It's Not Density Altitude, but

B

Bob Collins

Guest
When I came up with the Density Altitude theory affecting bullet flight years ago, it explained many things to me in relationship to bullets hitting a target. I could tell the difference in conditions because when I use a chronograph I always had the chronograph at the target – there is nothing I wanted to know going on at the muzzle. This satisfied me for a short time, but I could still not explain why bullets did not follow the DA as well as I expected. About three years ago I was working on “why” and decided it was not the DA but the total amount of water in the air. Relative Humidity is just that – a percentage of water in the air due to temperature, altitude, and barometric pressure – mainly – let’s keep this simple, this is where density altitude is figured. It could be raining with a high relative humidity, say the 90’s, but the air where you are shooting could be as dry as and desert – 3 to 5 grams of water per cubic meter. The DA won’t help you here.
Grams of water per cubic meter tells you how “thick” the air is, the thicker the air the slower a bullet moves through air, and forces within the air (wind direction and speed) can “push” a bullet around more as the air is thicker. Now keep figuring – thicker air slows a bullet down so it is in the barrel longer, so the “tune” of a gun changes as the length of time it takes for a bullet to exit the barrel changes. What you are doing when you moving a tuner or changing the amount of powder in a cartridge is getting the end of the barrel to be still when the bullet exits. Otherwise, if the barrel is moving (vibrating) when the bullet goes out, the barrel “hits” the bullet on its tail end creating bigger groups.
I had a hard time conveying this to shooters until one of my friends a Navy Chief (in charge of F-18 repairs) came up with a simple way to prove the theory. As air gets thicker the speed of sound increases. Here was my answer, (the Speed of Sound) I had been looking for to explain so many things involving bullet flight. The why that bullet went there it did became clearer. Next my Chief’s two biggest points come to play quickly, barrel twist rate and vectoring of the wind, you will have to do your part to obtain a bunch of X’s.
Here are two different reading I did last year and my Chief’s reply.
1. 65.9 degrees, 51.3% RH, 29.90 pressure, 1.2031 and 98% Std. Air, 8.29 grams/water per cubic meter - SOS is 1098.686 FPS.
2. 69.2 degrees, 53.2% RH, 29.87 pressure, 1.1936, 97% Std. Air, 9.536 grams of water – SOS is 1096.929 FPS.
Just the temp difference doesn't change the speed much at all due to the RH and water content. Also, remember we are doing these calculations with 0 wind factor, so there is no resistance/vectoring. These were readings from an indoor match.
3. Speed of Sound at Summerville, SC – hurricane Sandy off shore. 65.6 degrees, 75.9% RH, 29.68 pressure, 1.1929 of Std. Air 12.10 grams of water per cubic meter. SOS is 1091.61 FPS . Outdoor match on October 27th, 2012
Just for your information, I shot a 40X, 6 o’clock firing pin, 17 twist barrel with the same ammo. Indoors it shot terrible, but outdoors it shot fine. SOS?

I hope this helps explain group sizes and the ever changing results we get when we go to matches. It sure helps while trying to obtain extreme accuracy to know some of the factors involved.
Bob Collins
 
Bob,
1. Air density decreases with increasing humidity, because H20 (molecular weight 18 g/mol) is lighter than N2 (the primary component of air, molecular weight 28 g/mol).

2. Speed of sound decreases with increasing air density. Speed of sound c = sqrt(k p/rho), where k is the specific heat ratio, p is pressure and rho is density.

3. Speed of sound decreases with increasing humidity, all else being constant, because k = 1.33 for water vapor, but 1.40 for air.

4. Using the ideal gas law, speed of sound can be expressed as a function only of temperature and humidity as c = sqrt(k R T), where R is the ideal gas constant and T is temperature. This equation makes it easy to see that the effects of pressure and density are combined into an effect of temperature alone.
see http://www.engineeringtoolbox.com/speed-sound-d_82.html

5. The objective of tuning a barrel is to get the muzzle angle to be rising as bullets exit so that slower bullets, which exit the barrel later, are launched at a higher angle. If the angular velocity of the muzzle is right, fast and slow bullets hit the target at the same elevation.

6. The weight of air in the barrel is tiny compared to the weight of the bullet, regardless of its density. The more important factors in producing different muzzle velocities (and barrel exit times) are varying powder charge weight, and varying powder and barrel temperature.

Hope this helps,
Keith
 
Bob I was reading Brian Litz book Applied Ballistics for long range shooting about SOS changes, he used 59 degrees F. compared to 100 degrees F. it was 1116.45 fps to 1159.74 That would explain a lot about using the same load in these different Temps. I seem to shoot better in about 60- 70 degrees in the rain. So now I'm really going to make my self nuts by looking at the forecast and picking a load that will work, any other Ideas. Thanks

Joe Salt
 
mks said:
Bob,
1. Air density decreases with increasing humidity, because H20 (molecular weight 18 g/mol) is lighter than N2 (the primary component of air, molecular weight 28 g/mol).

2. Speed of sound decreases with increasing air density. Speed of sound c = sqrt(k p/rho), where k is the specific heat ratio, p is pressure and rho is density.

3. Speed of sound decreases with increasing humidity, all else being constant, because k = 1.33 for water vapor, but 1.40 for air.

4. Using the ideal gas law, speed of sound can be expressed as a function only of temperature and humidity as c = sqrt(k R T), where R is the ideal gas constant and T is temperature. This equation makes it easy to see that the effects of pressure and density are combined into an effect of temperature alone.
see http://www.engineeringtoolbox.com/speed-sound-d_82.html

5. The objective of tuning a barrel is to get the muzzle angle to be rising as bullets exit so that slower bullets, which exit the barrel later, are launched at a higher angle. If the angular velocity of the muzzle is right, fast and slow bullets hit the target at the same elevation.

6. The weight of air in the barrel is tiny compared to the weight of the bullet, regardless of its density. The more important factors in producing different muzzle velocities (and barrel exit times) are varying powder charge weight, and varying powder and barrel temperature.

Hope this helps,
Keith
Click to expand...



Thank you Keith. There are a great number of misconceptions about air density; huh? :rolleyes: You sure have a way with words and the patience of Job.

Guys and gals, when MKS speaks, listen carefully. He knows what he's talking about! :)


Gene Beggs
 
Ahhhh, the vast gulf betwixt the "what" and the "why" :) . . . . . . . "stopped muzzle" anybody???

Thank you Bob Collins for the WORK you've done in taking notes and collating results.
 
Now, if someone can relate this to a workable and repeatable process for tuning and staying in tune throughout a given day??

I've worn out a couple of Kestrel meters in capturing and recording data over the last several years but I have not found a predictable relationship
between environmental changes and accuracy. I have found out that as the day progresses and the wind increases that MY accuracy goes in the tank.

But, with this new Angel Dust powder tuning is not needed, YHEA!!
 
JerrySharrett said:
Now, if someone can relate this to a workable and repeatable process for tuning and staying in tune throughout a given day??
Click to expand...


I hear ya Jerry. I'm patiently waiting(With Kestrel in hand) for a solution to staying in tune all day/weekend. The recommendations I've tried,don't always work like they're supposed to.

Sometimes,my rifle goes in and out of tune during a yardage. For those who don't shoot Benchrest,thats Five(5),Five shot groups at 100 or 200yds. It happens too frequently with Vt-133.



Glenn
 
Column of air in barrel?

I will avoid all of the issues of Air Density outside the barrel.
The thought about the air in the barrel would be this...
The gas (air) in the barrel at and prior to the first shot could be considered the same as the air outside. HOWEVER in a match when a shot is fired and then the chamber immediately reloaded what should be in the barrel is the remaining gases from the ignited or burning powder. It would seem that the volume or column of gas in the barrel would not be completely affected by the removal and insertion of another round but only to a minor amount unless there were enough wind to either blow directly down the barrel or across the muzzle enough to create a vacuum or in some other way disturb the column of air inside.
Just a thought.
 
Question in regard to tune.

There is some variation in opinions or maybe just in wording regarding tuning, but I have always had a curios question.
The question is not in regard to how a barrel vibrates or where it vibrates or where the bullet should be upon exit but rather that it seems that one general consensus is that whatever that point it should be the same.
If that is the issue, does tune equate to finding a point that the barrel vibrates uniformly to the point of the bullet being able to exit at the same point of vibration location?
 
Vern: From what I've witnessed at the range, is for every action there is a reaction, what I've been told and have tried, if the weather changes, or you change primers or what ever, its going make a difference. How much is probably going to depend on how vibration sensitive your rifle is!

Joe Salt
 
Vern said:
There is some variation in opinions or maybe just in wording regarding tuning, but I have always had a curios question.
The question is not in regard to how a barrel vibrates or where it vibrates or where the bullet should be upon exit but rather that it seems that one general consensus is that whatever that point it should be the same.
If that is the issue, does tune equate to finding a point that the barrel vibrates uniformly to the point of the bullet being able to exit at the same point of vibration location?
Click to expand...

No

The bullet exits where it will depending on velocity, thrown charges, velocity variations, ensure that this WILL be different shot-to-shot.
 
Vern said:
There is some variation in opinions or maybe just in wording regarding tuning, but I have always had a curios question.
The question is not in regard to how a barrel vibrates or where it vibrates or where the bullet should be upon exit but rather that it seems that one general consensus is that whatever that point it should be the same.
If that is the issue, does tune equate to finding a point that the barrel vibrates uniformly to the point of the bullet being able to exit at the same point of vibration location?
Click to expand...

If the muzzle is always in the same position, then slow bullets hit the target low and fast ones hit high. That is not tuning. See Kolbe's nice article at http://www.border-barrels.com/articles/rimfire_accuracy/tuning_a_barrel.htm
 
Vern said:
I will avoid all of the issues of Air Density outside the barrel.
The thought about the air in the barrel would be this...
The gas (air) in the barrel at and prior to the first shot could be considered the same as the air outside. HOWEVER in a match when a shot is fired and then the chamber immediately reloaded what should be in the barrel is the remaining gases from the ignited or burning powder. It would seem that the volume or column of gas in the barrel would not be completely affected by the removal and insertion of another round but only to a minor amount unless there were enough wind to either blow directly down the barrel or across the muzzle enough to create a vacuum or in some other way disturb the column of air inside.
Just a thought.
Click to expand...

Thinking out loud, I would expect the expulsion of gasses to create a vacuum in the bore allowing atmosphere to rush back into the bore. Also I could imagine a fairly straight walled case to act as a piston upon removal. Did a quick calculation on a PPC case volume and it’s approximately 1/5th to 1/6th the volume of a typical 6mm bore.
 
There are a couple of things that have not made it into this discussion, powder changing (in the measure, if one is not preloading) from exposure to changing ambient conditions, and that one of the features of a tuner is its slowing barrel swing to the point that bullets exit during the muzzle's rise, which offers some degree of compensating effect for differences in velocity, as may be seen in the material that Varmint Al has produced. If one assumes that the effect of a given amount of powder is constant, and it is not, that in itself could be the fly in the ointment. It may be that the new Canadian powders will show less change than 133, and be better candidates for tuner predictability. Time will tell.
 
I have the tuner and the all the other gear. Bottom line my 6 PPC is very difficult to keep in tune. I have to work the process each time I take it to the firing line.

At the same time, and conditions, my 30 BR barrel is a dream. Once set the tuner has never been adjusted again. It just always shoots bug holes or less. The only downside is the recoil and the price of the bullets.
 
Octopus,

I have found, with the Begg's tuner, on the 30 BR I do not have to be as precise in my placement of the tuner as compared to the 6 ppc. Meaning, if my spreadsheet tells me 3 o'clock and I turn it anywhere between 2:00ish and 4:00ish = I'm still ok. I can not get away with that on my 6 ppc. This corresponds to what you have found and what other NTUs (non tuner users) have found with the wide tune window of the 30's.

Francis - you hit the nail on the head with post #15
 
Most days the DA at our range changes over 1,000 feet from 9 AM to 2 PM.

The lowest I have recorded was Friday Mar 15, 2013 -498 feet. Many days the DA is over 2,000.

The true altitude at the firing line is 42 feet.

The DA changes have major effect on my 6 PPC but not my 30 BR.
 
It seems to me that the best shooters in the short range BR game pay little attention to density altitude. Shooters like Tony Boyer often tune at the range by looking at their target and adjusting powder charge according to what the group looks like on paper. Of course, this method requires a lot of experience. Gene Bukys uses a type of tuner that he terms a snubber since he does not adjust it at all during the match. When he installs a new barrel on his rifle, he tunes it without the tuner in a conventional manner. After he finds the best load, he installs the tuner/snubber and adjusts it until he equals or surpasses the original best load. At that point, he locks the tuner in place and does not adjust it further. He will change his powder charge as needed during a match. Evidently, he has this game figured out....if one looks at how he has shot for the last two or three years. Good shooting....James
 
I see the same happen

with Rimfire and Air Rifes, the tune shift thing. One of the problems is WIND almost always comes up along with the temp and decline in humidity, which leads one to question their ability to cope with it, (the wind) so one is not sure. I think it stands to reason that a Tuned rifle will shoot better in the wind than one out of tune.

What I have observed indoors is pretty much the same as outside, when the temp goes up and down, so does the accuracy.
 
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