to tap your fired case to use with your oal gauge. This will give you an accurate reading. I have been using this gauge for years. It used to be Stoney Point before Hornady. I usually bump the sholder back less than .001 with my bump die to make sure that its not too long. Works for me

[Woopig]

Texas10 do you use a compensation tool?

[daved20319]

Okay, guys, it's apparent I STILL haven't made myself clear, so I'll take one more swing at it. There are 2 different measurements under discussion. First is base to datum, i.e. case length from the base to a point on the shoulder based on SAAMI specs. You use a case comparator to determine this dimension, and it's used to set your shoulder bump.

Second is max bullet seating length to the lands, measured either from base to ogive or base to tip. As tip length can vary quite a bit, and really only matters if you're length restricted by a magazine, most use base to ogive. But here's where it gets muddy.

The Hornady OAL gauge and dummy case are used to find max length to the lands, but that length is referenced from where the case shoulder bottoms out in the chamber, NOT from the case base. So I have 2 cases, one measures 1.555" base to datum, the other measures 1.559". However, shoulder to lands (ogive) is the SAME DISTANCE, regardless of which case I use. But you MEASURE base to ogive, so there will be a difference of .004" between the 2 cases.

So if I STILL haven't made myself clear, I give up, as my only other option is drawing a picture, and I suck at art :rolleyes:. All that said, I think I've clarified the situation in my own head, the only question still remaining is, does it really matter? Later, guys, and thanks for sticking with this.

Dave

[JW]

Okay, guys, it's apparent I STILL haven't made myself clear, so I'll take one more swing at it. There are 2 different measurements under discussion. First is base to datum, i.e. case length from the base to a point on the shoulder based on SAAMI specs. You use a case comparator to determine this dimension, and it's used to set your shoulder bump.

Second is max bullet seating length to the lands, measured either from base to ogive or base to tip. As tip length can vary quite a bit, and really only matters if you're length restricted by a magazine, most use base to ogive. But here's where it gets muddy.

The Hornady OAL gauge and dummy case are used to find max length to the lands, but that length is referenced from where the case shoulder bottoms out in the chamber, NOT from the case base. So I have 2 cases, one measures 1.555" base to datum, the other measures 1.559". However, shoulder to lands (ogive) is the SAME DISTANCE, regardless of which case I use. But you MEASURE base to ogive, so there will be a difference of .004" between the 2 cases.

So if I STILL haven't made myself clear, I give up, as my only other option is drawing a picture, and I suck at art :rolleyes:. All that said, I think I've clarified the situation in my own head, the only question still remaining is, does it really matter? Later, guys, and thanks for sticking with this.

Dave

Your are correct and CF Junkie is correct ( that helps a lot I know)

You are looking for a starting point on the length and that is it. I do not think the .002-.004 difference you mentioned will come into play.

Typically you will adjust depth ( and powder), to what shoots the best groups anyway.
The only issue would be if you are planning to jam the bullets or touch the lands, but even at that length you are going to have to see what length shoots the best.
Enjoy the new rifle
Jack

[yobuck]

I'm going to try to save you a whole lot of time, effort and frustration. What really matters is finding a reliable starting point for bullet seating, and there is an easy way to do that.

First make sure the chamber and throat are clean. Then take one of your shot cases and neck size it. Back off your die if you need to so you don't bump the shoulder back. Then seat a bullet long. I prefer to seat and pull the bullet a few times to get a reliable but not too tight seating force. You can also rub a little die wax on the bullet if you find that works better.

It'll help to remove your extractor too, but it's not necessary.

Now chamber the round with a firm push on the bolt. If you've removed the extractor as suggested, open the bolt and tap the case out with a cleaning rod inserted from the muzzle end. Measure base to ogive and record that dimension. Repeat a few times to get a good average.

This is your hard jam measurement and a reliable starting point.

Back off that dimension by say, .020 and start your seating depth study using a moderate charge and working your way deeper into the case until your find paydirt. Berger has some good suggestions as to how far to move the bullet at each stage.

I'm shooting a 140 gr SMK at .020 off and it's extremely accurate.

This is the way it was done years back when i started loading.
At the time there were no special tools for hand loaders to use, such as OAL gauges, comparitors etc.
Some of us still use that method, and it is probably the best method overall.
However, since cartridge length is determined by the ogive of a particular bullet, it is necessary to find where that meets the rifleing.
Using the method as explained above is correct, however the bullet itself should be smoked with a match or coated with a sharpie in order to determine
when there is no longer any contact with the lands.
From that point, it would be correct to determine how far from the lands you actually are as you check the oal each time you seat it deeper.
A very faint mark on the bullet could be established as (kissing the lands), deeper settings would be the Actual distance from them, and of coarse (only)
pertains to the particular bullet being used.
I dont see where COAL means anything, other than for magazine length of the cartridge.

[257ack]

Here is the way that I do it: I use bushing dies and take the bushing out for this procedure. I shoot a fired case that has been necksized only until I feel the bolt getting a little stiff. I put the Hornady headspace (bump gauge) on my calipers and bump the shoulder back .002. I take that fired case that perfectly (fits my chamber) and tap the primer pocket to screw onto my straight oal tool. I screw that onto the tool making sure that it bottoms out on the oal tool. The neck has not been sized so a bullet will slide in it more firmly than the original Hornady modified case. I place the bullet that I am going to use in the mouth of that case. I then push the tool with that modified case into the chamber until I feel the shoulder bottom out in the chamber. I then gently push the rod of the oal tool to push the bullet in until I feel it touch the lands. I then put the bullet gauge on my Hornady tool and measure with my calipers. I then set my seater die to that measurment

[GrenGuy]

Dave, I understand Your question and Your concern. The answer to You question “does it matter?”, depends on the amount of accuracy You expect. If 1” at 100yds is acceptable, it doesn’t matter. I shoot IBS competition at 600/1,000yds, and in that case it does matter. Until I started shooting competition, I never had to full length size, neck sized only. Competition shooters who use the Hornady/Stoney Point system absolutely need a fired modified case to achieve acceptable accuracy. Cartridge Base to Shoulder Datum is Headspace, minus tolerance. For maximum accuracy, that tolerance is .002” for practically everyone. Too much Headspace could result in Fail to Fires. Your not in that situation. Using the equipment You have, because the case has not been fire formed, there will be that amount of error in Your final result. When I reloaded for hunting, that amount of error didn’t matter. Now that I compete, that and everything else does matter :-(. Ultimately, only You and Your targets can decide what matters. Good Luck!

[PhilC]

Using the equipment You have, because the case has not been fire formed, there will be that amount of error in Your final result.
Spot on. Obtain a 5/16 - 36tpi tap and 7.3mm drill bit, (tap/bit combos readily available on Amazon) and modify a case fire formed in your rifle, or, send a fire formed case and $9 to Copper Creek Cartridge Co.

Then again, the method described by Yobuck has been used forever and, other than donating a case and bullet, is free.

[yobuck]

Spot on. Obtain a 5/16 - 36tpi tap and 7.3mm drill bit, (tap/bit combos readily available on Amazon) and modify a case fire formed in your rifle, or, send a fire formed case and $9 to Copper Creek Cartridge Co.

Then again, the method described by Yobuck has been used forever and, other than donating a case and bullet, is free.
My cases as a rule dont last very long anyway due to primer pocket expansion.
So i always have some to use for making dummy rounds.

[daved20319]

Thanks again, guys. I think I have a handle on what the issue is and what I'm going to do about it. Namely, compare dimensions of my gauge case vs. fire formed, and make adjustments as needed, double checking everything as I go, of course. Getting ready to load some test rounds, now I just need to figure out the best technique to economize component use while still getting reliable results. Later.

Dave

[Robinhood]

Then there is bullet R.O.

[yobuck]

Then there is bullet R.O.
Yes there is, along with lots of other things that can cause misses.
I had the (pleasure) of having a few serious 1000 yd Williamsport shooters as friends.
I was constantly being reminded that i would never shoot well the way i loaded.
But well enough to kill a deer @ 1000 yds and well enough to win a match or even a relay @ 1000 yds are two different things.
And we always somehow managed to kill more deer than they did.
By cheating. lol

[geezerhood]

Pardon me for not responding to your original thread question(s), but I thought you might find these videos by Eric Cortina helpful and they might give you some answers as well. Using many of his suggestions has simplified my reloading process and improved my handload accuracy.

Finding Jam
https://www.youtube.com/watch?v=a7vjgEgnhHk

Chasing the Lands
https://www.youtube.com/watch?v=oRXlCG9YZbQ

Chasing the Lands Part 2
https://www.youtube.com/watch?v=9FKq8Jj8YEI

Setting your Shoulder Die
https://www.youtube.com/watch?v=htvk1UYOXm8

Seating bullets to final depth before match
https://www.youtube.com/watch?v=43XTH8Pm9yA

Load Development Seating Depth Test
https://www.youtube.com/watch?v=rFI1DyaBwho