After reading some of joeb's answers and explanation of Greenhills Formula I am totally baffled. It seems to me(at least) that the stability of a bullet has to due with how fast it is rotating, not the twist that got it rotating. If a 30 caliber bullet leaves a 12 inch twist barrel at 3000 f/s, it should be rotating 3000 times a second. Let's say the bullet is stable at that rotational speed. Now lets send the same bullet through the same bullet, or a lead bullet of similar shape and weight, through the same barrel at 1500 f/s like we do with our cast bullets. It is now only turning 1/2 as fast but we know from experience that it will shoot well at that velocity. Do bullets have that much tolerance in rotational velocity or am I missing something? I make no claim to mathematics or engineering backround but would like someone to explain errors in my thing.
Twist rates for dummies
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- Last Post 21 December 2018
the cg has to do with the center of mass in the bullet ... the mass is in the bullet .
the center of pressure has to do with PRESSURE on the bullet by outside forces, such as the thousand mile per hour headwind the bullet sees on it's trip to the target.
if you hold the bullet in your hand it doesn't have a center of pressure ( except from your hand ... and that pressure depends on where you pick it up ) .
***************
with all this turmoil, it is a wonder the dang bullet ever gets all the way down to the target. amazing ...
ken
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After looking at NOE bullet diagrams and using a design which is offered in solid and hollow point, I found the answer. The cg moves closer to the base in a hollow point; the cp stays the same.
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Larry,
After reading the NOE information several times, could you (or someone) explain how to do the cg and cp balancing acts. Why do we need a scaled up cardboard cutout for cp?
If we hollow point a bullet, do cp and cg move the same direction or opposite?
THIS IS KINDA INTERESTING.
Cp is explained on the net, serious math involved, not for the feint of heart'
BUT,
https://spaceflightsystems.grc.nasa.gov/education/rocket/rktcp.html
includes
Mechanically determining cp
For a model rocket, there is a simple mechanical way to determine the center of pressure for each component or for the entire rocket. Make a two dimensional tracing of the shape of the component, or rocket, on a piece of cardboard and cut out the shape. Hang the cut out shape by a string, and determine the point at which it balances. This is just like balancing a pencil with a string! The point at which the component, or rocket, is balanced is the center of pressure. You obviously could not use this procedure for a very large rocket like the Space Shuttle. But it works quite well for a model.
which is an explanation of how to find Cg, not Cp, so even NASA is having trouble with Cp. As far as I've ever found, there's no simple way to find Cp, which changes for a bu7llet as it goes through the air.
To estimate Cg more easily make a copy of the bullet drawing, draw a line thru the middle lengthwise, and measure from the cut to edge of the bullet at various measured points a lotta times, sum them up and get the avg, that's Cg.
Or, measure your half bullet dwg. length, (Ex 5"), get excel to write a list of random #s, "Y coordinates, (Ex 2.1, 4.1, 1.3, …from 0 to 5"), measure the center to edge, avg both and that's Cg.
Or, just understand that for any bullet-shaped item, Cg is ALWAYS < 50% of total length from the base. And, that knowing Cg is never going to be of use in the shooting world.
However, talking about such things lends one an aura of knowledge and sophistication that fools some of the people, some of the time.
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Larry,
After reading the NOE information several times, could you (or someone) explain how to do the cg and cp balancing acts. Why do we need a scaled up cardboard cutout for cp?
If we hollow point a bullet, do cp and cg move the same direction or opposite?
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That's all well and good Frank because that's exactly what I've been agreeing with you about.....and trying to support......
"What I'm trying to show, with my targets, bullets and powder charges, is that the 13 gr powder charge, shown stable at 100 yards with the M43, was still gyroscopically stable, while being push around by transonic forces at 200 yards and the transonic forces, where causing the tipping, not instability."
Exactly what I've been saying and my posted targets demonstrated also. Yes, there is other games afoot besides loss of stability that can cause the bullet to pitch, yaw or tumble.......or fly off away from the intended line of flight.
LMG
Concealment is not cover.........
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LG,
What I'm trying to point out, is that you stated or inferred that my 1.19 long bullet had marginal stability.
What I'm trying to show, with my targets, bullets and powder charges, is that the 13 gr powder charge, shown stable at 100 yards with the M43, was still gyroscopically stable, while being push around by transonic forces at 200 yards and the transonic forces, where causing the tipping, not instability. It would have recovered after it left the transonic range.
Frank
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Michael,
The event that I'm speaking of was shot in the UK. The US team won, I believe.
I did read the article, on that US Army event, probably 20+ years ago
Frank
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Michael,
First, if you where a member of the 2002 US Creedmoor team, you must have known Bill Crane. We shot together in the Springfield, Oregon Schuetzen matches in the 80's and early 90's. I believe he obtain the barrels for those rifles.
Hello, Sorry, I don't know the name Bill Crane. He was not at the match held in September of 2002, Whittington Center, Raton that I know of. No one provided rifles or barrels to us. All competitors provided our own personal equipment and ammo.
You must be confusing two events.
You mention stability and the Springfield rifle (45-70) at a 1/4 mile? I imagine you might be referring to the US Army shooting trials conducted sometime in the 1870's at Sandy Hook. Do a search, it is interesting. The 45-70 was shot to maximum range and the impacts were plotted for angle, distance and more. A good read!
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i don't have a dog in this flight
( see what i did there ? ... flight ... heh )
but about 20 years ago i bot a program that used REAL bc, then was advised that different bullets have different bc at different velocities and humidity etc., so
you need both the horse AND the cart to forecast what is going to happen to your bullet in respect to trajectory ... you need some of the results to improve your prediction .... apparently the same holds true for effects of spin stabilization ...
*****************
these threads on stability reminds me of watching the olympic bobsled runs .... starts out high, easy at first, then shortly starts bouncing from side to side, and all the time going faster and faster downhill ...
but really fun to watch, and just as we hit bottom , we discover that we have gleaned something from the effort after all ..
****************
oh, and just like the bobsled run, i choose to go the safer route and be a supportive observer rather than a participant ... ( g ) .. thanks for the show ...
ken
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Frank
"LG, it's you that use and promote the Miller formula. It gives no allowance to bullet shape. That something that I'm try to help people understand, with real time experimentation and my stability program."
Again, you're making an erroneous assumption and then make an incorrect statement based on that assumption. I use several twist formulas including Miller's. I do not "promote" Miller's formula for cast bullets because the constant in it is based on long for caliber VLD type bullets. I have stated that in these threads. The Greenhill and Miller formulas were the ones mentioned in these threads by joe, YOU, myself and others.
In case you've not noticed I have been supportive of your contention that the on target results don't always coincide with the formula's answer, especially when the constant, which considers the shape of the bullet, is not correct. Suggest you take rmrix's advise and try some real long range shooting. You might find it interesting and instructive.
LMG
FYI; I'll add that many programs also require a BC which does consider bullet form and is used in the down range ballistics part of the programs (includes stability in some). The BC is calculated or entered using a "standard form" of which there are several. The G1 form is most often used with most bullets. It's use gets you very close in most instances if the other input data is correct. However, for use with long VLD type bullets the G7 form is considered more accurate.
Concealment is not cover.........
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This 11.5 gr target, above, was my first, 200 yard target, after sighting in, when I first built the rifle. The mold was on loan to me. I kept going up in my powder charge as I progressed and at the Modesto match, I made my final powder charge increase, to 13.5 gr #9. My next 10 shot group target, at my home range, was 1.44 @ 200. Then a few months later I shot a .463, 5 shot group @ 200. I haven't change the powder charge since. All with the 1.19 long bullet.
Frank
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Michael,
First, if you where a member of the 2002 US Creedmoor team, you must have known Bill Crane. We shot together in the Springfield, Oregon Schuetzen matches in the 80's and early 90's. I believe he obtain the barrels for those rifles.
If you've read the ASSRA thread, you know that I've had Rotor Cuff surgery (rt shoulder, of course) I tore it shooting my Borchardt 45/70, in a BP match in 1995/6.
The only way that I might compete, is with a 38 and that, I'm investigating. But, it seems that the 38's (other than one or two) aren't doing well past 800. It would be a great teaching experience though.
LG, it's you that use and promote the Miller formula. It gives no allowance to bullet shape. That something that I'm try to help people understand, with real time experimentation and my stability program.
I do have empirical documentation of the 13.0 gr load. No, I did not chronograph the 12.0 gr load, it had to get to the 200 yard target, with it's 10 gr of extra weight, at a much lower velocity and all your assertions of CP, would make it less stable at any range and velocity.
I have another target at 11.5 gr and a 1.21 long bullet, showing stability at 200 yards. I'll post it too, when I can.
Frank
Got it. It wasn't the size but, I changed the file name. It seems (maybe) if the system rejects a file, you can never post it again. I don't know why it would have been rejected, in the first place?
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Frank, I am thinking, ….dangerous, I know, that you have developed great interest in black powder long range shooting and you could test your theories buy barreling your old Roller with the 458 -14 twist and shoot the matches next summer in the PNW. That would be a good lead into the MT match in September 2019. I may try to make that one if I don’t draw a BP ML elk tag. We can ask to get squaded so that I can pull and paste your targets in the pits.
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frank .. don't know but if the pic is " too big " it is rejected. ... too many pixels ...... you might need to take less dense pics or edit it skinnier in a photo editor.
or send it to my email, i will give it a whirl. [email protected]
ken
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Larry,
Regarding your (damn good BTW) target with the 2 "fliers" - what were mirage and/or wind conditions like? Out beyond 200 yards anyway wind, particularly wind near the target that the shooter may not be aware of without good enough wind flags, can move shots around quite a bit. Great work that you tracked the velocity and know that velocity variation was not the "root cause".
This thread should be re-labeled something like "Twist effects for the Advanced Shooters and Thinkers".
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Frank
As I said earlier, stability isn't hard and fast........you have 2 different bullet designs there......different nose profiles, different lube grooves, and different locations of the CG to CP, the meplat on one (moves the CP forward), different BCs, etc.
Have you the actual chronographed velocities? With that knowledge we can compute where the bullets (within a reasonable distance) could have been buffeted or otherwise adversely affected in the trans sonic zone. Without that knowledge we are only making assumptions which, as we've seen, if based on "I don't know for sure.. or I think.....or I assume...." most often aren't correct. Thus if you have the actual velocities ?
LMG
Concealment is not cover.........
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Ok, we'll use "marginally stabilized".
If my 1.19 long bullets are, marginally stabilized. How is it that I can shoot a .13 longer bullet, at a lower velocity and still have "good" stability?
Still can't post a picture. Monitors HELP!
Frank
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Frank
Your assumption as to "LG's assumption that my tipping bullets, caused be transonic conditions, were on the verge of being unstable" is incorrect.
What I said was; "Bullets can also be marginally stabilized, fly point forward and give some semblance of "accuracy" as seen in examples posted by joe and Frank."
I did mention some bullets can lose stability when dropping back through the trans sonic zone in a later paragraph. Perhaps you misconstrued one with the other in making the incorrect assumption?
LMG
Concealment is not cover.........
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Well, I was able to post only two more pictures, I'm not able to post the target but, the target has a 200 yard group, about .8 tall and 2.5 wide. Edit; I measured it and it's actually 1.99 wide.
I will post it ASAP, when the forum let's me.
Again, monitors, what is up with not being able to post pictures?
Frank
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Now, since I'm back to being able to post pictures. My rebuttal to LG's assumption that my tipping bullets, caused be transonic conditions, were on the verge of being unstable.
This target was shot from the same rifle. The bullet is the NOE, 33 caliber version of the 311365, 1.32" long. .13" longer than the bullet, that is tipping so much in the target I posted, that bullet is 1.19 long.
This bullet is heaver by 10 gr and the powder charge, is a full 1.0 gr less at 12.0. I did not chronograph the load but, it has to be going through the 200 yard target, at below the speed of sound.
Pictured are the two bullets and the target fired with the 1.32 long bullet.
Frank
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