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Bullet Stabilization Questions

 
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  #15  
Old 01-20-2004, 05:58 PM
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Re: Bullet Stabilization Questions

If bullet path vortices had anything to do with it it would happen with all loads and not just some.

The only other thing that I can come up with is for some reason or another the bullet has not gone to sleep. The Sierra manual states that the BC for 0-100 yards and in some cases 0-250 yards is noticably lower than the overall BC. This is due to the bullet exiting the muzzle is wobbling for a time, ussually caused by the gasses smacking the back of the bullet and or runout in a loaded round. As the bullet is in its free enviornment the rotation of the bullet causes it to stabilize. This is the point when it goes to sleep, or as some say, settle down. If you run it down a barrel with too tight a twist the bullet stabilizes too quickly, when that happens, it stays closer to the angle it was departed at. When that happens the BC is not nearley as high as it couold be because of the increased bearing surface of the bullet due to its not being pointed in the same direction its traveling. This causes rapid deceleration, which from everything I have seen can cause a loss of accuracy because the bullet is not asleep at that point. Once again this is only a theory.

If the bullet was perfectly stable upon exit of the muzzle it, in theory will be accurate from 1-300 or 400 yards. After that the scenario described above would cause too much velocity decay and the "waking up" of the bullet causing the bullet to wobble and eventually hit sideways. If the bullet is allowed to stabilize in its free enviornment, in theory, it would be most accurate from 300 to 1000 yards or when ever the bullet hit the transonic wall, when this happens the bullet will wake up and wobble and eventually go unstable.

If any of you have experiance with shooting a 308 win at 1000 yards, and I use the example of the 308 because it is not the easiest load to find loads for it that will even reach 1000 yards. Ask your self this: Have you ever had a load for your 308 that was .25 MOA at 100 yards and .25 MOA at 600 yards and sideways at 1k?? I have not. Have you ever had a load that would shoot .25 MOA at 100 yards and the 1 full MOA at 600 yards and even reach the target at 1k? I have not.

What I do know is that if my bullets are truly asleep, they will reach the target at 1k. If it reaches the target at 1k it is always a load that is very accurate all the way. The loads that are very accurate at 100 and 300 but not at 600 never make it to 1000 yards. This has to be from the bullet waking up, so to speak.

Regards

[ 01-20-2004: Message edited by: meichele ]

[ 01-20-2004: Message edited by: meichele ]
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  #16  
Old 01-20-2004, 07:20 PM
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Re: Bullet Stabilization Questions

After asking a question about group sizes needed to win in a short range BR match, I was given the following statistics. I had asked what was an average top 10 Match agg at 100/200 yards in the Light Varmint (10.5# class). The answer was .180" for 5 5 shot groups at 100, and .205 for 5 5 shot groups at 200. That means that they shoot, on average, about 12% bigger at 100% farther.

Think about that. A PPC shoots almost no different at 200 than it does at 100, Yet, noone shoots them for LR. And what's more, just like it's said above, they dont even weigh powder out when they load these things. It's just too small an error at the ranges they shoot. While at 1K, it's a must.

If anyone had all the answers about how a bullet flies, They'd be writing a book. I've heard so many tales of how a gun shoots real well at one range and real bad at another, I couldn't count them all. I have done it and seen it myself lots of times. There's just too much going on after the bullet leaves the barrel to have an explaination for it all. About the only answers you're gonna get on this question are opinions and estimates. Every one of them may be correct or incorrect to some degree, or under a particular set of circumstances and not under another.
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  #17  
Old 01-20-2004, 07:27 PM
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Re: Bullet Stabilization Questions

Quote from Wyo:
Someone please explain to me how if the rifle shoots into one hole at 100 why it might not shoot that well down range? If it is holding .5 MOA at 100 what would cause it to hold 2 MOA at 500? taking out all human variables.

I don't think I've ever seen this set of circumstances. Out to about 500 or 600, I've never seen a gun open up that much. I have lots of people come to my house to shoot at just those ranges and in about every case, they seem to either hold the MOA accuracy level, or get better. I've seen .75" guns @ 100 shoot 2" at 400 but other than shooting sucky factory loads, never seen the opposite like 15" at 400.
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  #18  
Old 01-21-2004, 02:59 AM
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Re: Bullet Stabilization Questions

Phil,

I think I might have gotten a fair amount of accuracy at 100 yards, but about 15" at 400.... but that was with the 240 SMK in the 11 twist 30-338. Unstable [img]images/icons/grin.gif[/img]

I know what you mean though, a 10" group at 500 yards, there's something serious wrong there.

That 10% increase over 100 yards -
That's 100% increase over 1000, but like you said, they just throw charges and shoot.

Still, I'd expect they'd be trickling if the felt that ES was the cause of the increase???
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  #19  
Old 01-22-2004, 08:53 PM
MAX MAX is offline
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Re: Bullet Stabilization Questions

michele, in regards your .308 experience:

I'm not expert in this and am shooting from the hip by not researching your particulars. That said:

1. BC does not have particular effect on gyroscopic stability(GS). GS is a function of twist/bullet length. GS may exist in one atmospheric circumstance and not in another, even with the same bullet/twist and velocity, due to differences in air density.

2. GS is inverse to velocity as the bullet travels downrange. This is because of reduced aerodynamic loads and resulting moments, and the fact that rotational velocity of the bullet decays VERY SLOWLY. Because GS is inverse to velocity, so to is tractibility(bullet nosing over to follow the flight path). This contributes to a reduced BC as does the increase in coefficient of drag due to Mach number decay.

3. Highly stabilized bullets suffer more from yaw of repose that bullets with low GS. As I understand it, a point can be reached in high GS scenarios that will cause a bullet to tumble because of the issue of yaw of repose and intractibility. In other words, it gets a bit too much sideways, and aerodynamic moments overcome GS.

Apparently there are two paths to the same result. Read on:

4. There has been discussion here in the past that stipulated bullets with low GS may tumble due to turbulence associated with transonic flowfields. I have not read authoritative sources on that issue, and remain undecided.

5. Speculation: Your 1:12 twist may be a bit slow for the "cold dense air" and your resulting GS low. It is common for bench shooters to use different twist rates at different altitudes due to this. Harold Vaughn discusses this at some length in his book, regarding the 6mmPPC and the use of 14" or 16" twist rates for a FB spitzer of 60 grains or so. By adding a little bit of velocity, your bullet may have had a bit higher GS or remained supersonic to POI, or both.

In any case, BC has no direct relation to GS. The cause of your bullets getting sideways lays elsewhere.

If I spoke incorrectly on this, I'm sure one of the guys or gals here will correct me, but that is MY take on your experience. The Physics of Exterior Ballistics is intolerant of extremes. JMO
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  #20  
Old 01-22-2004, 11:08 PM
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Re: Bullet Stabilization Questions

I understand that BC has little relation to GS. However, if I cant keep the BC high it WILL go unstable before it reches 1k as it doesent retain enough velocity to reach that far before hitting the transonic wall. [img]images/icons/wink.gif[/img] I also understand twists and their relashionship to apmosperic conditions. I sometimes hunt with differant bullets during winter months than summer for that reason.

Also, the 11.25 twist doesent do any better in even a little warmer air with those particular bullets. They are just not designed for 1k shooting. As I have learned, some bullets are made for 1k and some are not. The 150 nosler BT and the 168 SMK are NOT made for 1k shooting. This is partly why, and I do say partly, why they dont reach 1k when fired from a 308 win. Other bullets like the 175, 155 and 178 have gone to 1k with FLYING colors [img]images/icons/smile.gif[/img] [img]images/icons/smile.gif[/img] Mostly because thats the job they were made to do.

Most of what I said earlier are things I am doing research on and have NOT YET arrived to any exact conclusions. Most are thoughts to ponder and help others to peice together their own puzzle. The big picture is more complex than anyone has ever thought. Everthing has a relationship in ballistics. Even GS and BC. Without good BC, GS will decay faster than it should and without good GS the high BC just will not be there.

As for most of the rest, we will maybe in time truly understand it all.

MAX, thank you for your insight.

[ 01-22-2004: Message edited by: meichele ]
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Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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  #21  
Old 01-23-2004, 12:05 AM
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Re: Bullet Stabilization Questions

interestingly enough, I have talked to a few guys who make bullets and to a man, each has tried shooting bullets backwards. Even at 1000 yds, they see no difference in accuracy. That would lead me to beleive that bullet shape has nothing to do with the bullets stability. I've got no first hand experience in this however.

Making a projectile fly is more complex than just smashing some lead and copper together. I will admit though that I rely on Walt Berger to handle that part of things. I've never shot bullets backward but that day will come.

I aggree with Max on the GS/BC Non-relationship. I would also say that there is a place where a bullet goes to sleep for a period of time and if you can make sure your target is in the bullets way at that point, you will shoot good groups. Finding that spot is not so easy.
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