Help figuring out .223 accuracy discrepancy

[GlockTerrier]

rcmodel: Is this evidence enough?

Maybe Thanks, rcmodel - I don't doubt your word - just wanted to be clear about the kind of POI shift I had in mind and keep some healthy scientific scepticism about the value of limited sampling in personal observations....

Let's assume that the short line bullet-dependent POI shift objectively exists and the precession/yaw/wobble-induced "bullet walk" is indeed responsible for it. What follows is another hairbrain speculation of mine - so bear with me, if you can, here...

This may have an interesting application for a quick selection of best perspective round for the rifle and estimation of a bullet quality - as they leave the barrel of your rifle for an autonomous ballistic flight (keeping in mind that a specific gun during firing process "imprints" itself on the bullet affecting to various degree its aerodynamic qualities)

During load development the attention is usually on the size of the group - and this is a measure of consistency in the walk. But if the "bullet walk" theory is even approximately correct the POI position itself is also an important observation that tells us about amount of walk - or an amount of precession/yaw/wobble of the spinning bullet in flight.

If all the rifles have their rifling twist in the same direction - the bullet always walks in the same direction - let's say - to the right. Up or down I'd think is less important and is probably related to the shape and mass distribution that changes amount of cumulative positive or negative overall aerodynamic "lift" the bullet gets during a revolution due to assymetrically rotating yaw/precession angles periodically changing its "angle of attack". (if Rcmodel also makes planes he will understand me )

So - if I shoot several groups with different loads/bullets without changing the rifle zero - the one with a group average POI to be farthest to the left has a minimum amount of walk (or precession/yaw/wobble induced lateral travel to the right) and should be closest to ideal "truest" bullet! Again - we're talking about the aerodynamic quality of bullet as it is "imprinted" with your gun during firing - and not its "original" out-of-the-box condition. And this is exactly what we're looking for!

Now, I'm not saying that that the "leftest" group will be most accurate - although it very well may be. The walk on average can be small - but still not consistent enough. This may very well be related to gun "imprinting" now rather than to the bullet that already proved that "on average" it flies "truest". What we have here is potentially the best candidate among tested - so we may start working on fine tuning "imprinting" factors for it- speed, jump to the lands, barrel rifling qualities (this would explain the effect of some "firelapping" techniques on accuracy), ramping and chambering mechanics etc. Of course, lack of constency still can be caused by the bullet physical inconsistency due to manufacturing tolerances - then very little we can do and we can go on to fine tuning the next most accurate group on the left as the next best candidate.

Overall, if this works - tracking short-line (~100y) average POI position may be an easy additional guide to significantly accelerated accurate load development. Of course, if one already shoots one hole like disneyd does with his 60gr V-Max load - one can stop right there...

I'd be interested to hear personal obeservations or published results that may add credence or disprove this speculation about "leftest group to have most accuracy potential". (assuming that all rifling twists in the same direction - I have no idea if this is true, actually... If yours twists in the reverse direction -it would reverse the rule to the "rightest" group)

Thanks!

 

[crux]

Anybody got a world class air rifle or pistol? It would be interesting to see POI impact shifts of pellets of the same weight, and different designs. Just brain farting..........

 

[GlockTerrier]

crux: Anybody got a world class air rifle or pistol? It would be interesting to see POI impact shifts of pellets of the same weight, and different designs.

Thanks, crux! Very clever suggestion! (too bad I don't have such rifle )
The only thing is that I have no clue on dependency of the size of assumed effect relative to speed/size...

The supersonic flight, very fast rotation and longer distances for shooting may create an effect easily measurable in inches. On the other hand slower subsonic flight, slower rotation and shorter distances may create a much smaller effect in POI that can be easily lost in the noise of random shifts of POA (point of aim)...

The slow subsonic "bullet" may have to be larger (like baseball or golf or tennis ball) to create "curved ball" effect due to rotation. (which balls of course "curve" for a different reason - friction of the surface) But who knows.. - anyway, the suggestion is certainly would be interesting to check on - Thanks!

 

[rcmodel]

First, you have to accept that "bullet walk" is whats causing it.
I don't!

Change in POI is caused by differences in barrel harmonics with different bullet weights, bearing surfaces, etc.
Simple as that.

Bullet walk is always present at long range, but it is always consistent with an accurate load.

I have watched too many .30 caliber bullets through a spotting scope doing the corkscrew on the way to the 1,000 yard targets to believe "bullet walk" has any effect on accuracy.
They walk all over the place for quite a ways, but they still all hit pretty much in the same group.


rcmodel

 

[peterotte]

Here's one homegrown hairbrain theory of mine if anybody cares:

You got that one bang on GlockTerrier. A good analogy. The bullet spin and yaw causes a spiraling flight path that can dampen down or get worse. I wonder whether the muzzle crown could influence boat tail yaw. I suspect that the small chamfer one often finds on a crown could be making matters worse. There is a web-site that describes the phenomena.

Check this one out!

http://www.nennstiel-ruprecht.de/bullfly/index.htm#Figures

 

[GlockTerrier]

Change in POI is caused by differences in barrel harmonics with different bullet weights, bearing surfaces, etc.

Well, the "barrel whip" theory as the answer to all ballistic questions as pertinent to reloading doesn't satisify me - YMMV, of course. (In fact I tend think it was overhyped for reloading masses and may be almost totaly irrelevant for load development and is primarily of interest for the rifle designer...) It certainly doesn't offer any convincing insights to the question posed by Disneyd and significant lateral POI shifts I've observed after the switch to different bullets.

1. Reloading public somehow decided that they can "time" the bullet speed in the barrel "to catch the barell muzzle" at the point of minimum amplitude. Maybe they are not aware that the physics of the barrel whip *always* makes the muzzle an "anti-node" in deformation waves - the point of maximum whip amplitude and speeds.

2. The physics of rifle barrel whip concentrates its energy mostly in the vertical plane - leaving significant *lateral* POI shifts for different load/bullets completely unexplainable.

3. The parameters of the whip are defined by almost constant resonant parameters of materials, construction and masses involved and should be very stable. To expect to change them just by changing the load within normal working ranges is as likely as to expect change the tone of the guitar string from E to C just by playing a bit softer... I'm sure that Disneyd's barrel "whips" absolutely the same for all it counts with 55gr@3100 as it does with 60gr@3000...

4. If the barrel whip was a major factor in accuracy we'd have heavy short pistol-length barrels with rifle scopes blasting the heck out of long varmint rifles in accuracy.

5.

Redneck2: Taking today's ammo (bullets) in old rifles gave sub 1" groups.

I wonder if barrel whip theory was used in 60s to explain their group size - what would they say then if they knew what we know in 21st century?

In general I find the whole preoccupation with rifle-related factors in working on accuracy of a modern firearam ammo a bit obsessive.

Yes, some guns are constructed in general more accurate to aim and shoot than others, barrel whip, floating tubes and all. But ultimately it's not the gun that punches the holes hundreds yards downrange - but the bullet, - and it has yet to get there on its own and a lot can happen on the way in the supersonic flight through dense supercompressed air. Funny thing is that we know what even a light breeze of air can do to POI at mid range distances but are still peering into the gun in search of answers to subMOA accuracies...

 

[GlockTerrier]

peterotte: Check this one out!

http://www.nennstiel-ruprecht.de/bul...ex.htm#Figures

Wow! Thanks, Peterotte!
As always all the good thoughts were invented before us

http://www.nennstiel-ruprecht.de/bullfly/fig24.htm

If a bullet flies stable (gyroscopically and dynamically!) and the transient yaw has been damped out, usually after a travelling distance of a few thousands of calibres, the bullet´s axis of symmetry and the tangent to the trajectory deviate by a small angle, which is said to be the yaw of repose .

For bullets fired with right-handed twist, the longitudinal axis points to the right and a little bit upward with respect to the direction of flight, leading to a side drift to the right. The yaw of repose, although normally measuring only fractions of a degree, is the reason for the side deviation of spin-stabilized bullets.

Note that this is fraction of a whole degree, not a fraction of a minute of a degree

http://www.nennstiel-ruprecht.de/bullfly/yawrepf.htm

The repose angle of yaw (or yaw of repose, also called equilibrium yaw) is the angle, by which the momentary axis of precession deviates from the direction of flight (see figure ). As soon as the transient yaw induced at the muzzle has been damped out for a stable bullet, the yaw angle d equals the yaw of repose.
The magnitude of the yaw of repose angle is typically only fractions of a degree close to the muzzle, but may take considerable values close to the summit especially for high-elevation angles.

The occurrence of the yaw of repose is responsible for the side drift of spin-stabilized projectiles even in the absence of wind. The spin-dependent side drift is also called derivation.

It can be shown that for right-hand twist, the yaw of repose lies to the right of the trajectory. Thus the bullet nose rosettes with an average off-set to the right, leading to a side drift to the right.

Now at least we can get our terminology in order...

From the formula for the yaw of repose (boy, this is a mouthfull, I kinda liked "bullet walk"... ) it is proportional to axial (or polar) moment of inertia of the bullet and the rate of spin and in reverse proportion to a forth degree of a bullet speed.... So inconsistent moments of inertia (bullet quality as it leaves the muzzle) and speeds are primary factor in inconsistencies of a "bullet walk", sorry - "yaw of repose" Different bullet weghts, shapes, construction - different moments of inertaia - different POI laterally.

Now it would be great to find some numerical /experimental data on actual amount of the efffect...

Some more:
http://www.exteriorballistics.com/ebexplained/5th/42.cfm

And more:
http://www.tuffsteel.com/Ballistics/bullfly/fig25.htm

 

[Wildfire]

>223 accuracy

That AA 2230 is good powder and what I use. The 1 in 8 is maybe a little fast for "that" 55 grainer. The 60 sounds much better. If accuracy is what you want, max loads are seldom the accurate ones. The OAL is up to the Magazine. If you are talking about freebore. you may get away with what ever that magazine will allow. As a rule .008 to .015 bullet jump is best, but you gun may not allow that. Yes, the flat based bullets are usually more accurate at shorter ranges. But fail at around the 300 yard mark.
If you are getting 1/2" groups with that bullet stick with it and refine the charge and seating depth. My Varmint rifles are 1 in 14 twist and will not toss a 60 grainer. Faster the twist the heavier the bullet.
Hope I didn't hit the wrong page.

 

[Walkalong]

ALL bullets wobble a bit until they settle down. Then they must mantain enough rotation to remain stable all the way to the target. The best twist for pinpoint accuracy is one with the slowest twist that will sufficiently stabilize the bullets used for the distance shot. That will settle them down sooner than if they were fired in a faster twist barrel, BUT, a faster twist than necessary barrel can still shoot very, very, well.

Different weight bullets shoot to different points of impact, in general. Nothing mind boggling or strange about it.

Most factory barrels are a bit faster twist than needed for some ammo to accommidate several different bullet weights fired in the same gun at different velocities. They shoot just fine.

 

[rcmodel]

1. Reloading public somehow decided that they can "time" the bullet speed in the barrel "to catch the barell muzzle" at the point of minimum amplitude. Maybe they are not aware that the physics of the barrel whip *always* makes the muzzle an "anti-node" in deformation waves - the point of maximum whip amplitude and speeds.

Then why does the Browning BOSS work so well to tune a barrel?

2. The physics of rifle barrel whip concentrates its energy mostly in the vertical plane

This simply isn't true.
Barrel whip can and does occur at any point around the 360 degrees of the barrel.

There is nothing magical about the forces that cause it to only cause it in the vertical plane of the barrel.
A fully bedded barrel or one with only forend tip pressure might be more inclined to move vertically.
A free-floated barrel is free to go any direction the stress in the steel wants to make it go.

But I can see you already have your mind made up, and anything I say isn't going to change it.

Good luck on your quest for the magic bullet that doesn't change the barrel harmonics and stray off POI when you change its weight, bearing surface area, peak chamber pressure, and velocity.

I'm outa here!


rcmodel

 

[FM12]

Take a lesson from the benchrest shooters: up to about 300 yards use, flatbase bullets. The bt dont have time to settle in or "go to sleep" and be stabilized in short (<300 yds) to be as accurate as flat base rounds.

 

[GlockTerrier]

rcmodel: why does the Browning BOSS work so well to tune a barrel?

I don't own BOSS and cannot attest how well it works beyond the Browning marketing hype, not everything I've read from customers was a success testimony (I personally was skeptical, since the most of potentially useful compensating the whip blast happens a bit late to my taste - *behind* the bullet that already left) - but if it does indeed works well, this only confirms what I said: whip is primarily of a concern to a rifle designer and a rifle construction factor - rather than ammo designer...

Quote(GlockTerrier):
2. The physics of rifle barrel whip concentrates its energy mostly in the vertical plane

Rcmodel: This simply isn't true.
Barrel whip can and does occur at any point around the 360 degrees of the barrel.

Your categorical statement doesn't contradict mine. Yes, the whip does have both lateral and vertical components - but *mostly* the barrel whip in a well constructed rifle is contained in a vertical plane. Which whip your BOSS adjustable muzzle break try to compensate *most* - vertical or lateral?

For example: this patent for muzzle-break device explains the whip very well:
http://www.patentstorm.us/patents/6923292-description.html

The second and third aspect of transition ballistics is vertical and horizontal or lateral barrel whip; studies indicate this event provides a significant psychological portion of perceived recoil by the person discharging the firearm. All gun barrels tend to bend down during rest to a degree determined by their form and rigidity factors. This condition is most prevalent in light shoulder fired firearms; but is also a factor up to cannon bore barrels. Vertical barrel whip is that action that takes place as the projectile moves rapidly through the bore; causing an upward rise of the barrel that attains its maximum the instant the projectile exits the barrel.

Concurrently; rotation of the projectile as defined by the rifling helix imparts a counter torque to the barrel and the vertical vector of this torque amplifies barrel rising whip. Horizontal or lateral barrel whip has a similar effect; but of far less magnitude than vertical whip. Lateral barrel whip is mechanical and is most often caused by poor manufacturing techniques that result in a lateral bend in the barrel and greater attention to barrel manufacturing accuracy can relegate lateral whip to insignificance in actual practice.

If anybody has different sources of data I'd be interested to get a lin to them too...

rcmodel: .. I can see you already have your mind made up, and anything I say isn't going to change it... I'm outa here!

Sorry to see you go, I hope you'd change your mind... Certainly, just simply "anything one says" isn't going to change anybody minds... I was just looking to a bit of collective thinking and learning : arguments and contr-arguments based on facts, observations, logic and laws of nature... My mind is not set on anything - I'm just looking for the best and plausible explanations of the facts known to me and observations that I've made myself - and already learned something (Peterotte - Thanks!) Spinning bullet's "yaw of repose" is a very interesting effect that may potentially answer some of of my own questions regarding consistent lateral shifts of POI when going from one bullet to another and can be helpful in the bullet selection process.

 

[Walkalong]

Is Mind F****** a gun related non problem considered gun porn?
Whoops. I'm outa here too.

 

[GlockTerrier]

Is Mind F****** a gun related non problem considered gun porn?

I'd vote: yes - but only by those who have to examine what they use for their brains...
Thanks for your important contribution to this thread!

 

[peterotte]

2. The physics of rifle barrel whip concentrates its energy mostly in the vertical plane - leaving significant *lateral* POI shifts for different load/bullets completely unexplainable.

I'm glad you mentioned this one, GlockTerrier. My Anschutz shifts its POI laterally 'sometimes'. But, this rifle has a third mounting point just forward of the breach on the barrel. The screw fits into a dovetailed 'nut', which isn't even tight! Or should I say it had a third mounting point. It's gone as of now.

 

[peterotte]

disneyd , it would be real interesting if you could test those same loads at different distances to see what happens. (Same scope setting of course). Even comparing them to the 60gr at different ranges. If you were able and/or inclined to.
Would that be asking too much?

 

[peterotte]

causing an upward rise of the barrel that attains its maximum the instant the projectile exits the barrel.

That's interesting, GlockTerrier. Thanks for that very informative post!

 

[GlockTerrier]

For those if us who doesn't terribly mind "gun-related non-problems" I think may be interesing this document:

http://www.firearmstactical.com/pdf/fbi_observer_sniper_manual.pdf
FBI Academy, Quantico, Virginia: "Advanced Rifle Training for the Observer/Sniper"

Although not always technically correct wrt some ballistic effects they do provide some rule-of-thumb data for an effect of imperfections in the bullets:

"A center of gravity displacement of 0.0005" in .308 match bullet can increase group size approximately 1MOA. This is called "radial" error. Axial and radial errors complement each other". Axial error can be induced by non-concentric ammunition or non-square flat bases. Axial error of 0.001 increases a group size ~1moa.

 

[peterotte]

Just for interest. I have discovered that my Anschutz hornet fore-end sometimes lightly touched the barrel on the side. That would explain the lateral POI shifts I have observed. Fixed it now but need to do some serious range testing!

 

[Walkalong]

I'd vote: yes - but only by those who have to examine what they use for their brains...
Thanks for your important contribution to this thread!

Thanks. I couldn't resist.

 

[GlockTerrier]

Walkalong: Thanks. I couldn't resist.

I can easily see that... But us geeks also want to have some fun when we're not at the range or the reloading bench

BTW, I've spent some more time reading about the bullet flight trying to get a feel for the size of various physical effects..

Alas, it would seem that the "yaw of repose" in ideal bullet by itself is very week effect amounting normally to 0.1MOA at 100 yards and growing to 1MOA at 1000yards for light calibers bullets and is pretty much negligible unless we're talking long-range artillery shell...

On the other hand it is my understanding that very little tangible known of effects of "non-ideal" bullets on their flight and effect of its imperfections to static and dynamic stability in flight. My idea of a "bullet walk" had more to do with increased instability in flight due-to imperfections of shape, surface and mass distribution in the bullet that results in increased bullet scatter rather then pure spin effect of rotating ideal bullet's "yaw of repose".

FBI Academy sniper manual cited above gives some sketchy emperical data for these types of the effects but there's no reference to their source or how this was measured.

I have some of these cannelurred bulk 55gr Hornady's. I also have a bullet comparator (ogive-to-base) that I haven't used yet...

What may be interesting to do is to measure anything we can realistically measure about the bullet consistency: OAL, weight, base-to-ogive - for a sizable sample of bulk and match bullets and compare variances, ranges etc. (Granted, we really need to get a rotational momentum characteristics of the bullet - but even these can be better than nothing. )

Then to make three identical loads for three strings of fire:

One - with sorted bulk bullets, sorted to "match bullets" tolerances
Second - bulk bullets selected with typical "bulk" deviations.
Third - close by weight "match bullets".

Shooting and comparing groups from these three strings without touching rifle zero do not prove anything as what we measure is only indirect evidence of potential rotational inconsistencies and is related to the out-of-box bullet rather than parameters of fired, rifle "imprinted" bullet - but still may be interesting as a general idea of the relative importance of bullet parameters consistency...

If anybody made experiments like that already - it'd be interesting to hear what was found...

 

[Walkalong]

As a Benchrest shooter I can say what we talk about is:

Best Barrel. (Hart, Spencer, Shilen, etc.)

Best Action: (Stolle, Farley, Hall, etc.) Mine's a Rosenthal.

Best Bullet. (Berger, Barts, Knight, Gentner, etc.)

Results on target are all we are interested in, and we are probably not smart enough to figure out how it gets there anyway.

I imagine the High Power shooters know a lot more about those things. We don't care.

 

[GlockTerrier]

Best Barrel. (Hart, Spencer, Shilen, etc.)

Best Action: (Stolle, Farley, Hall, etc.) Mine's a Rosenthal.

Best Bullet. (Berger, Barts, Knight, Gentner, etc.)

Amen to that...

However, the subtle point that intrigued me a lot in this discussion was that once one has best his barrel and action thing figured out - one has an ideal platform for experiments and research in rifle-imprinted bullet flight and evaluation.

Want to know how this spinning bullet flies after being imprinted by the rifle and launched? Just fire it from it and you'll see....

Now the geeky reloading question is can we extract more information about the ammo from the results on paper? Is only group size meaningful? What about POI shift, if any? Etc..

 

[redneck2]

A center of gravity displacement of 0.0005" in .308 match bullet can increase group size approximately 1MOA.

I didn't elaborate, but in the Handloader article, Barsness used a machine that measured bullet quality. Juenke (sp?) maybe. Don't remember now. Anyway, very minor imperfections made for big differences in accuracy.

Just dropping a box of bullets on a concrete floor from the height of a few feet would affect accuracy.

 

[Walkalong]

Want to know how this spinning bullet flies after being imprinted by the rifle and launched?

Nope.

Learning to read wind flags is much more important.