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Old March 20, 2024, 01:18 PM   #301
tangolima
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Quote:
Originally Posted by stagpanther View Post
If the twist in the bore is reversed (left instead of right)--is the jump/fall of the aerodynamic jump also reversed? Reminds me of climb vs conventional cut in milling.
Cross wind from the right, left-hand twist. POI shifts to low-left. Not sure it is called aerodynamic jump. But yes, it is reversed compared to right hand twist.

-TL

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Old March 20, 2024, 01:44 PM   #302
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Quote:
If the twist in the bore is reversed (left instead of right)--is the jump/fall of the aerodynamic jump also reversed? Reminds me of climb vs conventional cut in milling.
If the twist is reversed then the forces acting on the bullet will be changed. "Aerodynamic jump" will be in the direction the bullet needs to equalize the aerodynamic forces working on it.

It will angle into the wind always but the specific angle will change due to the change in vector addition along the the axis.

The angle will change but the direction of the jump is determined by the direction of the wind with the bullets nose turning up wind.
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Old March 25, 2024, 02:37 PM   #303
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We have been dancing around the observation (aerodynamic jump?) without going into the original question. How does a spin stabilized projectile steer into the wind?

It is obvious on fin stabilized projectile such as an arrow. Say cross wind is from the right. It acts on the fin (feather/empennage) to turn the arrow's tail to the left, and its head to the right.

For a bullet, it is quite on the contrary. The wind pushes its tip to the left and it will tumble. Apparently spinning keeps it from happening. But how exactly?

The "aerodynamic jump" we observe is a byproduct of this how.

-TL

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Old March 27, 2024, 03:49 PM   #304
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https://openstax.org/books/universit...of-a-gyroscope

Quote:
The "aerodynamic jump" we observe is a byproduct of this how.
It adds a force vector to the vector addition.

See:

Quote:
See, here's the thing. If it has canceled that airmass velocity then it isn't moving at the same velocity as the air mass.
Quote:
Why does it have to move at the same velocity?? Nothing says that it does move at the same velocity but it must equalize those forces and therefore it will take on the velocity of the air mass it is traveling in. That is principle of aerodynamics. Even the Large Transport Category Aircraft in the Airlines must obey it.
https://thefiringline.com/forums/sho...&postcount=294


Quote:
The wind pushes its tip to the left and it will tumble.
Not when the Center of Gravity is behind the Center of Pressure in a properly designed bullet.

Last edited by davidsog; March 27, 2024 at 03:57 PM.
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Old March 28, 2024, 08:40 AM   #305
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TL,

I've got the stabilization answer written up and will post it when I've had time to make a couple of illustrations to go with it. Meanwhile, I'll leave you to contemplate why we can spin stabilize bullets with their center of mass (CM, aka, the center of gravity (CG)) behind their center of pressure (CP), as is the case with most modern bullet shapes, or with the CM and CP in the same place (e.g., a double-ended wadcutter (DEWC)) or with the CP behind the CM (e.g., a hollow-base wadcutter (HBWC)). Like the DEWC, A round ball also has its CM and CP in the same place but doesn't suffer from the in-flight airstream trying to turn it as elongated shapes do. For the round ball, the spin only needs to provide enough gyroscopic stiffness to prevent it from rotating on an inconsistent spin axis, as it gets pulled off course by the Magnus effect if it rolls out of a smooth bore, turning in random planes.
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Old March 28, 2024, 09:13 AM   #306
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A round ball also has its CM and CP in the same place but doesn't suffer from the in-flight airstream trying to turn it as elongated shapes do.
Subsonic and travels in a corkscrew pattern thru the air as it attempts to balance the forces.

The relationship of the CG to CP is dictated by realm of flight the bullet primarily travels in.

The nature and formation of normal shock determines the relationship.
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Old March 28, 2024, 09:16 AM   #307
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Pretty good video here on some of these issues if you understand what he is talking about....

https://thefiringline.com/forums/sho...d.php?t=618375

The language he uses is talking about specific conditions and has specific meanings that are not intuitive. Easy to misunderstand and draw nonsense conclusions.
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Old March 28, 2024, 09:39 AM   #308
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Unclenick. Finally you are joining the discussion. Your illustrations are the best as always!

Yeah I have contemplating all those scenarios. Round ball is an interesting one. Adding to the list is airgun pellet. It is fin stabilized, and yet it shoots terrible without riflings. Its "aerodynamic jump" is funky too. It remains the most interesting one on my list.

Thanks.

-TL

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Old March 28, 2024, 11:32 AM   #309
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I've been traveling a bit and then been busy trying to catch up.

A few years back, I wrote my own point mass solver from the ground up using the basic equations of classical mechanics together with published drag functions because I wanted to see some of the intermediate results not displayed in commercial software (vertical and horizontal deceleration rates and that sort of thing). So, I've been in the weeds with trajectory calculations. I also read McCoy's Modern Exterior Ballistics and was fortunate to have survived triple integral eigenfunctions for particle position probabilities in quantum mechanics in school without developing PTSD, so McCoy's math didn't intimidate me. So I've been in the weeds with that, too, and have previously written a verbal description of stabilization mechanics over at shootersforum.com. I just wanted to get a little deeper into it here and show the origins of aerodynamic jump.
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Old March 28, 2024, 12:08 PM   #310
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I like McCoy's book. I also have book written by Donald Calucci. They are the main sources of information. I also came across books and papers authored by ballisticians from behind the iron curtain. One is from Albania. He has some equations that I haven't seen anywhere else.

-TL

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Old March 28, 2024, 02:31 PM   #311
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Geoffrey Kolbe is a good fellow to communicate with about litterature. From his posts, I infer he's got quite a ballistics library.
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Old March 29, 2024, 03:43 PM   #312
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stagpanther: heh yes.
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Old April 10, 2024, 12:02 AM   #313
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While Unclenick works on the illustrations, let's look at an easier puzzler, shall we?

Real story. The turret wheel of my scope wore out. Clicks had become mushy. I fixed it with some epoxy creativities. But in the process I lost the zeroing. I was eager to shoot next day, so I did the barrel sighting in the garage in the middle of the Sunday night.

I picked a screw head on the garage door as reference point. The scope was mounted on an AR upper 5 yd away. Bore on the screw head. Cross hair on the screw head. Good to go.

Next day. Target was a 24"x24" cardboard 100yd away. I knew the it was going to hit high, so I aimed at the lower edge of the cardboard. 3 rounds, all off paper. Dust on the berm showed poi way high. 40 - 50 moa high.

Why does it shoot so high? I used the same trick to bore sight my Remington 700 in .30-06. It was on paper, even it kicked much harder than 5.56.

What happened?

-TL

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Old April 10, 2024, 05:43 AM   #314
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Quote:
While Unclenick works on the illustrations, let's look at an easier puzzler, shall we?

Real story. The turret wheel of my scope wore out. Clicks had become mushy. I fixed it with some epoxy creativities. But in the process I lost the zeroing. I was eager to shoot next day, so I did the barrel sighting in the garage in the middle of the Sunday night.

I picked a screw head on the garage door as reference point. The scope was mounted on an AR upper 5 yd away. Bore on the screw head. Cross hair on the screw head. Good to go.

Next day. Target was a 24"x24" cardboard 100yd away. I knew the it was going to hit high, so I aimed at the lower edge of the cardboard. 3 rounds, all off paper. Dust on the berm showed poi way high. 40 - 50 moa high.

Why does it shoot so high? I used the same trick to bore sight my Remington 700 in .30-06. It was on paper, even it kicked much harder than 5.56.

What happened?
Rapture effect from the eclipse?

Close-in sighting like you did I usually do only for leveling of the scope's reticle--I wouldn't count on it for accurate representation of actual bullet trajectory--despite the many products that claim they can do that for you. Did you try contacting the scope manufacturer prior to home-repair? Scopes are the one firearms product I won't attempt to goof with as they are just too tricky and dependent on how they are assembled.
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Old April 10, 2024, 05:44 AM   #315
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Range-Error-Ratio (simplest* math game)

Barrel is off (pointed up) from scope line of sight by 1.5" (nominal scope height) at 5 yards.
Assuming minimal bullet ballistic drop through first 100 yards, that's a (100/5)*1.5" = 30" error at 100.

Trajectory table calcs to confirm, but that's my initial *pre-coffee fingers & toes methodology.

.

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Old April 10, 2024, 05:51 AM   #316
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Note that the military M4 zeroing target is used at 25 yards.
That cuts the 1.5" LOS error down to only 100/25 * 1.5 = 6" high at 100.
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Old April 10, 2024, 09:17 AM   #317
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We are close but not quite. Here are a few hints.

1. The method works (on paper at 100yd) for conventional type of rifle (Rem 700). The problem seems to have something to do with AR type.

2. M4 25yd zero is not for 100yd. 300yd if I remember correctly.

For the scope, that is another story I can write up. It is a cheap no-name scope but it works quite ok. It is not worth contacting the manufacturer, if they are still in business somewhere in the far east. It is a good candidate for tinkering. The repair worked. The scope is back on line pulling the plow.

-TL

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Old April 10, 2024, 09:56 AM   #318
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This is not AR-specific in effect.
Rather... pure geometry for any rifle with a scope offset above the bore:



The reason the military uses a 25-yard zero is that it flattens the bullet climb-out
to where you have a point blank (+/- 4") battle zero out to 300 yds
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Old April 10, 2024, 10:06 AM   #319
tangolima
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Quote:
Originally Posted by mehavey View Post
This is not AR-specific in effect.

Rather... pure geometry for any rifle with a scope offset above the bore:







The reason the military uses a 25-yard zero is that it flattens the bullet climb-out

to where you have a point blank (+/- 4") battle zero out to 300 yds
Your graph shows what I have with REM 700. It is on paper at 100yd (error less than 24"). But with my AR, the error was 40"-50".

-TL

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Old April 10, 2024, 10:09 AM   #320
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Measure your actual scope height above the bore.
If 2" your 100-yd impact will be 36" high
(Same error if 1.5" but only 4 yards)

If 2.5"/5yds . . . 45"

Very little change at those close-in zero geometries makes a big difference

Last edited by mehavey; April 10, 2024 at 10:46 AM.
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Old April 10, 2024, 10:49 AM   #321
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Ahha. That's it. 1.5" above bore on rem 700, 2.5" on AR. 1" at 5yd adds 20moa. The method needs modification.

I will sharpie a dot above the screw head by the amount (sight height - 0.5"). Bore on the screw head. Sight on the dot. It will be on paper. Guaranteed.

Very good. Let's go down into bore sighting rabbit hole a bit more. I never use any those laser gadgets. How about them guns that don't allow me to see through the bore. I have my method. What is yours?

-TL



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Old April 10, 2024, 11:31 AM   #322
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tangolima: grab a pair of sizors and open them slightly not the distance apart of the cutting ends and the handle ends, then open them further and look. that's the difference in scope hight above the bore. as scope hight increses poi -vs- los increases. at a given range, the liength of the sizors
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Old April 10, 2024, 11:32 AM   #323
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you are aiming at the hinge, the top handle is the scope and the bottom handle is the chamber. think about it.
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Old April 10, 2024, 11:41 AM   #324
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seems i should have updated before typing. sorry.
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Old April 10, 2024, 01:13 PM   #325
tangolima
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Quote:
Originally Posted by georgehwbush View Post
you are aiming at the hinge, the top handle is the scope and the bottom handle is the chamber. think about it.
I don't quite get the picture. Can you elaborate further? Thanks.

-TL

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Last edited by tangolima; April 11, 2024 at 01:04 AM.
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