Altitude Density & Temperature Change

GsT,
I’m using a Chrony Beta Master about 10 feet from the muzzle. Sorry, I didn’t write down the SD.

Obviously, the barrel does warm up with each shot, but I didn’t see a pattern of velocity change with each round during the string.

Lee
 
Boyd Allen said:
I get a little over .0035" but this is a calculation that is new to me. Looking up the coefficient of thermal expansion of 416 steel, in the range of 0 to 100 degrees Celsius it is 9.9 microns per meter per degree Celsius. The difference in temperatures in Celsius works out to 16.6 degrees, a micron is .000039 inches. 21.75 inches converts to .552 M. Check my math and procedure. I am not an engineer. Boyd
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Boyd,
Correct. That is about 1/8 turn for a tuner with 32 pitch.
 
K Hope said:
Question: Should I take that to mean Temperature is the only atmospheric condition that changes burn rates?
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Yes, and maybe humidity. Really, it is the temperature of the powder, cartridge and barrel that are important, but these follow atmospheric temperature when we are talking about changes from morning to afternoon. Atmospheric pressure presents a back pressure on the front of the bullet, but this is really small compared to combustion pressure. Atmospheric density presents some weight of air that must be pushed out of the barrel in front of the bullet, but again, this is really small compared to the weight of the bullet.
I don't doubt that humidity can change the sliding friction between bullet and barrel. Probably more important for rimfire than centerfire.
 
I disagree, and let me tell you why. Some time back Jim Borden told me of a time when he went to a major match, (national or Super Shoot I forget) preloaded. Over a period of several weeks he loaded up batches of ammo until he had the total that he would be taking. All of the loading was done at the range 50-60 (I forget) rounds per session. He would change the powder charge until he got the velocity that he wanted and then finish loading that batch. During that time their was quite a bit of fluctuation in ambient conditions, fronts and rain mostly which had effects on the humidity that the powder (133 I believe) was exposed to. At the end of it all the total difference between the weights of the powder charges used to load to the same velocity was 1.5 gr. The thought was that tune is velocity related which is why he loaded to the same velocity rather than by weight. Before the match the fellow that he would be traveling with expressed concern that this approach may have been a mistake, so cartridges were pulled from the various batches, mixed and test groups fired. They were just as he had hoped. By loading to a particular velocity and ignoring the difference in charge weights between batches, the result was as he expected, the accuracy was excellent. Some time ago there was a very well done article in Precision Shooting magazine that I have a scanned copy of somewhere, that reported on the effects of powder moisture content on velocity. The method was clever and the results surprising. Several different powders were tested. The differences in velocity between bone dry and maximum saturation (2% if memory serves) was quite striking, much greater than anyone that I know would have expected.
 
It’s Confusing because the data is bogus

Lee Hachigian said:
Last weekend, we shot a 200 yard Score Match at WWCCA. Since it was a fun match and cold, most of us come preloaded. I asked the guys if I could set up a chronograph. I had loaded up 50 rounds of V-133, weighing each round for my 6ppc. It was somewhat mild this day with temperature starting at 37*, which resulted in a lot of fog (we still had plenty of snow on the ground). The match started a little late due to the fog (we couldn’t see the rings), so the first shot was fired around 11:00 am, and the last shot close to 1:30 pm.My average velocity for the first match was 3390 fps. By the 5th target my avg. velocity was 3324 fps. The temperature went from 37* to 39.5*. The Density Altitude when from -667 to -280. I didn’t write down the humidity.Yesterday afternoon, I went to the range to fireform some brass. Again all the rounds were weighed. I shot 10 rounds every half hour. Starting velocity was 3089 fps, and ending velocity was 3090. DA went from -220 to +156, and temperature went from 45* to 49.5*. Humidity went from 60% to 48%.I can’t draw any conclusions why the velocity changed during the weekend, but didn’t yesterday. Any thoughts?Lee
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Chronos can be affected greatly by ambient lighting. Check out this link... http://www.accurateshooter.com/gear-reviews/led-lamp-kit-for-shooting-chrony-chronographs/
 
Boyd Allen said:
I disagree, and let me tell you why. Some time back Jim Borden told me of a time when he went to a major match, (national or Super Shoot I forget) preloaded. Over a period of several weeks he loaded up batches of ammo until he had the total that he would be taking. All of the loading was done at the range 50-60 (I forget) rounds per session. He would change the powder charge until he got the velocity that he wanted and then finish loading that batch. During that time their was quite a bit of fluctuation in ambient conditions, fronts and rain mostly which had effects on the humidity that the powder (133 I believe) was exposed to. At the end of it all the total difference between the weights of the powder charges used to load to the same velocity was 1.5 gr. The thought was that tune is velocity related which is why he loaded to the same velocity rather than by weight. Before the match the fellow that he would be traveling with expressed concern that this approach may have been a mistake, so cartridges were pulled from the various batches, mixed and test groups fired. They were just as he had hoped. By loading to a particular velocity and ignoring the difference in charge weights between batches, the result was as he expected, the accuracy was excellent. Some time ago there was a very well done article in Precision Shooting magazine that I have a scanned copy of somewhere, that reported on the effects of powder moisture content on velocity. The method was clever and the results surprising. Several different powders were tested. The differences in velocity between bone dry and maximum saturation (2% if memory serves) was quite striking, much greater than anyone that I know would have expected.
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By luck I was just getting started Benchrest in 05 and had the privilege of a special invite from JB. At the 05 Super Shoot I loaded in his trailer were I was also introduced to a number of his close friends including JJ. Since that time for better or worse I have been hooked on the study of NC. I have not done the testing that others have done and have been criticized for over theorizing but I do believe humidity does change burn rates.
 
Have any of you read the article that I mentioned in my previous post? Since the magazine is no longer in business I suppose that I could post the article if you have not. Unless someone thinks there would be a problem with that.
 
Boyd,
I was thinking preloaded ammo, in which case atmospheric humidity doesn't change the powder. The cartridge is sealed well enough that the moisture can't get in or out. But if we consider ammo loaded in high humidity in the morning and loaded in low humidity in the afternoon, then I agree with you.

Keith
 
mks said:
Boyd,
I was thinking preloaded ammo, in which case atmospheric humidity doesn't change the powder. The cartridge is sealed well enough that the moisture can't get in or out. But if we consider ammo loaded in high humidity in the morning and loaded in low humidity in the afternoon, then I agree with you.

Keith
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As I see it and take it for what it’s worth you are correct in that it is the powders temperature that has the primary influence on burn rates however moisture not only reduces the powders strength it also reduces sensitivity through ignition and flame spread delays. So even pre-loaded (with elevated moisture content) depending on chamber temperature (how many rounds shot), the length of time cooking in the chamber and starting and stopping times I think you will have increased fluctuations in pressure and velocity.
 
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