Bullet Stabilization Questions

[Michael Eichele]

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 ]

 

[4mesh063]

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.

 

[4mesh063]

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.

 

[Brent]

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

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???

 

[MAX]

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

 

[Michael Eichele]

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.

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

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 ]

 

[4mesh063]

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.

 

[BountyHunter]

"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."

****, sure hope I get some of those "backward bullet shooters" on my next relays. That is one thing, I have got to see to believe!!!!!

BH

 

[BountyHunter]

JR

You trying to take all the fun out of it.

I still want them guys on my relay if they want to try it, but at other end of the line.

BH

 

[MAX]

Well now, backwards bullets eh?! One of my specialties for "short range shooting"!

I've yet to compute drift or drop, but a Hornady 148 gr HBWC loaded backerds over 3.0 gr of Bullseye will flat screw up an armadillo! Field tested out to about 10 yards so far. Shoots to same POA too!

Actually, I did load some 40 gr. Speer spire points backwards once for a Hornet whilst trying to develope some squirrel loads, and they shot well out to 50 yds but I didn't try further.

The "going to sleep thing"... According to Vaughn and a few others, most bullets exit the muzzle with at least two(epicyclic) precession modes, one fast, the other slow, and the resultant motion is a rosette pattern. Sort of a long arc interspaced with short loops. The short loops are the fast cycle precession which is the one that damps out for the most part around 200 yards or so, depending on GS. The slow cycle mode has a dimension of that may shrink in circumstance of high GS, or grow somewhat with marginal or low GS. Slow precession cycles do not null, and are continuous for the duration of the bullet's flight. At ranges typical of benchrest competition these issues are of small import, however, the precession cycle represents yaw from flight path, hence DRAG. It reduces BC. This is one reason that short range bench shooters may prefer flat base bullets and slow twists, whereas you fellows might like to err on the side of faster twists and higher GS. Do I need add High BC?

All yaw or precession cycles have a resultant value, that is, an average value. Even though a bullet may have an epicyclic(dual) precession mode, or tricyclic(3 or more) mode, and yaw of repose is in play, it all boils down into a single resultant value. As velocity changes, so too does the GS and this in itself results in more "trouble". How do we ever hit anything with a bullet?

This is one reason the 6DOF programs were developed and are so useful in ballistics research. Hope you have a supercomputer handy...

To clarify one comment I made above, I do not know how to quantify or express "turbulence" that may be associated with transonic flight. The region of highest DRAG that will be encountered in the atmosphere occurs between Mach .75 thru approx. Mach 1.3, and for most forms drag DECREASES as Mach increases above this speed range. I know things happen when a bullet slows thru this range, but not what or why.

It's Miller Time...

 

[JR]

Bounty Hunter,

When he says there was no difference in accuracy, maybe he meant didn't hit the target whether the pointy end was out or in, therefore neither was more accurate than the other..ha..

Right..here's my take on why some shoot better at 1K than 100yds..when a bullet exits the muzzle, it leaves with a slight angle of yaw, where a force acts upon a point on the Center of Pressure, which wants to make the bullet want to overturn on that axis and, well tumble..The spinning motion of the bullet should negate this effect, bringing gyroscopic stability, and the bullets precess around it's center of gravity, giving you nice neat little 1/2 minuter's.

Basically, the more wobble the bullet has as it leaves the muzzle, the longer it will take for the spin to correct..If the spin can not correct in the distance you are shooting, due to barrel whip, due to imperfections in the crown which deflect the bullet slightly off a reasonable angle of yaw, aerodynamically the bullet is presenting more surface to drag forces, which will in turn affect the BC, which will lead to spotty shooting..

Also, if you have a reasonable 100 yds shooter that won't group at 1K could be because the bullet is understabilised, or even overstabilised..WHOAH!! ha..

Keyholes at 1K are often due to the bullet's stability thru the transonic zone, a region right at the speed of sound, which is one the most horrendous afflictions to flight. Not just necessarily that it must be going 'x' fps, but have a bullet with a high BC and a spin rate to match, to cut thru the cheese.. using .223's(AR) shooting 90gr JLK's at 1000, have found that a 1/6.5" twist is preferable to drive this round thru accurately..1/7" just doesn't seem to cut it..It is preferable to remain supersonic at your yardage..ha..

but, I digress with my hoo-ha..

JR

 

[Michael Eichele]

I think MAX has made some pretty good sense here. I also have used 3-5 grains of bulls eye!! I filled 2 cases with epoxy resin up to the bottom of the neck and drilled out for a flash hole. Sounds like a cap gun. He he he!!

I think some bullets will stabilize quicker than others. I have noticed higer BCs in some bullets between 100 and 300 where as others didnt have much of a BC till after 300. An interesting side point, in the Sierra manual, they disscuss what each bullet was desighned for. Some are stated to have an optimum accuracy range of 100-600 yards, like the 168. Others are 100-1000 yards like the 155 and 175. Some are 300-1000 yards like the 180 and 190. I think the 240 was 600-1000 yards. Obviously bullet design and ballance have a little to do with it. Come to think of it my experianc with the 168 and 155 and 175 and 190 seems to be concistent with their statments. Also as far as bullets being fired backwards, the twist will stabilize them the same. They might wake up faster though. If you think about it, they might stay stable longer because of the forward weight. Ever shoot an arrow w/o vanes?? It works because of the forward weight. It might not be as accurate but it does work.

[ 01-24-2004: Message edited by: meichele ]

 

[demarpaint]

I would like to see those backwards bullet targets myself.

 

[Michael Eichele]

I am a little curious. Some here have said in 1 post that there is NO relationship between GS and BC. In others its stated that BR shooters use differant twists to offer differant GSF for shorter ranges.

If this is the case than it obviously matters what the GSF is to get a another BC.

Many shooters and even some bullet manufactures have done tests to show that in most cases using typical bullets that when a major change in GS occurs, a change in BC ocurrs as well. This would indicate that GS has a relationship with BC.

Think of this. If a bullet continued to wobble slightly for 600 yards before it settled down, what do you think that would do to its BC?