Two statements that come up from time to time are that a dished breech plug will increase muzzle velocity, and that accuracy decreases as the flash hole size increases above 0.062. I conducted a series of experiments to test these hypotheses by chronographing muzzle velocity.
Dan Phariss supplied the barrel and did all of the necessary machine work. Thank you Dan.
Disclaimer Only one chronograph was injured or killed while collecting these data.
Breech Plug Face Shape Experiment Experiment Conditions:
38 .50 caliber barrel (Getz)
70 grains Goex FF
Coned flash hole
0.062 flash hole diameter
15 from muzzle to chronograph
Spit lubed 0.018 pillow ticking patch
0.495 Hornady ball
80 90 degrees F
Flat faced breech plug
Breech plug face dished in hemispherical shape with 1/2 ball end mill.
Sample Size 15 for each breech plug
Table 1
This was the first time I had chronographed black powder loads, so I wasnt sure what to expect. For these data, the standard deviations were 27 and 30 feet/second, and the ranges were 100 and 112 feet/second for the dished breech plug and the flat faced breech plug respectively. Ive seen similar standard deviations and ranges with factory loaded centerfire ammunition, so I feel confident that my loading procedures were reasonably consistent.
The mean muzzle velocity for the dished breech plug and the flat breech plug were 1,609 fps and 1,522 fps, a difference of 87 fps. This difference is statistically significant, that is, the difference in muzzle velocity is real. These results are very robust you can expect to get a statistically significant increase in muzzle velocity in other calibers and with other loads.
A cautionary note: the difference in muzzle velocity of 87 fps we found in this test applies only to this barrel, breech plug shape and load. The exact gain in muzzle velocity you may get with your barrel and load would have to be measured.
This is an easy experiment to do. Lots of folks have chronographs. Should anybody decide to repeat this experiment, Id be very interested in your results.
Velocity as a Function of Flash Hole SizeExperiment Conditions:
38 .50 caliber barrel (Getz)
70 grains Goex FF
Coned flash hole
15 from muzzle to chronograph
Spit lubed 0.018 pillow ticking patch
0.495 Hornady ball
80 90 degrees F
Flat faced breech plug
flash hole diameter (inches):
0.0620
0.0670
0.0730
0.0785
0.0820
0.0890
0.0960
0.0995
Sample Size 5 for each flash hole size
In discussions with Dan about factors that affect accuracy, he has mentioned that bench shooting lore contends that accuracy decreases as flash hole size increases above 0.062. Obviously, the best way to test this is with a machine rest, but I dont happen to have one. However, we can look at two variables that are known to affect accuracy with a chronograph. These variables are average muzzle velocity and variability in muzzle velocity. As a rule of thumb, decreases in muzzle velocity and increases in variability of muzzle velocity will tend to decrease accuracy. So, this experiment was intended to address two questions
1. As flash hole size increases, does muzzle velocity change?
2. As flash hole size increases, does variability in velocity change?
The statistic used to analyze the data for question 1 is called regression analysis. For this type of analysis, we change one variable, in this case flash hole size, and record the effect on muzzle velocity. The data are plotted on a graph, with the flash hole diameter on the X axis and the velocity on the Y axis. Heres the raw data:
Table 2
And heres the graph:
Figure 1
Surprise, surprise, surprise as the famous philosopher Gomer Pyle would say. I had expected that as flash hole size increased, velocity would decrease at a linear or perhaps exponential rate. I most certainly did not expect velocity to increase when the flash hole size changed from 0.062 to 0.067.
The first order of business is to look at this unexpected result. What happened? These data were all taken during the same session. Powder, patches and balls were all from a single lot, the same chronograph was used, and the ambient temperature didnt change appreciably during the session, so changes in experiment conditions can be ruled out.
Possibilities include:
1. This is a real phenomenon, and we will see this same result with other barrels and other loads.
2. This is a real phenomenon, but it is a characteristic of this particular barrel and load, and we will not see it in other barrels or with other loads.
3. This is an artifact of random variation in the data, and is not a real phenomenon.
#3 seems unlikely to me, as the data are reasonably consistent; however we have insufficient data to come to a definitive conclusion.
#1 and #2 these seem to be the most likely possibilities, but well have to do a bunch more testing to find out if one of these hypotheses is correct.
Until I can explore this issue a little further, I excluded the 0.062 flash hole date from the analysis. So, heres the graph without those data, and the regression line.
Figure 2
Regression analysis gives us two very useful things, an equation for the best line through the data, and a value known as r squared.
r squared is a very useful number, because it tells us how good our model is. The values for r squared range between 0 and 100%, and tells us how much of the variability in y is explained by changes in x. With a value of 0, there is no relationship between the x variable and the measured effect. With a value of 100%, all of the variation in y is explained by changes in x. Obviously, the closer r squared is to 100%, the better the model is and the more we know about the relationship between x and y.
In this case r squared = 54%. That is, 54% of the variation in muzzle velocity is explained by changing the flash hole diameter. The remaining 46% of the variation is known as unexplained error.
I had hoped for a substantially higher r squared value, so I was a little disappointed by this result, although not too surprised. In virtually all of the flintlock data that I have looked there is an unusually high amount of unexplained variability. I dont know why, but this seems to be the nature of black powder and flint ignition.
The regression equation is
Velocity = 1,704 (2,432 x Flash Hole Diameter)
This is handy because now we have a predictive model. If we want to know what the expected velocity is for any given flash hole diameter, we simply perform the calculation. The following table shows the flash hole size, calculated average velocity for each flash hole size, change in velocity from the previous flash hole, and the cumulative change in velocity.
Table 3
As can be seen in the graph and the table, as the flash hole size increases, velocity decreases. Good. This is what we would expect. The data shows though, that velocity doesnt drop off too rapidly. At 0.082, velocity has only dropped off 36 feet/second from the 0.067 flash hole. But by the time we have gone from 0.067 to 0.0995, a total of 79 feet/second has been lost.
Looking at the graph, it appears that variability in velocity is consistent from 0.067 to 0.082 but above 0.082 variability appears to increase substantially. However, with only five samples at each interval, there are too few data to test statistically.
To answer question 2 does muzzle velocity variability increases with flash hole size - I did an additional experiment to get sufficient data to test statically. All experimental parameters are as above, except that 15 shots were measured with 0.0625 and 0.0995 flash holes.
Table 4
Note that the total range in muzzle velocity was 112 feet/second for the 0.0625 flash hole, compared to 165 feet/second for the 00995 flash hole. The muzzle velocity standard deviation was 30 feet/second for the 0.0625 flash hole, and 48 feet/second for the 0.0995 flash hole. This difference was statistically significant that is, variability in muzzle velocity really does increase as flash hole changes from 0.0625 to 0.0995.
To summarize this set of experiments:
1: Dishing the face of the breech plug will increase velocity. These results are statistically significant and robust. With the experiment parameters used in these experiments, the gain in velocity with a dished breech plug was 87 feet/second. The exact gain that you may get with other caliber and load parameters would need to be measured, and cannot be projected from these data.
2. Between 0.067 and 0.0995, muzzle velocity decreases as flash hole size increases. The velocity difference between these two flash holes was -79 feet/second. It appears that enlarging the flash hole to 0.082 will not have a substantial effect on velocity muzzle velocity loss at 0.082 was only 39 feet/second.
3. Variability in muzzle velocity increases from 0.0625 to 0.0995. The difference in variability with these two flash hole sizes is statistically significant. From the graph, it appears that variability in muzzle velocity between 0.0625 and 0.082 is similar, and that above 0.082 variability starts to increase substantially. There are however, too few data to make a definitive statement except at the extreme flash hole sizes.
4. The decrease in muzzle velocity and increase in variability of muzzle velocity as flash hole size increases support a hypothesis that increasing flash hole size could decrease accuracy.
