What say you?

BS, they are spin stabilized not laser guided

Sounds like a parallax issue to me.

My take, based on my limited knowledge of the various sciences, is that they should be at the most stable they are going to be upon leaving the muzzle (at their highest speed). I would think that any spinning projectile, moving forward, would only tend to become less stable as it decelerates. I will await someone more knowledgable than myself to see if my pseudo-educated guess is correct.

Hell if I know…but this is an interesting discussion.

The stability factor actually does increase at range. The angular velocity of the projectile decreases more slowly than the linear velocity, so the aerodynamic forces on the bullet decrease faster than the gyroscopic stabilization.

Read up on bullet coning motion. Supposed to be its greatest at the muzzle. Settles out down range. Several good research articles on it, even Sierra admits it’s possible.

And there you have it, just that for which I was waiting. You (I) learn something new every day. Very interesting.

The aerodynamic forces acting on the nose are what tend to destabilize a bullet in flight so anything that reduces those forces helps the bullet maintain its nose-on flight. As mentioned above, slowing down makes it more stable because the spin rate decays at a much slower rate then the velocity. Climbing in altitude with less dense air helps as well. Mule Deer has addressed this pretty well a few times.

Swifty52 has it right.

Aerodynamic forces make the bullet spiral very slightly, out to about 200 yards.

Interesting. Makes me wonder if little vortex generators on the plastic tips of some bullets would smooth out the air flow over the bullet?

I didn’t read all the comments but I’m of the opinion that the bullet rotates around its center of mass going down the barrel and a slight yaw maybe induced from the chamber to engaging the rifling. After leaving the barrel I believe that it takes a little while for it to start rotating around its center of gravity and have found that bullets from good barrels and a good load that’s been worked up tend to shoot better at distance when measuring group size

That is showing degrees of yaw or where the nose of the bullet is pointing, not the flight path of the bullet. The diagram on the right is showing where the bullet actually went. It started diverging as soon as it left the muzzle and continued to do so. Once the deflection starts, it won’t get any better. There is nothing acting on the bullet to make it “go that way to hit the target”.

👌
The video is the answer the first part of the question. And you correctly answered the second part. Bryan did a lot of experiments using shoot through targets to prove that group size doesn't shrink with distance.

i have wondered this some too i have noticed this just a little with my 257 Weatherby mag. with 100 gr. bullets at 3800 FPS. that at 100 yds. bullets seem to be very slightly key holed and at 150 yds. and beyond these bullets seem somewhat perfect bullet holes. is this possible ? would different barrel twist either faster or slower make a difference ?

I recently published an article on flat-base versus boattails:

1) Boat-tailed bullets do tend to destabilize more when leaving the muzzle than flat-based bullets, due to more powder gas blowing by the angular base during muzzle exit. This is why short-range benchrest bullets are flat-based, or have very slight boattails.

2) But te reason higher-BC bullets tend to group better at longer ranges is not primarily because they take longer to "stabilize," but because they drift less in wind.

3) Most shooters (especially hunters) don't get this: Instead they assume groups are relatively larger at ranges beyond 100-200 yards because bullets are becoming "destabilized" as they slow down.

4) This is actually the opposite of what happens, because bullets in flight are affected by two different forces: air pressure on the front end, which tends to destabilize bullets, and gyroscopic stability from the rifling twist. Velocity slows much faster than bullet spin, so bullets fired from a sufficient rifling twist become MORE stable at longer ranges, due to lower pressure on the nose--at least until they drop to around the speed of sound, which doesn't happen with typical hunting bullets.

5) Wind-drift increases at about twice the rate of range: A bullet that drifts half an inch at 100 yards in a 3-mph wind will drift around 2 inches at 200 yards in the same wind--and 8 inches at 400 yards

So yes, boattails can be less accurate at shorter ranges, but become more accurate at longer ranges due to less wind-drift. And most hunters have no clue how much even a 3-mph breeze (often consider "calm air") can drift a bullet even at 100 yards. Since wind is rarely absolutely steady, groups "open" up at longer ranges, unless the shooter can compensate accurately.

MD nailed it. There’s also - changing from above the speed of sound to below the speed of sound - note that 22 LR target ammo starts at below the speed of sound.
If I knew the reason for this, I forgot.

My head hurts taking all this in...how about an amusing anecdote for those of us in Rio Linda? Some years back when the Commonwealth countries were still using .303's in competition, it was common knowledge that the Mk7 .303 174 gr ball round was barely capable of holding 2,5 moa at 100 or 200 yards. But, it got better at 300 and , Katy bar the door at 600, theory was the damn thing straightened up and flew right. Embarrassing several competing countries, including us Yanks.

The groups didn’t get smaller but the MOA did, right?

that`s what my 257 Weatherby does i shoot better groups farther out with bullet holes perfect even with my 257 Creedmoor and yes i was using boattails always . thanks again john ! Pete53

It's really a curious thing how a bullet settles down to an optimum after it leaves the muzzle. The way I understand it is in part as has been said here several times already...diminishing overturning moment due to velocity decay.

That's only a part of the story, though. That overturning moment is "frustrated" by the fact that gyroscopic precession always forces the bullet to react by moving 90 degrees in relation to the direction the aerodynamic force would cause it to move (that is what causes the coning motion). It's like a cat chasing its tail. Since the aerodynamic force (overturning moment) is constantly diminishing, the coning motion diminishes also until it is essentially gone and the bullet finally takes on what Nennstiel refers to as the yaw of repose. It's fascinating stuff.

Smaller groups at longer distance or smaller angles of dispersion?

Thanks for your reply MD.You have a way of explaining this that even a Luddite(me) understands.

That's the way I read it

That's not quite the whole story. The Brits harnessed a phenomenon called "Positive Compensation" which meant that Enfield's could be tuned to release bullets at different points of the vibration depending on the velocity of the bullet. The slower bullets would exit the muzzle when the muzzle whip was high and the faster bullets would exit the muzzle at a point when the barrel whip was lower. This would cause the slower bullets to NOT impact lower as you would expect, and the bullets would converge at a given range (but they had terrible vertical at short range). This apparently was a byproduct of the rear locking action and the two piece stock. Creighton Audette mentioned that the M14 compensated slightly at 600 yards as well.

The effect worked in the vertical plane only and Enfields were tuned or "regulated" to take advantage of that effect at 1000 yards commonly. The most well known "regulator" of Enfields is G.E. Fulton.

The effect only worked when there was a wide variation in velocity as was present in the .303 ammo of the day and in Brit 7.62 Ball with large spreads like Radway Green (RG) ammo. This effect was negated when match ammo with decent velocity spreads started being used. I'd heard of Regulated Enfields being used in the 1992 Palma without good effect. I think the Enfields were hung up for Palma matches there after due to match ammo taking over in Palma.

A few years back, Brian Litz wanted to study any potential rifles that grouped better at longer distances that at closer distances. He put out an offer of a expense paid trip to his incredibly equipped range to do instrumented firing on acoustic targets. Thought there were a few that claimed to have such rifles...no one stepped up to put up.

Harold Vaughn addressed the phenomenon is his book "Rifle Accuracy Facts" calling it "Poor Statistics".

The one explanation that did make sense to me was scope parallax. If your parallax is set for long distance, the sighting error induced by parallax at the closer distances could account for the bigger groups.

If anyone wants to read more;
https://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/1979098/1
or here;
https://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/9497148/1

Here's the Challenge that Litz put out there;
https://www.longrangehunting.com/threads/applied-ballistics-shoot-thru-target-challenge.144359/

Last I checked with Brian, there were no takers.

There are some guys here that think their rifles shoot better further out as well. I shoot a lot at the 100 yard line for a couple reasons: My club range is 100 yards and we shoot a lot of targets there during competition. We shoot 10 shot groups for score and 10 shots at close range tells us much more about our rifle, load and how well we are shooting. I also shoot a lot of longer range and almost all the time, when I have a 1/2 moa rifle at 100 yards, it is still 1/2 moa at 4,5, 600 and 1,000 yards. Something I find that has a huge impact on group size and POI is the wind. I guess If I used scopes with crappy parallax, I could see that as a huge problem, but the NF scopes I use don't have these issues. Lately I've been shooting silhouette varmint matches where the targets are as far out as 800 yards. One of the smaller targets (a rabbit) is at 600 yards, so it pays to have a rifle that shoots at least 1 moa. Sub moa is even better. The only time you are really going to see a difference in accuracy because of bullet yaw is when the bullet passes through the transonic state and in most cases, that is well beyond a 1,000 yards in most cases. You guys that are seeing some issues don't have your load and rifle dialed in. If you shoot better groups at longer range, that is either a fluke or you have a parallax issue..
6.5 creedmoor:




22-250:




None of my rifles shoot better further out. It doesn't make any sense that they would. And to truly assess something like this, you would have to be shooting in a vacuum and no one here does that..

Chris F, thanks for that. I had the wrong idea...I thought (?, heh, heh) it had something to do with gyroscopic stability improvement...obviously not. Beach Boys clearly knew a lot about the Enfields, the wrote a song about it I think..."Good Vibrations".

Great topic!!

Thanks to those adding in!!

OP I think is referring to bullet Yaw as the bullet exits the muzzle. It does take time for some bullets to "go to sleep".

Aberdeen Proving grounds takes pictures of bullets as the exit the muzzle, clearly demonstrating "Yaw". Particular VLD designs take longer to spin on their own axis, while a round nose bullet runs on it's axis very soon after exiting the muzzle.

While a bullet is in Yaw, the BC is less, or course.

Not exactly. If a bullet’s stability is insufficient to damp the limit cycle yaw, drag increases and the bullet’s sensitivity to wind and air currents increases, which can degrade accuracy even before the bullet reaches the transonic range. Of course, non-zero limit cycle yaw does also decrease dynamic stability as the bullet passes through the transonic zone, as well. And transonic can occur even at 800 yards or closer with common .308 and .223 loads.

Good discussion here. One aspect of bullet stability affecting dispersion (increase in group size) that I haven’t seen mentioned yet is bullet concentricity and imbalance. Depending on other sources of dispersion, this may get “lost in the noise”, but dispersion at the muzzle due to bullet imbalance is proportional to rotational speed, the degree of imbalance of the bullet, and at distance this dispersion becomes proportional to flight time. While dispersion due to parallax and other aiming error is linear with distance, dispersion due to bullet imbalance is exponential with distance. And increased stability increases this dispersion.

Dispersion at long distance is really the net effect of multiple sources of dispersion, some of which offset each other. Increased stability can increase group size due to bullet imbalance, but decrease group size due to reduced drag, wind drift, velocity variation, and dynamic destabilization as the bullet goes through the transonic range. Whether increased stability will lead to larger or smaller groups at long distance partly depends on how concentric the bullets are, how much the the BC changes with increased stability, how much wind variation there is, how much muzzle velocity variation there is between shots, and how the bullet behaves in the transonic regime.

Now in response to the OP, the first part of the question deals with how long it takes for a bullet to stabilize. There are two components of gyroscopic stability, nutation and precession. Nutation is fast and small circular motion, precession is slower and larger. Together they make up what is known as the epicyclic motion that the bullet’s tip traces out (Google it for some decent visualizations), and result in less stability than a bullet with no epicyclic motion. Bryan Litz has done some modelling and testing, and has found that nutation typically damps out by 100 yards, but precession can persist much further, resulting in the limit cycle yaw.

My rifle shoots better when I fire fewer bullets.

Jordan, you hit it dead on. This is one of the papers I have been reading. Although Sierra has different examples and Berger cites the Sierra studies this guy goes right along with what you stated. No excerpts just a link and photo. It’s a good read to me but maybe a little boring to you. Pic is courtesy of Brian Litz.

https://www.researchgate.net/publication/224927152_A_Coning_Theory_of_Bullet_Motions

Thanks for posting. I have been on board with this since reading Varmint Al's website that explains the phenomena. This is why Long Range Audette Ladders work well for load development.


Long Range Tune and Barel Harmonics

"Bong Test" - hmmmmmmm. Where are those Doritos at anyway?

If anyone wants to read up on the “coning motion” of bullets, use epicyclic swerve or bullet swerve in your favorite search engine.

I believe evenDr Mann discussed this a hundred plus years ago. The Bullet’s Flight

If more Campfire members bought Bryan Litz's excellent books and actually read them, we wouldn't get nearly as many posts asking about this sort of stuff.

But then, isn't the Internet all about "free advice"?

If everyone bought your books (and actually read them), then no one would be asking any questions at all...and we wouldn't need a forum!

But then again, we'd want a forum so we could argue about stuff.

Yes....the map on what works for Sg and the factors that contribute to it has been well drawn by many over the years. JBM's excellent site is a great resourse.

Add to the above "free advice" and Urban Legend that need to go the way of the marsupial wolf: bullet weight (it's the length, not the weight that matters), 'cheek weld', 'incipient case seperation', 'excessive head space', and my favorite....'if I do my part'.

Good shootin' -Al

Thanks, Al--and RiverRider!

I use both Berger's and JBM's calculators--but prefer JBM's twist-rate when using plastic-tipped bullets, as they have an offset for 'em. As the late Don Miller discovered after he came up with the twist-rate formula most computer formulas are now based on, plastic tips don't count nearly as much in overall length as "metal."

John

Agreed on the JBM data! Dovetailing from the late Dr. Robert McCoy's work on Sg (McDrag and McGyro), JBM has done a wonderful job. I find it much more reliable than other programs that consider an Sg of 1.5 as the lower threshold for stability.

JBM's bullet data base alone is very impressive. And as a particular bullet evolves, JBM stays up to date with the changes.

Good shootin' -Al