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Charge weight vs volume, in filled cases - mind blowing discovery?

 
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Old 02-17-2014, 12:42 PM
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Charge weight vs volume, in filled cases - mind blowing discovery?

This sprang up out of another thread. In the interest of not hijacking THAT thread, I have started this new one.

Quote:
Originally Posted by Max Heat

Lets start out by being in agreement that the closer you are to having a 100% filled case, the more efficent, in terms of velocity vs powder charge, the round becomes. From what I have found when working at (or very close to) 100% capacity, a change of say, 1% in the case-filling VOLUME of the charge makes MORE of a difference in velocity than a 1% change in the charge's WEIGHT! I have discovered that when using cases having randomly varying weights, the velocity spread will be considerably tighter if the charged is measured by dropping a marked or scribed rod into down into the case and perfectly matching the height (relative to top of the neck, so case lengths must match) level of the charge in each case, rather than by perfectly matching the weight of each charge. Does this blow your mind? But such measurements are only accurate enough when the case is filled to top of shoulder/bottom of neck minimum. This method appears to effectively negate variations in velocity caused by case weight (and thereby internal volume) differences.

Now that I have gone public with my theory, for which I only have a small amount of supporting data at this time, I welcome anyone to try replicating it, and either concur (agree) or digress (disagree) that this is indeed the "case"!


Quote:
Originaly Posted by DocB

Consider my mind blown!

I can see the validity in your method. It is a very interesting technique.

What do you use for a rod that's small enough not to displace the volume of the case?

DocB


Quote:
Originally Posted by Max Heat

The rounds were 7mm rem mag, and the rod was the one used to knock the case out of the sizing die and seat the primer, on a "Lee Loader" reloading kit for that round (referred to by Lee as the priming rod).

The theory should apply "across the board" however, to ANY round. But the larger the diameter of the rod, without it being so tight in the neck that it's free movement might be restricted, the more accurate the volume measurement will be. It also needs to be a measurement that is "within" the neck, for maximum accuracy of the measurement. So we ARE talking no less than FULL cases, for the technique to achieve the maximun result. Filling the case is simply matter of finding exactly the right powder. Also, the finer the line is that is marked on the rod, the more accurate the measurement will be. That's why I mentioned "scribing" the rod.

There IS a "variable" that could prove difficult to keep under control though. And that would be the "settling" factor, which obviously could have detrimental effects on the consistancy of the "true" volume of powder that is in the case. So absolute consistancy in the way each case is loaded & handled, will go a long way in assuring that the technique will work to it's full potential.

One factor that I have given some thought to (but not tested as of yet), is how the weight of the measuring rod might affect the settling factor. The rod I was using is steel, about 5 1/2" long, and almost large enough in diameter to completely fill the ID of the case neck. I don't know if a rod that is made of a light weight material would offer more consistancy or not. It could be that the weight of the steel rod might actually HELP to equal out any case-to-case settling differences - I don't know.

PS - I will admit that I did stumble upon this "discovery" purely by accident. If I hadn't been chronographing those shots, I would never have known.
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Old 02-17-2014, 06:13 PM
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Re: Charge weight vs volume, in filled cases - mind blowing discovery?

Quote:
Originally Posted by Max Heat View Post
1% in the case-filling VOLUME of the charge makes MORE of a difference in velocity than a 1% change in the charge's WEIGHT!
Quote:
Originally Posted by Max Heat View Post
This method appears to effectively negate variations in velocity caused by case weight (and thereby internal volume) differences.
It's a good observation but you're missing a variable/correlation in matched case volume.

What you're seeing falls inline with my contention that brass weight in no way correlates to actual case volume, which matters most to load performance due to load density at a given charge weight -and this side of 75Kpsi loads.
In other words, if we're to use charge weight for consistency(instead of your rod), we need to be paying attention to actual case volume(H20 capacity), and NOT brass weight.
With this, and consistent powder charging, load density should measure well with your rod.
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Old 02-17-2014, 06:25 PM
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Re: Charge weight vs volume, in filled cases - mind blowing discovery?

ADD: Some of this is of course dependent on case design and load density also.

If you want PROOF of density over powder or brass weight try this experiment:
Chamber a round with the barrel pointed up and slowly lower to a rest. Note the velocity on firing.
Now chamber a round with the barrel pointed down, and slowly raise to a rest. Note the velocity on firing.
The lower your fill, the greater the difference, regardless of same powder and brass weight.
You'll see the same from cases weighing the same, and charged the same, but of different H20 capacity(affecting load density).
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Old 02-18-2014, 04:58 AM
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Re: Charge weight vs volume, in filled cases - mind blowing discovery?

More info:

When I was using that method, which was between 1 1/2 and 2 years ago, the charge weight in the 7RM case was around 75gr of Reloader 19 - definitely a hot load. Bullets were Nosler 120gr ballistic tips. I haven't located the sheet with the actual chrono data on it yet. At the time, I do remember posting something to the effect of: "80% of the shots were within 5fps of 3680". I remember thinking that was pretty amazing , as I never saw such a low ES in ANY of my chrono data prior to that.

I haven't shot the RM since then, as my attention has shifted to the 7RUM, which I purchased not long after that. But now that I am settled on a powder that makes use of the full case capacity of the RUM (RL33), I am in a position to gather similar data. But with the local range now closed due to all of the snow that has been piling up around here recently, it may be an unknown amount of time until the testing can comence.
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Old 02-18-2014, 05:26 AM
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Re: Charge weight vs volume, in filled cases - mind blowing discovery?

I would also like to add that with my own experimenting wth case weight and case capacity, that the differences in case capacity were far less affected if the charge density, or put in other words the packing scheme of the case over actual charge weight variations made more of a difference to low ES and SD numbers. I used a 'swirl charge' technique of getting the powder to settle to the same height in the case by tilting a powder funnel and pouring the powder into the case so that the powder swirled around the axis of the funnel and made the powder settle into a denser column, as compared to being just dumped by the measure. The difference was substantial between 'as dumped' from the measure to being 'swirl charged' by the funnel.
Variations in velocity were dramatically reduced and the load was easier to tune with less rounds fired down range. It appears to work best with compressed loads, either heavily or mildly compressed slower powders have the largest differences.

Cheers.
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  #6  
Old 02-18-2014, 07:41 PM
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Re: Charge weight vs volume, in filled cases - mind blowing discovery?

Interesting thread. If volume is driving the performance, would a typical powder throw give you the same result without measuring using a rod in the case? It basically fills a chamber of known (set) volume and dispenses it when flipped. Any measure with a baffle should keep any "packing effect" to a minimum.

Maybe worth adding this method to your testing and see how it compares.
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Old 02-18-2014, 10:59 PM
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Re: Charge weight vs volume, in filled cases - mind blowing discovery?

Quote:
Originally Posted by varmintH8R View Post
Interesting thread. If volume is driving the performance, would a typical powder throw give you the same result without measuring using a rod in the case? It basically fills a chamber of known (set) volume and dispenses it when flipped. Any measure with a baffle should keep any "packing effect" to a minimum.

Maybe worth adding this method to your testing and see how it compares.

But the thrower's set volume, which does not vary (once set), is intended to represent a set weight. What I'm talking about is using each case's own volume to dictate exact amount of powder, who's weight is allowed to vary [slightly] from case to case, maintaining a consistant fill level of the case.

At that time I saw those amazingly close chrono #s, I didn't have quite as much equipment to work with as I have now. I've been loading my RUM rounds individually by weight, searching for the optimum powder type & charge weight. But now that I've settled in on the powder, and almost narrowed down to the exact "base" charge weight that I want to go with (right around 102 or 103gr, which IS into the neck of the case), I'm going to have the powder measure attached to my progressive press to drop a charge into the cases. Then make final tweaks to each charge, according to readings from my "powder cop" die, before sending the cases on to the seating station, and out.

The powder cop does actually have a rod that "drops" into the case to allow visual verification of the charge level, before it moves to the seater. If the charge level is up into the case neck, the rod measurement will be roughly 5 times more accurate than a similar measurement would be if the charge level is below the shoulder. The rod uses an o-ring for reading the charge level. I think more accurate readings could be taken [and therefore finer adjustments possible] if a small, very thin washer [of appropriate size to fit the rod] is placed on top of the o-ring, with something of equal thinness that it can be lined up with.
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