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Comparison of MVs of 7 .30 cal. cases with 110, 180 & 250 grain bullets
Old 07-31-2006, 03:30 PM
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Default Comparison of MVs of 7 .30 cal. cases with 110, 180 & 250 grain bullets

Since it's easier to get to the computer than it is to the range, AND since I don't have rifles chambered in 7 different .30 caliber cartridges, I thought I'd do some more ballistic pencil-whipping.

It has long been "taken for granted" that "large" capacity cases, while they do make bullets go faster, do so with a non-linear increase in powder. The "large" cases are referred to as less "efficient" than "small" capacity cases. While, to my knowledge, the term "efficient" so used has never been specifically defined, it was always 'understood' that it meant that you can make bullets go faster with big cases, but you're gonna hafta use a lot more powder to do so. So I thought I'd just test that "truth".

I decided to use .308 caliber because it is so universally (in the US) considered the "yard-stick" by which every caliber must be compared. That galls me, but it is nonetheless the parochial reality. So, I chose the following cartridges. Their case capacities, in grains of water, follow in parentheses:

.308x1.5 - (38.00)
.308 Win - (56.00)
.30-06 AI ("Improved" because I wanted an increase in case capacity of about 15 grains between each case) - (70.00)
.30x.338 Win Mag - (85.00)
.300 Weatherby Mag - (98.90)
.300 Rem Ultra Mag - (113.00)
.300 Pegasus - (132.01)

For bullets, I chose the Speer 110 HP, the Nosler 180 Partition, and the Barnes 250 RN. This represents essentially the spectrum of bullet weights available in .308. Plus, there is a 70 grain difference between each bullet and the one adjacent to it.

While there are relatively slight differences in max chamber pressures between the cases, in order to keep things as simple as possible, "oranges to oranges" and all that, I kept the max chamber pressures all equal at 60,191PSI.

Again, in order to keep things simple, I kept seating depth uniform for all bullets and cases - 66.7% of the caliber, or .205".

Finally, I kept the bullet travel equal. In other words, I compensated for over-all cartridge length so that all bullets in all cases traveled exactly the same distance before exiting the barrel. Bullet travel was 23.807". This resulted in bbl lengths ranging from 25.1 in the .308x1.5, to 26.5 in the .300 RUM. Those are reasonable bbl lengths without getting into extra-long "custom" lengths.

The bullet travel of 23.807" was selected because I started with the .308x1.5 and the 110 HP and a muzzle velocity goal of 3000 f/s. In order to achieve this AT A SPECIFIC BARREL TIMING NODE, the barrel needed to be 25.1" long, leaving 23.807" of bullet travel. All other bullets and bbl lengths were adjusted to that length.

The results for each bullet in each case were obtained by caclulating the bbl timing nodes, and selecting the one that gave the maximum MV. In other words, the load was determined by the charge that gave the maximum velocity AT AN OPTIMAL TIMING NODE. In more 'other words', I made conditions optimal for every case with every bullet. In even more 'other words', I made things the best they could possibly be for each bullet in each case. There are no "disadvantages" to one bullet/case combo by having to "fit" to some other bullet or case's constraints.

I found the results intersting. (Obvious, I suppose, or I wouldn't be presenting the results here.) Before I present my personal conclusions, I'll present the graph of the results and let youse guys mull it over.

The graphs have 3 lines, one each for the 110, 180, and 250. In the first graph - Case Capacity vs Average Muzzle Velocity - the lines you see are the 'best fit' of the data points using a linear regression. The slope and intercept data are in the corners of the graph. The slope and intercept for the 110 is in the upper left; the 180 in the upper right; and the 250 in the lower right. The slopes are:

110 - 7.6727
180 - 7.5577
250 - 6.1822

The intercepts are:

110 - 2754.7
180 - 2166.6
250 - 2002.4

An "R squared" value is a measure of how well the regression line fits the data. The "R squared" value for the 110, (0.99), is very high. (A perfect fit "R squared" value would be "1.0".) The 250 "R squared" value, while not as 'tight' is still a good fit.

The second graph - Average Efficiency vs Case - is the "efficiency" of each bullet/case combo, with "efficiency" defined as the ratio of Muzzle Energy in foot-pounds to charge weight in grains. In other words, how many foot-pounds of muzzle energy do you get for every grain of powder. These models are 'second order' hence the square functions, but ignoring the math for the most part, note that the fits ("R squares"), are very good.

Have a look at these and I'll offer my conclusions after we've had a chance to discuss them a bit.

Attached Images
File Type: jpg OBT 30 cal efficiency 110 - 180 - 250.JPG (63.9 KB, 48 views)
File Type: jpg OBT 30 cal case cap 110 - 180 - 250.JPG (61.1 KB, 61 views)

Last edited by gitano; 08-01-2006 at 01:18 AM..
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.30-06 ai, .300 pegasus, .300 rem ultra mag, .300 weatherby, .308 win, .308x1.5, .30x.338, math, obt, optimal barrel timing

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