case capacity and powder charge ?

[rebs]

If charge weights are weighed precisely and you have cases with different capacities then doesn't that effect your precise powder weights ?
Say you weigh your powder at precisely 24.0 grains and load it in cases with difference capacities then your powder weights are not precise any more right ? They might as well be 23.8 or 24.2 etc depending on case capacity right ?

 

[Straightshooter1]

While case capacity does produce a difference, the issue is . . . just how much a difference in case capacity makes a noticeable difference given any fixed powder charge??? Is the difference significant enough to warrant the effort to sort?

Along with that, if the powder charge is at 100% of case capacity, does a .2 gr difference in case capacities make as much difference with the same cases charged to 85% of capacity?

It'd be interesting to me to see the science for this.

 

[Dusty Stevens]

Yes. You got it figured out. Almost comical what people worry about and theres something that obvious right there

 

[Zero333]

That's why we use brass from the same lot #. At least for the uber precision ammo.

Even for plinking ammo I prefer to use the same Headstamp brass.

 

[Ned Ludd]

Even brass from the same Lot# can have a wide variance in internal volume. Minor volume variance on the order of a tenth grain or two won't typically make a large difference; but the impact is also related to the specific cartridge used - the smaller the cartridge capacity, the greater the effect of internal volume variance.

The easiest approach to determine the magnitude of the effect is to measure case volume for a sample of your brass, then load up a few of the highest and lowest volume cases with a constant charge weight and measure velocity from the two extremes. If the difference between the average velocity values for the high/low volume cases is not greater than the ES of either set of cases, you won't be able to shoot the difference anyhow. If the difference in velocity between the two sets is greater than the ES, then you'll probably want to sort cases by volume...certainly for anything "important".

 

[mikecr]

Say you weigh your powder at precisely 24.0 grains and load it in cases with difference capacities then your powder weights are not precise any more right ?

No, its "precisely 24.0 grains" powder weight. Nothing changes what is given.

If you really want to know whether capacity variance with a given charge affects pressure, then without further qualification, the answer is maybe.

 

[Texas10]

If charge weights are weighed precisely and you have cases with different capacities then doesn't that effect your precise powder weights ?
Say you weigh your powder at precisely 24.0 grains and load it in cases with difference capacities then your powder weights are not precise any more right ? They might as well be 23.8 or 24.2 etc depending on case capacity right ?

Your powder weight stays the same, but the percentage of case volume that is not filled (free space) with powder obviously changes, sometimes to the detriment of your load.

Case in point: 223 Rem, Federal Case, 25.2 gr Varget, 69 gr Lapua seated .005 into the lands. This bullet really shot better jumping .005, but trying to seat it further into the case compressed the charge so much the bullet would not hold the seat, and would creep out and throw fliers.

If I could find a case with a larger internal volume, I could keep this great shooting combination, but after trying drop tubes and other charge settling ideas, I finally decided to drop down .7 gr. to a lower node as it just was not a workable combination.

Another bad property of free space is combustion efficiency. A well known factory 308 load used by the military had a bad habit of suffering delayed ignition and over pressure events. Not the end of the world in a bolt gun but in a mini-gun spewing thousands of rounds a minute, a delayed ignition event means a cartridge going off after it's ejected, and that's not something you want to be anywhere near when it happens.

Testing revealed it was related to free space inside the cartridge. Changing propellants and charge weight cured the issue.

 

[Mulligan]

If you look at the bullet holes, you can see that 5 bullets were colored blue and 5 red.
All 10 loaded exactly the same, powder measured on an FX=120i, bullets same lot of Berger 108 Hyb
Brass is new Norma 284 win. Primers are all the same lot.
The loads with the blue bullets are the 5 heaviest cases in the box and the red are the lightest 5 in the box.
Does case volume cause vertical like variations in powder charge..... yes it does.
The was shot round-robin at 600 yards.
CW

 

[ebb]

The difference in case volumes doesn't change the charge weights. The charge weights the same no matter what case you drop it in to. What does change with different cases is the expansion ratio, but only slightly. Is the change enough to effect accuracy? It has to have some effect.

 

[T-shooter]

Different case capacities can vary the pressure and muzzle velocity even though the powder charge is exactly the same. I tested a few loads in this link:

http://forum.accurateshooter.com/threads/bto-015.3980992/

 

[Straightshooter1]

Different case capacities can vary the pressure and muzzle velocity even though the powder charge is exactly the same. I tested a few loads in this link:

http://forum.accurateshooter.com/threads/bto-015.3980992/

Hmmm??? I don't think that's in question. It's more about how much case capacity makes a notable difference. In that link you show increments of .005 in seating depth. But, what does that translate into how case capacity is measured (e.g. grains of H2O)? And of course, that too will vary according to the case diameter one is measuring.

 

[T-shooter]

Hmmm??? I don't think that's in question. It's more about how much case capacity makes a notable difference. In that link you show increments of .005 in seating depth. But, what does that translate into how case capacity is measured (e.g. grains of H2O)? And of course, that too will vary according to the case diameter one is measuring.

Varying the seating depth has somewhat the same effect as a different internal case capacity. A powder load is whatever you measure it at. But it may be possible to vary the powder load based on the case capacity to maintain a constant velocity. I think the question was that a certain charge weight would act like it was slightly more of less when the loaded case capacity was considered.

 

[Straightshooter1]

Varying the seating depth has somewhat the same effect as a different internal case capacity. A powder load is whatever you measure it at. But it may be possible to vary the powder load based on the case capacity to maintain a constant velocity. I think the question was that a certain charge weight would act like it was slightly more of less when the loaded case capacity was considered.

I measured 10 .308 PPU cases that were full sized with the shoulder bump to the same dimensions and necks trimmed to the same case length. The 10 cases measured from 54.5 grains to 54.9 grains of H2O. What would you say is the difference in case volume does that .4 grains be in terms of inches of seating depth for these cases? .005", .010", .0005" . . .??? If the volume difference of .4 grains only amounts to .0005" in seating depth, is it really of any concern? At what point would it start to be worth giving attention to?

 

[Straightshooter1]

But it may be possible to vary the powder load based on the case capacity to maintain a constant velocity.

Actually, I've used the software QuickLoad to actually do that.

I just might go back to QuickLoad and interpolate to get the answer to the question I've posed above . . . since I can change variables like case capacity, case volume, powder load, etc.

Addendum:

Well, it was easier than I thought using QuickLoad. Here's what I came up with . . .

Loading a .308 case measuring 2.005" in length and 175 Nosler CC's with a bullet length of 1.250 and using 41.0 gr of H4895 at a temperature of 85°F. Also, calculating the velocity out of a 26" barrel.

* 54.9 gr of case capacity and Seating Depth of .375"
Pressure: 47,002 psi
Velocity: 2688 fps

* 54.5 gr of case capacity and Seating Depth of .375"
Pressure: 47,728 psi
Velocity: 2696 fps

* 54.9 gr of case capacity and Seating Depth of .396"
Pressure: 47,718
Velocity: 2696 fps

So . . . for this particular cartridge, the .4 gr of measured case volume difference equates to a change in seating depth of .021". . . . for a velocity change of 8 fps. I'm not sure how to quantify the difference in jump to the difference in volume might due to change in pressure curve effecting harmonics . . . except to actually print it on paper. But, at least the numbers give one some idea of the relationship.

Note: I could have changed powder load only to get velocity close to the same, but pressure difference would have been wider than what I did with seating depth.

 

[Ned Ludd]

QuickLoad will show a pretty much linear response of change in velocity with respect to case volume variance (i.e. change in seating depth, using a longer/shorter Lot of bullets, high/low case volume within a single Lot# of brass). However, for very small changes in effective case volume such as are observed with successive seating depth increments, the actual change in velocity is typically smaller than the SD for a 5-shot group meaning, effectively, it's not a statistically different value. For example, if two individual 5-shot group average velocities differ by 6 fps, but the ES/SD for each group is 20fps /7.0 fps, the two average velocities are not statistically different. It's pretty easy to discern when two (or more) average velocity values are actually statistically different. In my hands, changing the seating depth of [jumped] bullets in .223 Rem or .308 Win cases by as much as .010" in either direction generally doesn't cause a statistically significant change in velocity. If you change the seating depth by approximately .030" or more, the change in velocity starts to become statistically significant.

Clearly, if you're changing the internal case volume, regardless of the mechanism (i.e. changing seating depth, bullet length, etc.), pressure will change, and, by extension, velocity will change. However, there are additional factors such as the accuracy of most chronographs, accuracy of charge weight measurement, bullet weight, neck tension, small sample size (i.e. 5 shots or less), barrel heating, powder selection, primer selection...the list goes on and on, that also affect average velocity measurements. When case volume variance becomes so large as to outweigh the other major factors that introduce velocity variance, it will start to show up as a larger change in velocity between sample sets than the SD of an individual velocity sample set. IMO - small changes in case volume (i.e. +/- 0.1 or 0.2 gr) are not typically the cause of large variance in average velocity. It is usually due to one of the other factors. Case volume variance of ~0.3 - 0.5 gr (or more) starts to become noticeable with a typical chronograph. Again, the smaller the case, the larger the potential effect on velocity of a given case volume variance. Also critical is the intended purpose - some disciplines are clearly less tolerant of case volume/velocity variance than others.

As I mentioned above, if you think case volume variance might be a concern, the simplest and most direct way to test it is to select a few cases (i.e. ~5 or more) from the extremes (high/low volume), load them up identically, and go measure velocity. If the case volume variance is sufficient to give you average velocity values that are statistically different (i.e. - the difference between the two average velocities is greater than the SDs of the two individual sets), then you may want to start sorting cases into groups with smaller volume variance.

 

[Straightshooter1]

Excellent points Ned Ludd.

Of course, in my little run through with QuickLoad to see what the effect is alone, all those other factors you mention is assumed to be constant . . . though in reality, they aren't as you well point out.

 

[T-shooter]

There are so many variables that it is difficult to get accurate results without measuring every component. The test I reported where not measured except the total C.O.L. and the powder was within .02 grain. For instance, the variance in just the bullets from the same lot can be quite a bit as shown in these for a .223 measured with a Hornady comparator (which is .010 smaller than the bullet diameter) from each end. I didn't check all in the box, just 25 or so looking for the shortest and longest overall. By loading measuring from the tip, the amount inserted into the case would be .011" more. If loaded from the approximate ogive to bearing surface intersection, the bullet would be .013" deeper into the case. Then you may end up with a longer bullet and a smaller capacity case compounding the problems. I've never checked cases in one lot but I bought a couple hundred Hornady .308 Match cases and they averaged 7% larger inside after they were full length resized and prepped. I had to start over on my loads. The group size grew by 300%.