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Old March 22, 2012, 10:25 AM   #1
Wyoredman
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Cartridge Over All Length questions

Shooters and handloaders,

In my recent quest to "get smart" about my reloading and turn myself into a handloader, I have posted many questions here on TFL.

Not to long ago I posted a question about cartridge sizing that is still an active thread and the conversation is teaching me more with every post. With that said, I now have another question reguarding COAL.

I worked up some loads for my .243 win Remington 700 using H4350 and Sierra 100gr SPTBTs. I seated the bullets to the depth recommended in the Sierra manual of 2.650". After shooting the load work up, the 40.0gr charge (near max in the Hodgdon manual) was best with a 5 shot group of 0.892". All other test loads grouped over 1.0". I observed no pressure signs with 40.0 gr.

Last night I placed a bullet in the chamber of my rifle and held it agains the lands, placed my cleaning rod in the barrel and made a mark. I then did the same with the rod against the bolt face. When i measured the differance (using callipers), I came up with a measurement of 2.703".

I want to load another test batch of ammo with the bullets seated to 2.700", 0.050" longer than the book COAL recommendation.

My question is - Should I start over with the load work ups, or can I just try and fine tune the 40.0gr charge at this longer OAL? I would guess that pressures will be less and I will be safe, but I am not sure. Any help would be appriciated. Thanks in advance.
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Old March 22, 2012, 10:39 AM   #2
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Figure peak pressure can go up about 20% if you seat the bullet out to touch the lands. Reducing the powder charge 10% will reduce peak pressure about 20%, so 36 grains is what you want to use when you are out that far.

Take a look at Berger's procedure for finding a seating depth sweet spot. When you find the best one, work the load back up. Then, if you want, try a couple hundredths either side of center to see if further improvement occurs. The problem is that the process is interactive, with change in depth changing pressure and barrel time, affecting the charge weight tuning.

Note that it is not uncommon to find more than one seating depth sweet spot, with one not too far from the lands, but another when the bullet bearing surface is around one caliber into the case neck. One may be better than the other. It doesn't happen with every bullet and chamber combination but it happens often enough to look for, as the shorter COL's don't usually have the magazine feed issues the long seated ones can.

Below illustration is a measured 18% change reproduced with kind permission from Jim Ristowe at RSI. The upper three pressures are with the bullet touching the lands, while the lower four traces are with the bullet 0.030" off the lands. Same powder charge. Different bullet nose and chamber freebore and throat shapes affect this differently as they vary the amount of gas bypass that occurs before the bullet obturates the bore.

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Old March 22, 2012, 10:55 AM   #3
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Wow! Thanks for the link. I knew I could get an answer that made sense.
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Old March 22, 2012, 12:53 PM   #4
CS86
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Quote:
Take a look at Berger's procedure for finding a seating depth sweet spot
I took a look at this and thought it was pretty interesting. He talks about finding the sweet spot for the COAL by work up loads with the different COALs, but what about powder? There is a sweet spot with powder also, and this changes when you start to play with the COAL, doesn't it? I would think that you would be testing lots of rounds by changing the 2 perameters. Each COAL should have its own load work ups shouldn't they?

I didn't realize that chaning the coal by .005 would make that much of a diff, but according to Berger's procedure it can make a substantial difference.
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Old March 22, 2012, 01:09 PM   #5
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CS86,

I am going to test the H4350 with a seating depth of 2.700". I have a chronograph, so I will post my results.

According to the Berger link, I should be seeing higher pressures with the longer COAL, so my velocities should increase some for each load also. It is going to be interesting. Looks like I have a good day of testing ahead of me!
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Old March 22, 2012, 01:31 PM   #6
mehavey
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Quote:
...going to test the H4350 with a seating depth of 2.700".
Wyoredman, I recommend against that as your starting position for two reasons:

- The Cleaning Rod OAL measure is a lot better than nothing, but it is also imprecise
by many thousandths (especially considering how tiny a thousandth is....) *

- Bullet ogive/lands contact point vary "a couple" of thousandths relative to bullet length
even in the same lot, and you are targeting only 3-thou standoff -- iffy at best. **

I suggest you start at a measured 20 thou off (which is pretty standard), and work in 5 thou at a time while watching the chrono and the brass.


* Suggest you get the Hornady OAL gauge if you want to get precise distance-to-lands measurements

** Suggest you also get a Hornady Comparator set if you want to control your play close to the lands ***






*** No. I don't own Hornady stock.

Last edited by mehavey; March 22, 2012 at 01:38 PM.
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Old March 22, 2012, 01:57 PM   #7
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Mehavey,

At 0.020 of the lands, my OAL would be +\- 2.680" assuming my chamber measurement of 2.703" is close. This is 0.03" longer than the 2.650" OAL the Sierra manual has listed. Pretty much right in the middle of the allowable (by my measurements) length.

I just seated a dummy round at 2.700" and used a match to "smoke" the bullet. I tried it in my rifle and it does not show any contact with the lands.

I have tested the H4350 at the 2.650" length, why can't I start the test over at the 2.700" length and stop if I see pressure signs? If I start at the recommeded starting load in the manual, shouldn't I be safe?

My reasoning is this: Once I have data from the minimum (Manual) and the Maximum (my measurement) COAL, I will have a better idea of which side of the length scale to concentrate on for maximum accuracy. If the 2.650" cartridges produce beter groups than the 2.700" ones, then I can start moving up from 2.650". If the 2.700" cartridges show beter accuracy, I can start moving down from there. I was thinking this would be a better process of elimination.

If I start at 2.680" and see better performance, then which way do I go on my next test? Increase or decrease length? See my reasoning? Thoughts?

Or, i could use the berger test with only one powder charge and differing lengths. But I was trying to test powder charge variation and length in the same test, as they both have a birect effect on accuracy. If the same charge turns out to be accurate with both OALs, then I am on to something, correct?
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Old March 22, 2012, 02:00 PM   #8
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I'd start at 2.680 and move in 0.005" at a time.
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Old March 22, 2012, 02:02 PM   #9
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I do see your point. This stuff could be work
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Old March 22, 2012, 02:20 PM   #10
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So are you going to work your loads up with each COAL, or only use one measurement of powder for each COAL?
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Old March 22, 2012, 04:10 PM   #11
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I,ve got the base work-up with the 2.650" OAL so I think I will do a whole other set of charges with the 2.680". Mehavey has me convinced that 2.700" may not be safe.

This way I can surmize how the length affects the accuracy and velocity at the different charges. I will let you know how it comes out.
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Old March 22, 2012, 04:11 PM   #12
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My suggestion has been to find the COAL that gives you best groups w/ a "reasonable" charge (first)
Then play with the powder charge around that figure.

Others start at 0.020 standoff and find the optimal powder charge
Then work in and out on the COAL.

Whichever you choose, do it systematically
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Old March 22, 2012, 04:17 PM   #13
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Quote:
...2.700" may not be safe.
It might not be so much a safety issue (though as UncleN notes it could be), but more
a "...are you into -- or are you out of..." the lands. Sitting at 2.7 might
find you both, and that can get very inconsistant.


postscript: My AR shooting 55grainers loves to have the bullet as much as 10 thou
into the lands -- but the powder charge was worked up against that condition.
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Old March 22, 2012, 05:03 PM   #14
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Quote:
My suggestion has been to find the COAL that gives you best groups w/ a "reasonable" charge (first)
Then play with the powder charge around that figure

This is just my 2 cents coming from a pretty amature loader, but... I've worked up loads for my 223 bolt with .15 backed off of the max COAL using the hornady gauge. When working up loads you can see a good difference in accuracy between .5gr powder increments. Given that Berger's procedure says you can see substantial accuracy change between .002-.005 COAL difference I would think that for each COAL you would have to work your loads again to see which pair the best. This makes me think that you wouldn't really benefit form working with a single powder load and just changing the COAL.
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Old March 22, 2012, 07:25 PM   #15
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Quote:
...wouldn't really benefit form working with a single powder load and just changing the COAL.
I don't necessarily disagree, but I tend to search for smallest/most consistent not necessarily final best) groups within a range of OALs; lock that OAL and then play with the powder charge around it to improve the groups further to the next stage of performance.

Others might do it the other way`round: some random OAL first (e.g., 20 thou off the lands) then find the most consistently-performance powder charge at that OAL -- then play with the OAL.

Adjust Powder -- Adjust OAL -- Adjust Powder -- Adjust OAL -- Adjust powder -- Adjust OAL ...........
Pick your starting stage and cycle from there.
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Old March 23, 2012, 09:39 AM   #16
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Wyoredman,

Again, I am the fan of transfers, standards, verifying, bullet hold and the running start, I want my bullet to have the jump, and, I am the fan of ‘time is a factor’.

When adjusting the seater die and accuracy is very important, I make transfers, I use the transfer to move the dimensions from the chamber to the seater die, I do not have Redding seaters with the indicator on top, all of my seater dies are adjustable in thousandths of an inch, not a problem for me but my seater dies come with that feature at no extra cost.

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Old March 23, 2012, 10:00 AM   #17
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All right guys, Another snag in the process.

Last night, I was making up some rounds for this test. About half way through I opened a new box of Sierra bullets and after seating the first one, realized that the points on the bullets from this new box are not as "long" as the old box. (Both boxes are the same type and product #.) Now I have a problem measuring COAL!

The new box bullets seem to be 0.02" shorter than the "old" box bullets!

Is there a way to figure COAL from the ogive without the comaparator? I ordered one today, but I wanted to shoot this test this weekend. Damn the luck?
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Old March 23, 2012, 10:04 AM   #18
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My problem with the cleaning rod method is that it introduces variables which are irrelevant to the relationship between your handload and chamber. Adjust your seating depth to give at least 0.005" standoff from the leade of the rifling, with the longest of 20 measured, loaded cartridges. If you crimp, be sure to measure them after the crimp is applied because it often adds up to 0.005" in OAL to a cartridge.

In my experience, 0.005-0.010" of standoff still gives excellent accuracy while eliminating the potential problems associated with seating out to engage the rifling. Be advised that I am not a benchrest shooter and I have authored no widely-read chronicles on the subject pf precision reloading.
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Old March 23, 2012, 11:16 AM   #19
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Quote:
The new box bullets seem to be 0.02" shorter than the "old" box bullets!
Exposed lead tip? (If so, not unusual.)

The homemade comparator guage for a 243Win would be a 1/2-3/4" square block of aluminum
w/ an exact 6mm hole drilled into it.

`Know any friendly machine shops?


.

Last edited by mehavey; March 23, 2012 at 11:23 AM.
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Old March 23, 2012, 11:36 AM   #20
CS86
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Quote:
the points on the bullets from this new box are not as "long" as the old box.
Would the difference in points matter so much? I would think you would seat at the same depth as you original settings because the seating die pushes the bullet down using the cone shape piece in the die. surely the bullet shape hasn't changed and its just the tip. hopefully this makes sense to you guys.
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Old March 23, 2012, 12:08 PM   #21
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Yep, exposed lead tips. Thanks for the heads up on the machine shop.

CS86, the differance matters on how to determine where to set the seating die. Do I use the long tip bullets to determine 2.680", or do I use the shorter tiped bullets? Measuring to the point, there is a 0.02" variation! The seating die uses the ogive to push the bullet into the case, not the tip, but I am measuring OAL from the tip.

BTW, I just measured more bullets from the same box, the variation is within each box!

I need a bullet comperator!
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Old March 23, 2012, 12:20 PM   #22
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Set the die up using whatever bullet you measured the 2.700 distance-to-land dimension with.

- Adjust for that 2.700 figure and mark the slot position of the seater stem
- Adjust stem down to 2.695 and mark the slot position again
- Adjust stem down to 2.690 and mark the slot position again
- Adjust stem down to 2.685 and mark the slot position again
- Adjust stem to 2.680 and load five of whichever bullet box

...... at this point the seater ogive contact it set. It doesn't know/care anything about the bullet tip

- Adjust stem back up 2.685 mark the load five (of whatever box)
- Adjust stem back up 2.690 mark the load five (ditto)
- Adjust stem back up 2.695 mark the load five (double ditto)
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Old March 23, 2012, 12:24 PM   #23
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Thank you, sir. I don't know why it seems so simple when someone else explains it to me.
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Old March 23, 2012, 06:26 PM   #24
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Why not keep it real simple and use the load book that applies to the bullet you use. I did not read through the whole thread, but screwing around measuring COAL with a cleaning rod is not good. I have made a few chamber reamers in my time and if you are really into it you COULD figure the ogive contact point +/- pretty close with the chamber prints. It would give you the diameter of the bullet at contact. The reason that load books can give you an OAL is because they are figuring on their bullet.
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Old March 23, 2012, 09:46 PM   #25
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Not necessarily. They usually give a COL to stay in SAAMI spec and keep a good hold on the bullet. Your throat can be quite different from another fellow's and best accuracy seating depth in it can be quite different from what it is in that fellow's gun.


Wyoredman

I think what Mr. Stecker at Berger wrote is a bit confusing in its wording, but here is how I understand it:

When Mr. Stecker says a meaningful change is 0.002" to 0.005", I believe he means that is the limit of useful resolution of seating depth adjustments. In other words, that's the kind of round-to-round variation in ogive distance off the lands you normally find after seating bullets, so attempting to make adjustments any smaller for any purpose whatsoever is pointless.

I don’t think Stecker meant you would necessarily see a statistically significant change in precision (group size) from making an adjustment as small as 0.002” to 0.005”, but even if you can, he seems to think it’s beside the point. He says they’ve found seating depth sweet spots are at least 0.030" wide, so all you really need is to test at seating depth increments narrow enough not to skip over a sweet spot that wide. That's a 0.030” step.

I’m not sure how generally true the above is. It applies to their VLD long secant ogive bullets, obviously from their testing, but I’m not convinced a tangent ogive or a short ogive can be counted on to behave the same in all guns. I’ve seen 0.010” make a difference, but that was in guns already shooting cloverleafs, and as I’ve explained elsewhere, small improvements tend to be masked when applied to large groups.

It seems reasonable to me that once you locate a sweet spot you should use smaller increments of seating depth either side of it to find where it’s ends lay, so you can center your loads in the middle of it. That will give you some tolerance slop if you have length consistency issues. In some guns it may also yield a bit of further improvement, but no guarantees there. The bottom line, to my mind is, if you think you have a gun that would benefit from a really small change in seating depth, it still makes sense to use the Berger approach to narrow the search before investing in ammunition to do a load workup for each small change.

If you follow Berger's target recommendation to start by working a load up for best accuracy with a bullet jammed 0.010" into the lands, that’s a logical starting point. That’s because a load that’s safe there should be safe throughout your test range. It will see its highest pressure jammed into the lands, and as you seat it a little deeper in search of a sweet spot, the pressures with that same charge will tend to get lower until you get deeper into the case than you likely need to go. The Berger test range should not take you south of that point. The old Dr. Lloyd Brownell study has an example of a round nose bullet's peak pressures in .30-06 getting steadily lower as it is backed away from the lands until it is 0.250" deeper than touching the lands. In his case there was only about 10% more pressure when touching the lands than at minimum with a constant charge weight, so it's also an example of the bullet shape and chamber shape affecting the whole phenomenon.

Bottom line, if you first work up a best precision load with the bullet jammed 0.010” forward of the contact point, then that same charge will be safe through your trials seating deeper up to Berger's recommended 0.150" maximum distance off the lands. That makes it a good alternative to just using a standard minimum load from a manual.

But you still have another issue, and that is that as the pressure drops the velocity drops and the barrel time increases, and that can move you off a sweet spot for powder charge or move you onto one. Thus you wind up crediting seating depth with a change that could have been made just with powder charge adjustment alone. So, what's a fellow to do to try to get these two variables segregated?

The obvious thing is also the least practical. You seat the load deeper in four steps, as recommended, then go through a whole powder charge accuracy load work up for each step to see which does best. Then if you make a fine adjustment, maybe have to go through the process with the change yet again. I'm no more thrilled with that prospect than you likely are. It's a lot of work, but also, if you can't wrap your work up in a single day, you complicate life with different conditions and other factors you don't really need in the way.

A better idea seems to me to be to take advantage of your chronograph. Create an accuracy load with the bullet jammed in the lands, and get the mean velocity for that load. Now shoot at least three rounds at each of the other depths and see if you can see one of them tightening. Take the tightest one, or if there is no clear winner just pick the middle and adjust the charge up to match the velocity of the tuned load that was in the lands. Assuming that’s a bigger charge, take the difference and multiply it by the square root of 2 (1.4 is close enough) and add the result to the original tuned charge. This should come very close to matching the barrel time of the tuned charge. It should give you a comparison of the two seating depths alone.

So, let’s make an example. You load a .308 in a Winchester case, with the 175 grain MatchKing jammed 0.010” into the lands and find 41.4 grains of IMR4064 gives you best accuracy at a velocity of 2560 fps. You next seat a bullet 0.160” deeper, and find you now need 42.8 grains of 4064 to get to 2560 fps, matching the velocity of the first load. The difference is 1.5 grains. You multiply 1.5 grains by 1.4, giving you 2.1 grains, and add that to 41.4 grains. You now have 43.5 grains. This will be a little faster than the original velocity, but it should be about right to produce the same barrel time as the original load, so it should already be pretty well tuned. You can do the same at the other seating depths, and it should come out close enough that you get a pretty true comparison of the effect of the seating depths themselves. You can re-tune the final load to be sure, but you won’t likely find you need to go more than a couple of 0.7% steps either side of it.
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