Bullet stabilization

[Fiddler Green]

This thread has had some posts removed but will be un-locked in case anybody has any information to add.

...

OK, first, Yes, I know it's true. It just doesn't make sense.

I've shot my 45/70 at 100 yards and the groups were....well, not worth measuring. Yet, the same load (61 grains of Goex cartridge and a 535 grain Postell) are really good at 450 yards. The story is: the bullet needs time/distance to stabilize. But, if it's off course to start with....how does it find its way back on course?!?

Explain, please.

Bruce

[Sir Charles deMouton-Black]

A projectile from a gun or firearm is like a gyro-scope, and "precesses".  Like a childs top, the axis of the projectile points slightly away from the axis of projection, and moves around, describing a cone in front of the projectile.  It is a very complicated phenomenon, caused by air pressure and gravity acting on the spinning projectile which is trying to remain pointed to a fixed point in space.  Each force causes a deviation at right angles to the force applied.

http://en.wikipedia.org/wiki/Gyroscope

http://www.google.ca/search?hl=en&defl=en&q=define:precession&ei=bOkxSqyWJIfCtAPcxtTXBg&sa=X&oi=glossary_definition&ct=title

However, at some point the precession becomes less.  In effect it "goes to sleep".  This may be what you are seeing.

My apologies for this clumsy attempt at a technical explanation, but I'm trying to recall my ballistics training from 1972! If you want more, google "external ballistics!

[Fiddler Green]

It still dosen't explain why a bullet, seemingly going off course at 100 yards, will be back on course at 450 yards.

Bruce

[Delmonico]

Don't know what causes it, don't really care, but by using a bullet as perfect as humanly possible cures it.

[Daniel Nighteyes]

It still doesn't explain why a bullet, seemingly going off course at 100 yards, will be back on course at 450 yards.

Makes perfect sense to me.  Can't explain it, but I can see in in my mind's eye.  Sort of like a ballistic cone with the "point" downrange.  The closer to the muzzle, the greater the projectile's ballistic variation.  As the projectiles get further downrange, the projectiles' individual variability steadily decreases...

Not a complete description, to be sure.

[Fiddler Green]

Makes perfect sense to me.  Can't explain it, but I can see in in my mind's eye.  Sort of like a ballistic cone with the "point" downrange.  The closer to the muzzle, the greater the projectile's ballistic variation.  As the projectiles get further downrange, the projectiles' individual variability steadily decreases...

Not a complete description, to be sure.

Yea, that's where I am, I know it works that way, but, it just doesn't make sense!

The bullet acts like a laser or heat guided missile, when it's first launched it needs to stabilize and pick up its track and then it flies to the target.  But, there is no guidance system on a bullet! 

Bruce

[Delmonico]

Think about it long and hard, the whole bullet is a gyroscope.

[Delmonico]

Go to your local toy store and buy one of those kids gyroscopes like a lot of us had over 40 years ago and play with it, you'll understand.

[Daniel Nighteyes]

Yea, that's where I am, I know it works that way, but, it just doesn't make sense!

The bullet acts like a laser or heat guided missile, when it's first launched it needs to stabilize and pick up its track and then it flies to the target.  But, there is no guidance system on a bullet! 

It helps a LOT that I've actually SEEN tracers do the "ballistic cone" thing that I was talking about.

As you know, the bullet is more-or-less rotating on its long axis, but what you may not be considering is that the rear part of the bullet is doing a kind of "hula" that initially causes a LOT of variation.  That variation makes the bullet's trajectory into an ever-decreasing spiral because, as the backside's "hula" gets less and less dramatic, the bullet's trajectory settles down until it eventually becomes a predictable parabolic curve.

There are a lot of reasons for this.  Boat-tail bullets do it less than "regular" bullets, but all of them do it to some extent.  I don't know if this helps one tiny little bit, but its the best I can do...

Regards from the Left Coast,

-- Nighteyes

[drcook]

I'll bet that if you push that Goex charge up towards 70 gr or a little more that it will shoot cleaner and
give you a little more consistency. Goex is not like Swiss, it needs a bit of compression to shoot well.
61 grains is not much compression. I will also bet that once you push it up a little and get it to settle
down, that Postell will shoot well at all ranges

dc

[Delmonico]

The qualilty of the bullets esp the bases is very important, as I've saif before, weighing bullets after visual inspections for flaws is important, once again a 1 gr difference lower in weight than the perfect standard for ones bullet and alloy means there is one or more voilds in a bullet that are about the size of a #8 shot.  If one will not shoot a bullet with visibles flaws that size, why do it with flaws you can not see?

Also a base needs to cut off perfect with no visible flaw in the base or a slight "bump" in the base.  The base of the bullet being the most important part.

[drcook]

What Delmonico said about bullet weights is 100% correct. Differences in weights can explain some of
the flyers that I see when I have spotted for different folks in the past. I have watched steel targets
being hit, then out of the blue, one comes a few inches to a foot or more higher (or lower). Voids also
help explain veering off to one side as they are gyroscopically unstable. Just like your car tire when it
throws a weight and causes the car to vibrate.

If a bullet has a void and manages to shoot straight, it will have a higher velocity than the
heavier bullets and change the point of impact.  Same with heavier bullets due to the mould
heating up cooling down etc.

I recently bought a RCBS electronic scale because it was on sale, I was trying to save a few dineros.
Brought it home and within the hour it was on its way back to the store. It wouldn't repeat itself.
I would weigh a bullet, then reweigh it, then try it again and each time got a different weight. Whether
it was the particular scale had an issue or the whole bunch, I decided to go ahead and get the one
I originally wanted.

I purchased a Myweigh Ibal 101. It cost 52.00 more with shipping and repeats itself, has double the
capacity of the RCBS and is overall, much much more scale than the little bit of more money suggests.

I now have batches of bullets that are within .5 grain of each other. I too was getting some
unexplained flyers when I would shoot over or under a steel knockdown target. Now I am going to see
how well this changes how I shoot.

here is the scale:

http://www.myweigh.com/scales/medium-scales/ibalance-101

if it turns out that anyone is interested, there is a gentleman on the Shiloh board that is a shooter of
course and sells these. (I have no monetary interest just sharing what I have found out and bought)

dc

[Delmonico]

drcook, it amazes me sometimes when someone had poor accuarcy with one of these rifles and wants to know if they should try a differnt bullet, primers, powder, case, rebarrel or who knows what else.  When you ask them how close the bullets weigh they look at you funny and say, "don't know, never weighed them."

If you don't have good bullets, accuarcy is not going to be something you have., a cast bullet needs to look perfect and has to weigh perfect or it won't work at distance.  I use 1/4 grain above or below 509 with my RCBS 500-Sil mould with 24-1.

I allow 1/2 grain on the Lyman 457122 I hunt with because distance will not be great, the target is fairly good size and them hollow points can be hard to cast.

[Delmonico]

The explination is simple as we've said, they don't stabalize till they get out at some distance and return to the center where they were launched from.  The better the bullet, the less it happens.

[drcook]

Now I wouldn't go and say that they aren't accurate at shorter ranges. I have competed at ranges
from 100 yards to 1000 yards with a reasonable amount of success. The below target was shot at
200 yards. That's 5 shots.

I start tuning my loads at 200 yds. My shooting partners and I strive for 2" at 200 yds. I am in the
process of tuning a 45-100. I am down at the 1/2 grain stage now. 

This is my bragging target. Wish I could shoot this well each and every time. Believe it or not, I came
in second place with that group.

These old style guns really will shoot well.



dc

[drcook]

there also must be taken into consideration the transition from "supersonic" back through the
turbulence when the bullet comes back through the sound barrier. I had a wonderfully accurate
load that shot 1 1/2 at 100 yds but wouldn't group well at 200, and by not well it wouldn't stay
on a target 18 inches high. so it was worthless for the competition I was loading for

in discussion with some of the other folks on a "bpcr" board, it was
suggested the bullets might have been going through the sound barrier and causing the 200 yd
issues. I never shot them out further, they could have settled back down.

i still don't follow what is meant by being more accurate at long ranges.  other than the 1 bullet
that my Stevens 45-70 wouldn't shoot well, my rifles are as accurate at 100 as they are at 1000.

black powder loads are more accurate at the longer ranges than smokeless loads in the same cartridges
due to black powder's more consistent velocities.

 

[drcook]

I was also thinking this through more last evening, and what we haven't discussed is rate of twist in the
rifle that is being shot.

I went and was doing some reading over on the Shiloh board and can say that I saw that most folks
are using at least 68 gr of powder under a bullet of that size. You may not be getting enough
velocity for adequate stabilization.

dc 

[Sir Charles deMouton-Black]

The Greenhill formula Calculator;

http://kwk.us/twist.html

I have previously used the version on Z-Hat;

http://www.z-hat.com/twistrate.htm

[Fiddler Green]

I was also thinking this through more last evening, and what we haven't discussed is rate of twist in the
rifle that is being shot.

I went and was doing some reading over on the Shiloh board and can say that I saw that most folks
are using at least 68 gr of powder under a bullet of that size. You may not be getting enough
velocity for adequate stabilization.

dc 

Were they talking by weight or by volume? I'm using weight as I have a digital scale. My load is 61 Grains, of Goex Cartridge, with CCI BR-2 Primers in Hornady, Starline and PMC cases (no, I don't mix them).

Again, I'm hitting targets at 450 yards with this load. I'm a former bench rest shooter, trained by the US Army to hit targets at long range and have practical experience at same and I'm the defending, local, Billy Dixon Champion. I don't claim to be Mike Venturino, but, I can hit a target.

I've clocked that load through a Pact Chronograph at 1,210 Fps. The issue is not accuracy at distance, I've got that covered, it's poor accuracy at close range that improves with distance that's got me baffled. One would think that a bullet, off course, would tend to stay off course, as that is the bullets new course. This would be true whether the bullet is wobbling or not. If a wobbling bullet is going off course and stabilizes, it would seem logical that it would continue on its current line of travel: off course. Yet, this does not seem to be the case. When a bullet stabilizes, in flight, it would seem logical that it has 360 degrees, or paths, that it could stabilize on and that would mean that you would have only a 1 in 360 chance (or 1 in 720, or 1 in 1440 depending on how far the distant target is) that it's going to head back toward the original intended path.

Yet, this seems to happen, everyone says it does, no one can really explain why.

Bruce

[drcook]

Bruce,

I and no one here is disputing or questioning your ability  just trying to answer the question and work
out the solution.

The charge is by weight. I don't know of any one shooting black powder in the BPCR,creedmore or gong
type matches who doesn't weigh the charges and drop them into the case with a drop tube. I weigh
all mine on a beam scale and then into the case through a 24" drop tube before compressing the powder.

What I do know is that black powder rifles are more accurate at long ranges THAN SMOKELESS cartridges
cartridges WITH THE EXCEPTION of dedicated tactical rifles and the tuned loads they use because
once a black powder round is tuned in, the variations in muzzle velocity can be in the single digits. So shot
to shot consistency is better.

When I competed at Alliance Neb in the 1000 yd match, the black powder shooters consistently outshot
the smokeless powder shooters due to this.

While I have no formal training, I have been coached and then developed my own techniques, solely
in black powder shooting. Smokeless is a whole nother animal and I have absolutely no experience with
it. 

I am not seeing any difference in accuracy from 100 yds to 1000 yds in
any of my rifles, except for 1 bullet that 1 of the rifles didn't shoot well. I am shooting rifles (Shilohs and
CPA Stevens, and a Pedersoli Sharps) in calibers 38-55, 40-82, 45-70,45-90, 45-100 and 45-110.

The 38-55's are my wife and daughter's rifles. They have a 1:16 twist barrel (a little slow I know) so I
had a 250 gr bullet mould built for them. We haven't shot this beyond 235, but at 100,150,210 and 235
the accuracy is there.

The 40-82 is just coming online. I had it built with a 1:13 twist Lilja barrel to stabilize bullets out to 1000
yards and to be a NRA sanctioned BPCR rifle for shoot at 200,300,385 and 500 meters. And at our local
ranges short course silhouette which is 100,150,210 and 235. 

the 45 calibers all shoot well from 100 to 1000 yds. They are all 1:18 twist barrels and I shoot 510 to 550
gr bullets.

So unless I am misunderstanding, I myself don't agree with the idea that they are more accurate at
long ranges than they are at short ranges.

For NRA sanctioned BPCR matches, the rifles have to hold a minimum of 2 moa to be competitive. I agree
with your thoughts it doesn't make sense. I think that at 450 yds (once again not questioning your
shooting abilities - just commenting on the load) that it is just one of those things that it happened to
group well at that range. Could be explained by visualizing a sine wave and the ocillations just happened
to coincide at that distance.

In tuning my loads, I have found that a 4 grain spread of powder can make a dramatic difference in how
these guns shoot. I am tuning the 45-100 again (new heavier barrel) for long range. BUT I will also shoot
it at the short range course. I started at 93, going 94,95,96,97 and 98 grains. The groups were big (8")
and then shrank to 3" and the expanded again with the 3" groups (at 200 yds) using the 96 grains of
powder. I am now going by 1/2 grain till I hit 2" at 200 yd which will be 1 moa.

Over on the Shiloh site there is a new discussion about kicking the amount of powder up.

As just an aside and to throw another wrinkle into it, we have a local club where the members shoot
old time muzzle loading slug guns. Yes muzzle loading. These folks will shoot 1 hole groups at 200 yds.
1 guy could write his initial within a 2" circle at 200 yds. They are shooting black powder and they are
loading by volume, instead of by weight.

regards,
Dave