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.22 bullet flight

.22 bullet flight

.22 bullet flight

(OP)
Gentlemen (and ladies):
I come to you from the world of International rifle shooting. I have become involved in learning why a .22 bullet, traveling at less than 1100 fps, does what it does in differing air densities. Specifically, we are attempting to decipher why flight stability (and hence group size) appears to improve when using "faster" ammo as the air density increases in response to altitude and/or barometric pressure, but slower ammo seems to improve groups when humidity goes up.

In other words, we think we have observed that the same ammo speed gives the best groups in a high or low DA independant of humidity; but once humidity rises, we see group size shrink by changing to slower ammo.

Please understand, I lied when I registered. -- Not an engineer of any type! But I am in the middle of this controversy and we need to know if there's an answer.

Is there something about humid air that changes flight stability in a decelerating projectile?

thank you!

Rifleman

RE: .22 bullet flight

Contrary to intuition, humid air is less dense than dry air at the same temperature and pressure. That is because the molecular weight of water is less than that of air (work it out yourself, assuming air is 20% O2 and 80% N2, and water is obviously 100% H20).


So, your results are entirely consistent. Denser air needs faster bullets, less dense air needs slower bullets.


Well, that was too easy....

Cheers

Greg Locock

RE: .22 bullet flight

(OP)
Greg-
Yes, but... what we're trying to figure out is: why does group size open up when the air becomes thinner due to humidity, but NOT when due to being at higher altitude, or the barometer dropping?
I would think that thin air is thin air, regardless of cause-- temp rising, barometric pressure, humidity, etc. But this doesn't seem to be the case.

As I said, humidity destabilizes flight, but the same high DA due only to temp does not.

Why?

Larry

RE: .22 bullet flight

Your second post doesn't agree with your first post.

"flight stability (and hence group size) appears to improve when using "faster" ammo as the air density increases in response to altitude and/or barometric pressure, but slower ammo seems to improve groups when humidity goes up. "

is what I explained.

Cheers

Greg Locock

RE: .22 bullet flight

(OP)
OK, I see the problem, I mis-spoke. Ignore my first post-- my second post asked the question correctly.

I didn't say accurately in the first post what I've observed. I meant to say that we've seen "faster" ammo shoot best at high AND low D.A., so long as the air is DRY.

But once humidity goes up, we need to shoot slower ammo.

Why does fast ammo still perform better in dry, but thin air?

Larry

RE: .22 bullet flight

First I have to say I followed this post yesterday and agreed with GregLocock, then you had to go and make the real world once again disagree with theory!!!

I have a few questions, can you give me the speed differences you are looking at, the altitude ranges, humidity ranges, and temperature ranges.

A possible theory:  You are shooting bullets in the compressible speed range (above 0.5 Mach).

I’m guessing that the higher your mach #, the more stable your shot as any compression wave in front of your bullet is tapered against it more adding some “support” to the sides of the bullet.

The speed of sound goes down as altitude goes up, it directly relates to temperature, assuming standard conditions.

So if you shoot the same speed higher up, you are shooting at a higher mach number because the speed of sound is slower up there.

Humidity has the inverse effect, as humidity increases, it raises the speed of sound and lowers your mach number.  

M= bullet speed / speed of sound at a given condition

Let’s see if this one flies…

Once you consent to some concession,
   you can never cancel it and put
    things back the way they are.
         ---Howard Hughes---

RE: .22 bullet flight

Isn't 1100 fps right on the hairy edge of Mach 1 at sea level?

RE: .22 bullet flight

(OP)
Blueskiesup-
First, remember that you have to simplify greatly, because I am NOT an engineer. That said...

Here's what's been observed:
At 1000-1050 ft. elev., with DA at about 700 I get good performance with muzzle velocity of 1078 fps. Groups open up as temp goes up. Above DA of 900-1000, groups are 1/2" or larger. (unacceptable)

At 400 ft. elevation, very good performance was seen with ammo at 1073 fps. under the following conditions: ( I do not have a DA for this day.) 75 F, about 75% humidity, and I don't have baro data for this session.

Another shooter has told me that in his experience, in a high DA (not sure of his definition, but let's say 2200+; remember this is for stuff happening on the ground) he has to shoot faster ammo as the humidity drops below 45%.

I know this is just anecdotal, and pretty sparse. The data set is very small. I'll collect more data for these same ammo lots next month in Colorado Springs, which is around 6000 ft., so it should be valuable data.

For now, we're wondering why humidity acting alone, in a high DA, seems to destabilize bullets. I don't know, is this enough data to help you at all?

Larry

RE: .22 bullet flight

(OP)
What did you mean by "compressible speed"?

Larry

RE: .22 bullet flight

What's DA?

I think you have too few data points to really pick a trend, but a random thought is that perhaps the incipient vapour trail on high humidity days is something to do with it.


Altitude
RH
Barometric pressure
Temperature
Bullet speed
Shooter

Can anyone think of any other likely variables?

Cheers

Greg Locock

RE: .22 bullet flight

(OP)
Greg-
DA is density altitude.

Larry

RE: .22 bullet flight

"compressible speed" - he means the velocity of the bullet is approaching the speed of sound, so that compressibility (of the air) begins to affect the bullet aerodynamics.

I think Greg might be close with his "vapor trail" explanation.  In high humidity, there may be condensed water droplets in the air (fog) or droplets may condense in the air just after the bow shock, as the flow is turned & expanded around the bullet (air is expanded below the local "dew point").  Big enough drops could deflect the bullet.  Dropping the bullet speed would reduce the relative amount of compression and subsequent expansion, thus fewer droplets forming.

RE: .22 bullet flight

RIFLEMAN:  Given the number of variables in this problem you probably are going to need quite a bit of carefully controlled and measured data.  The variables such as muzzle velocity, actual bullet weight, rigidness of the mount, relative humidity, etc. all enter into this.  Plus, I assume that as competition shooting the groups are tight.  

Good Luck
Dave

RE: .22 bullet flight

Guys...

Another possibility...

The .22 RF "bullet" is basic lead and/or copper-plated lead. To facilitate anti-fouling and projectile movement, each "bullet" is coated with a hard/thin form of grease specifically tailored for ammunition.

From my experiences, grease can/will absorb moisture. Since the coating is thin, it will tend to absorb/release moisture from/to the atmosphere as humidity varies. I believe You may find the differences "noted", in the effects of moisture in the lube.

Try this experiment to evaluate accuracy. Test 100-rds [or multiples there-of] of extremely high quality target ammo as follows.

50-Rds: Allow ammo to remain in a controled low humidity environment [5--10%, to dessicate the lubricant] for at least 2-weeks... then remove it and [IMMEDIATELY] shoot-it in a medium [40--60%] humidity test environmenmt.

50-rds: Allow ammo to remain in a controled high humidity environment [85--90% to absorb moisture] for at least 2-weeks... then remove it and [IMMEDIATELY] shoot-it in a medium [40--60%] humidity test environmenmt.

Notes:

Insure the "common" test weapon is "warmed-up" and cleaned with dry-patches just before each test [phase].

Use common procedures for both tests, to insure adequate controls for statistical purposes.

I'll "bet" You notice the differences indicated in previous emails.

Regards, Wil Taylor

RE: .22 bullet flight

I like that a lot more than the vapour trail theory.

How much does the bore wear in a competition gun?

Cheers

Greg Locock

RE: .22 bullet flight

(OP)
Greg- A couple hundred thousand rounds through a .22 rimfire barrel is no big deal. They last for years and years if you take care of them.

Actually, I like the vapor trail theory better, but what do I know.

Wil- that woud be a good test but it would be nearly impossible for me to set up in a controlled fashion. I can get access to a 50 meter range with a bench, but I don't think I could contol the humidity level. I could, however, wait until those specified conditions occur at an outdoor range, and then test in them, with the ammo at the waiting. That would be some months down the road.

Blueskiesup-
I'm starting to understand your post better. Let me see if I have this straight... and maye I'm combining theories here...

In higher humidity (all other factors being equal) Mach speed goes down.
When that happens, we get more compression in front of the bullet, which causes water to condense in the sides and trail. These water droplets are causing turbulence that de-stabilizes flight.

Did I get any of that wrong?

Larry

RE: .22 bullet flight

(OP)
And if I didn't get it wrong, why would water molecules (in the slip stream?) cause a stability problem?

Larry

RE: .22 bullet flight

actually have a feeling that your shots fly best at a certain Mach #, like 1.00 for example.

Take your shot speed and divide by the speed of sound for the conditions to get the Mach #.

What happens is the following:
As you approach Mach 1, a shockwave forms in front of your bullet.  This is called a bow shock.  The bow shock starts out with a very large radius and has a lot of drag.  As Mach # gets above to Mach 1 the bow shock gets more cone shaped in front of the bullet and has less overall drag.  There might be an optimal Mach number based upon the shape of your ammunition.  

Try calculating Mach # on various shots to get an idea.  You may be able to predict an optimal speed based upon the speed of sound in a given condition.

If you search google you can find a bunch of Mach # calculators that just require you to input atmospheric data, usually they are based upon standard conditions, some include relative humidity.  You will notice relative humidity and temperature effect Mach # inversely; this is where my original theory came from.

Once you consent to some concession,
   you can never cancel it and put
    things back the way they are.
         ---Howard Hughes---

RE: .22 bullet flight

Well, I dunno about bullet wax absorbing moisture, waxes are pretty much non-hydroscopic (i.e. don't absorb water).  Never heard of using grease on bullets, but then I never shot in competition -- is the lube on your rounds a grease or a wax?

Rifleman, I wouldn't worry about having a "highly controlled" humidity to do Wil Taylor's test.  Just put a dessicant pack (or two) into one ammo can with 50 rounds, and seal the lid.  Take another ammo can, put a wet/damp rag in the bottom, put in a couple of clips, seal lid.  Wait a day or two, then go test fire (bench mount, etc.)
Having a very dry vs. very moist bullet will tell you if the grease theory holds water.  If you see an effect from that test, THEN go back and do a test with "controlled" humidity conditions.  Or just start storing your ammo in dry cans.

RE: .22 bullet flight

Oh, and somewhere somebody got it backwards - humid air is less dense (lower molecular weight) than dry air at the same pressure and temperature.  Humid air thus has a higher speed of sound, i.e. a bullet with a given muzzle velocity will be flying at a lower mach number in humid air than in dry air.

The trouble with moisture in the air is not a molecular effect, but has to do with condensation and the formation of droplets.  A water droplet of even a few microns diameter is significantly heavier than an air molecule, and the impact with a solid body no longer follows the "laws" of gas dynamics.  Lots of random impacts with tiny (but not nano-scale) droplets during the bullet time of flight could increase the bullet CEP.  Condensation phenomena are generally ignored in most airframe aero analyses - the drops are tiny relative to the weight of a fighter jet.  But a .22 cal bullet is getting pretty tiny relative to a fighter jet too.  Condensation is considered when doing subscale supersonic wind tunnel testing - high velocity droplets have a way of sandblasting the nice, polished surfaces of tunnel models.  My $.02.

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