Barrel heat, effect on groups.

[270WSMANIC]

Delmar, My stock is walnut. I just tried the two bills while putting moderate finger thumb pressure on forend and barrel. I was still able to slide the bills.
Still haven't got to shoot anymore, rain stopped, not quit so hot and now I'm busy doing honey-dos. If I don't get to shoot soon going to start pulling hair out and I ain't got much to spare.

 

[ElevenBravo]

If you're giving adequate time for the barrel to cool down between shots, I'd look elsewhere first for the cause of this. Your powder could be temperature sensitive or the ammunition isn't correctly tuned for the gun, or it could be a shooter problem. Have someone load a snap cap or two into your rifle while you aren't looking and check for flinch.

 

[farmerboy78]

If you have ruled out a shooter issue and checked the rings and base, then I would look next at the bedding (pillars and action)and clearances for the barrel... if all that checks ok, I would check for copper fouling since you only have 30rds fired... sounds as you didn't break in the barrel... if all that checks, I would say that you don't have a straightened barrel and it walks as it heats...

 

[270WSMANIC]

No, I didn't do a barrel break-in. I did that 12 years ago with the original barrel, but have since read that barrel break-in is not necessary and does nothing, I guess there are two schools of thought??
With only about 30 rounds through the new barrel I need to shoot more before making any hard decisions on what to do.
As I said in my original post first two shots near touching the 3rd is the one leaving the group so unless I am only flinching on the 3rd shot I don't think flinching is the problem.
My last 3 shot group was with 2 quick shots near touching then after 15-20 min cool down I fired the 3rd shot. This gave me about a 5/8 group. That was all my ammo I had that day. If the tight groups are repeatable next time I shoot, by allowing cool down between shots I think I will have pretty good proof of a hot barrel problem. I will also try 3 shots from a hot barrel as a control test.

 

[farmerboy78]

It's proven that you have to break in a barrel, actually the throat area of the chamber... but you don't have to be silly with it...I just had a 6.5-06 built and it didn't settle in good until about 50-75 rds... and groups cut in half at around 100 rds... for my breakin, I fired 3 shot groups and cleaned while checking for heavy copper fouling... when I stopped seeing blue, I stopped cleaning for copper and my throat was broken in, at about 40 rds OR so... it's funny, you don't want the fouling at the throat, but you do need SOME copper deposited in the barrel for consistency. .. it is now a sub 1/2 moa rifle at 200 yds... I had to do my load development groups at 200 yds, because they all shot the same at 100yds...

 

[Bart B.]

I have never broke in a barrel and they all tested just as accurate as other used to win matches and set records who never break in a barrel. Barrels start wearing out from the first shot fired. Their accuracy level never changed from the first shots fired to the last 90% of their accurate lives.

A single 3-shot group tells you 19 out of 20 of all 3-shot groups will average somewhere between 40% of its size to about 250% of it. They are statistically poor to prove anything. On the other hand a 10-shot group's size means 19 out of 20 of them will fall between about 80% to 120% of their size. That spread's a lot smaller with 20 or 30 shots in a single group.

Short range benchrest few-shot group records are under .010 MOA. Their many groups of 10-shots each have aggregate average of about .200 MOA (means some are about .300 MOA).

 

[270WSMANIC]

Aim small to hit small

JMR40.Your post was the key to why my groups were not as small from the new barrel as they should have been, I thought it was all due to barrel heat. Turns out that under certain conditions I was having trouble seeing the bulls-eye clearly. My range is a path cut out of the woods. This time of year heavy foliage on trees means most of the day 100yd target is shaded while most of the day shooting bench is in the sun. The target I was using was a shootnsee 6in in diameter black with 1/2 in red bulls-eye. That day I was getting mostly 1 1/2 groups and I realized the crosshairs in my scope were tending to hide the red dot. I then changed to a different target with a 1in red bulls-eye. First 2 shots near touching 3rd opened group to 3/4in. I decided to fire my last 6 shots at the same bull with cool down every 2 shots. Since I was cooling the barrel anyway I walked to the target 2 shots touching, next 2 touching, next 2 ( I was almost afraid to look) still touching. I had a 6 shot group just over 3/8in center to center, in one ragged hole. With the 1 in bulls-eye I could see the crosshairs divide the dot in 4 equal parts and know I was holding center.
As you said in your post to hit small aim small. Thanks for the advice.
I love those shoot n see targets but I think from now on will spray paint a white 1 in or so square for an aim point.

 

[Trent]

Trent's info is good except for bedding the barrel.

Barrels vibrate away from any hard object touching them. That includes a pad between the stock fore end and barrel. As pressure amounts vary on fore ends with shooting position and recoil, anything touching both barrel and fore end transfers that pressure to the barrel. It's never constant.

Totally free floated barrels don't have such accuracy degrading from external forces.

Yes - but a proper bedding job can dampen harmonic vibrations on a barrel that is problematic due to uneven carbon or has stress in it from manufacturing.

My 10/22 is a good example. It's a match rifle, with a hammer forged bull barrel. Plenty of uneven stress in the barrel - it groups horribly when it is freefloated - and it grouped horribly with the factory laminate stock.

A 22LR is a special kind of pain, as normally a freefloated barrel on a centerfire will be settled down by reloading. You can normally find a sweet spot where the harmonics reach a neutral point at the time juncture of projectile exiting muzzle. I mean, that's essentially WHY we reload centerfire for precision shooting, so we can 'tune' the ammo to the barrel.

What most people who shoot / reload don't realize is them actual mechanics behind tuning a load for a rifle - e.g. what is happening that causes certain loads with a given powder / velocity to be inherently much more accurate than others with that powder, or velocity.

When you ignite a cartridge the barrel vibrates - the vibration depends on the material composition of the barrel, stress or lack thereof, contact with stock / bedding, trueness of the barrel to action, tension of the barrel nut, and so on.

So the barrel is vibrating while the gasses expand and the round is travelling down the barrel. Where that vibration is at the point in time the projectile exits the muzzle is critical, and how uniform your velocity is, is also critical (even more so on barrels with more vibration - thinner profile, etc..)

So you need a harmony of a couple of things for everything to work out and get that ultra-precise load everyone wants.

#1 - you need a load that shoots VERY consistent on velocity - a very low SD spread on muzzle velocity

#2 - you need that bullet to exit the barrel at exactly the right time where the vibration/harmonics of the barrel are absolutely neutral.

Otherwise you risk throwing the round in to a wider minute of angle, as the barrel is vibrating different directions as the round is transiting the bore.

Now measuring the vibration of the barrel in a precise enough fashion to predict what load will exit the muzzle at exactly the right time is pretty much impossible, even with high grade equipment. So we shooters do it the easy way - trial and error.

We try different projectile weights, different powders, different powder weights, different primers, etc, until we find that perfect combination that is very consistent AND exits the muzzle at the point in time there is a neutral harmony. (Ladder testing/etc)

Back to bedding and the 22LR situation; because it's actually relevant to see what bedding does

A 22LR you can't exactly reload for, to "tune" a load to your ammo. There are a few options to help though;

* muzzle-end adjustable harmonic dampers
* lot-testing of ammo
* adjustable bedding

As a case example I'll talk about my efforts to tame my match 10/22 rifle.

The 10/22 I have has a hammer forged bull barrel with no provision for a muzzle-end harmonic damper (it's not a "round" barrel and I don't feel like turning it down for one; I like the looks).

I don't have enough cash to do "lot testing" like the olympians to get a batch of ammo that naturally works with the rifle's harmonics.

That hammer forged barrel has plenty of uneven stress from the manufacturing process, and even cryogenic treating wouldn't relieve all of it. It shoots like crap with the barrel freefloated, and it shot like crap with the factory laminate stock! It shot like crap with every single type of premium ammo I tried.

So I bought an archangel stock that has an adjustable bedding system. You can move the block in a channel up or down the barrel, on the underside, and adjust the tension it applies against the barrel with a nut that you can torque.

This allowed me to mess around with dampening the barrel harmonics by moving a pressure point up and down the barrel's length, and adjusting the amount of pressure applied to the barrel.

Eventually through trial and error I found a point with a specific type of ammunition (RWS 50) that works perfectly and keeps me in the X ring at 50 yards 100% of the time.

Now translate that back to a conventional centerfire rifle that needs tamed;

Given if the barrel has sufficient stress, or other factors that make it "untameable", and no obvious crown damage to explain it, and the action is true and tight (e.g. you've ruled out everything)... and cannot be tamed while freefloated (many can't be)...

... Then you are left with a double engineering problem. You need to find a bedding contact point and pressure that will help dampen the harmonic vibrations to the point that you can find a neutral point with a specific load.

This can be pretty easily accomplished with a near-contact arrangement as he has, if he can slip a couple of dollar bills between the barrel and stock, then thin cork strips are the obvious way to go.

With a given load that shoots reasonably well, the OP can experiment with moving cork strips up and down the underside of the barrel, as well as adjusting the tension on those (by using more / fewer / different thickness layer) until the load settles down. Basically shoot group, move it, shoot group, move it; then go back and find the best group and shoot it again to confirm it wasn't a fluke.

Once you've found a contact point on the barrel where the harmonics are dampened, go back through the standard ladder testing to see if a load is found that will shoot to the level you need it to shoot.

(Basically it becomes a single point pillar bedding; just enough to dampen out the harmonics enough to do real load development... or to obtain the level of accuracy needed for the application - hunting, etc)

 

[Trent]

I have never broke in a barrel and they all tested just as accurate as other used to win matches and set records who never break in a barrel. Barrels start wearing out from the first shot fired. Their accuracy level never changed from the first shots fired to the last 90% of their accurate lives.

I've done it both ways and have seen similar results to yours.

The last 300 win mag barrel I shot out shot better at the end of it's life than it ever had at any point in time prior (mainly due to the now-longer throat helping to even out pressures) BUT it fouled horribly bad and I had to clean it every 5 shots to KEEP it shooting that way; which made it rather useless for f-class/etc.

A single 3-shot group tells you 19 out of 20 of all 3-shot groups will average somewhere between 40% of its size to about 250% of it. They are statistically poor to prove anything. On the other hand a 10-shot group's size means 19 out of 20 of them will fall between about 80% to 120% of their size. That spread's a lot smaller with 20 or 30 shots in a single group.

3 shot groups are indeed horrible for "proving" anything.

I use 5 shot groups in load development to quickly dis-prove, then revisit "good" groups with 10 shot groups to confirm that I'm on to something. Ultimately, 20 shot groups at distance is what I need to win matches; I'd rather burn the ammo ahead of time to confirm that I'm good, rather than THINK I'm OK and head out to find out otherwise.

Short range benchrest few-shot group records are under .010 MOA. Their many groups of 10-shots each have aggregate average of about .200 MOA (means some are about .300 MOA).

My match load for 300 win mag was a 10 shot best of ~.220 MOA at 100 yards during load development; with an average of .270MOA during additional testing.

I shoot that load in F-class and my lifetime average of 20 shot strings on that rifle / load is 195.6. Obviously I have a harder time keeping that 100 yard level of accuracy at longer distances

 

[lysanderxiii]

Ok I have a new douglas barrel on my Browning a-bolt, 24 inch 270WSM. After sighting the scope in, barrel was still a little warm and the day was fairly cool. I fired my last three rounds into 3/4 @ 100yd.
Another day this time temp in high 80's. first 3 shot about 3/4 again. Thats fine I'll take 3/4. After letting barrel cool down a bit 3 more till I had fired 3 or 4 groups. I'm going from memory now groups were something like 1in, 1 1/4in, 1 1/2, 1 1/4. I noticed that each group had 2 touching or nearly touching. Remember these 3 shot groups were fired with a cool down to warm between groups. Chorny showed 130 gr b-tips a bit over 3200 and I was 2 gr below max with mag-pro.
I usually don't look at my groups till the last shot is fired, but on my next group on another hot day; I watched the first 2 go into bout 3/8in and the third shot open to 1 1/4.
It looks like at least on hot summer days I need to shoot 2 and allow cooldown before the 3rd shot. I don't remember needing to do this with the old factory barrel??? Maybe it opened groups on hot days too and I have forgotten maybe chalked it up to me having a bad day at the bench???
I checked to make sure barrel still floated in walnut stock no problem there.
I'll be interested to see what y'alls experiences shooting groups with magnum rifles on hot days. Also wonder if some barrels are more affected by heat then others. Thanks

1) you'll probably need to shoot larger groups than three to get any idea of how big your group size is. The anomaly may not be the "flyer", but the two that touch.

2) Only after a few groups (five to ten) of 15 to 20 round per group, in a single sitting, can you compare "group size" on different days.

3) if the barrel is new, you'll need to break in in before any of this can happen....

 

[Bart B.]

Trent, if you need any bullet to exit the barrel at exactly the right time where the vibration/harmonics of the barrel are absolutely neutral, where in the barrel's whip cycle is that absolute neutral place place in the 360 degree cycle starting at the muzzle axis angle when the firing pin smacks the primer?

What's the harmonic frequency of that cycle relative to the resonant frequency of the barrel?

How do you prove that's where the bullets are leaving in its cycle when best accuracy's attained?

 

[Trent]

Trent, if you need any bullet to exit the barrel at exactly the right time where the vibration/harmonics of the barrel are absolutely neutral, where in the barrel's whip cycle is that absolute neutral place place in the 360 degree cycle starting at the muzzle axis angle when the firing pin smacks the primer?

What's the harmonic frequency of that cycle relative to the resonant frequency of the barrel?

The amplitude or frequency of vibration itself doesn't matter - as long as the barrel's business end is always in the exact same point in relation to it's start, at the instant the bullet leaves. It can (and will) still be moving.

How do you prove that's where the bullets are leaving in its cycle when best accuracy's attained?

The proof is in the groups! The other data points are irrelevant - and measuring them wouldn't get you anywhere, anyway! They will change from load to load and you can't predict what the next load would do until you measure it. Since you can't predict what any given load will do, as far as harmonics, you measure the end result - the groups - which tell you definitively.

This is merely the explanation as to 'why does this load group better than this other load' in any given barrel, assuming there is no material defect in the crown, improper shooting technique, or external factor (wind) errors contributing. I was also trying to explain how the barrel vibrations relate to bedding's impact on the precision of any given barrel (positive or negative).

An easier way to visualize the effect of bedding; think of a guitar string - or any long piece of metal, for that matter - when it is struck or jarred, it vibrates, buzzes or rings, right?

If you put your finger on the guitar string and strike it with the exact same amplitude as before, it makes a different (higher) pitch - the vibration is at a different harmonic.

If you take a long metal pole and whack it on the ground it vibrates and hurts your hand. Clamp a weight in the middle of the pole and whack it on the ground. Doesn't hurt your hand as bad. Vibrations are higher pitch (frequency) but less individual amplitude.

Same thing on a barrel. You put pressure on it with a bedding pillar and the amplitude and frequency of the harmonic changes. The vibrations are dampened (overall spread is decreased) and the chances of your bullet leaving the barrel in a specific point increase (as there are more neutral points; or peaks; or valleys). So instead of a larger scattered pattern now you start shooting smaller groups and you get clumps of shots.

Get the barrel dampened at the precisely right spot and with precisely the right tension (trial and error) with any given ammo (or reduce it to a point you can do meaningful load development), and you'll get good results.

Obviously it's preferable if you can skip all of that and find a barrel that is stress free, works perfectly consistently while free-floated.

If you have a non-high end barrel and you want to make it better in a conventional stock (e.g. typical hunting rifle) then a single pillar bed can help get you in to a better (more precise) situation.

 

[Bart B.]

So as long as the barrel's business end is always in the exact same point in relation to it's start, at the instant the bullet leaves, do you mean when the barrel's muzzle axis is at the same place as when it fired; pretty much straight out? And either on the down swing or up swing?

If so, that's the worst place possible. That's Browning's theory on how their BOSS works. The muzzle axis rate of change is maximum there and has the highest MOA per millisecond change at that point. The normal spread of the bullet's barrel time (case mouth to muzzle exit) will cause the greatest spread of departure angles at that point. If it happens on the muzzle axis down swing, that's counter productive to compensate for muzzle velocity spread. It'll string bullet impact vertically over a greater distance.

You must be unfamiliar with Geoffery Kolbe's and Varmint Al's finite element analysis of barrel whipping versus bullet exit as well as what the Brit's proved over a century ago about barrel whip versus bullet exit with their .303 SMLE's that shot more accurate at long range than short range. It's all caused by positive compensation of bullet exit angles at different muzzle velocities. All rounds in a given lot of ammo don't have exactly the same pressure curves, barrel time and muzzle velocity.

The amplitude or frequency of vibration itself matters very much. Along with barrel time, that's what controls the actual bullet exit angle relative to the line of sight. The barrel will be at its resting place (relative to the LOS) every 180 degrees of its whip cycle. If a given load has a 1.2 millisecond barrel time (about average), that barrel has to have the right resonant frequency and harmonic thereof to do that. It'll be on the up swing half the time; down swing the other half. Most barrels have a resonant frequency of under 100 Hz. When fitted to the action, it'll change a little bit. One cycle every 10 milliseconds.

Best accuracy happens when a barrel whips and wiggles the same for every shot. Totally free floated ones do that when there's enough clearance to the fore end. One with any contact with the rifle's fore end does not; nobody holds them exactly the same from shot to shot and that changes their whip and wiggle properties as variable pressures against the barrel gets transferred from an external force through that contact point.

 

[lysanderxiii]

So as long as the barrel's business end is always in the exact same point in relation to it's start, at the instant the bullet leaves, do you mean when the barrel's muzzle axis is at the same place as when it fired; pretty much straight out? And either on the down swing or up swing?

The barrel's muzzle does not have to be at the neutral point on bullet exit.

Shot ejection can occur and any point of the oscillation of the barrel, as long as it occurs at the same point each time. The fact that the point of impact changes with various loads shows that the bullet exit is occurring at a different point on the oscillation.

If so, that's the worst place possible. That's Browning's theory on how their BOSS works. The muzzle axis rate of change is maximum there and has the highest MOA per millisecond change at that point. The normal spread of the bullet's barrel time (case mouth to muzzle exit) will cause the greatest spread of departure angles at that point. If it happens on the muzzle axis down swing, that's counter productive to compensate for muzzle velocity spread. It'll string bullet impact vertically over a greater distance.

Marketing hype aside, all the BOSS is, is a weight on the end of the barrel that can be moved for and aft by calibrated amounts. This will change the barrel harmonics, and allow you to "tune' the barrel vibration to your liking.

As to where the barrel is at shot ejection and its velocity in the vertical and horizontal planes (the barrel will also whip in the horizontal plane as well), does not influence group size, but point of impact location. If the bullets all leave the barrel at the same point in the whip, they will all have the same vertical and horizontal velocity component added.

It is all about consistency...

You must be unfamiliar with Geoffery Kolbe's and Varmint Al's finite element analysis of barrel whipping versus bullet exit as well as what the Brit's proved over a century ago about barrel whip versus bullet exit with their .303 SMLE's that shot more accurate at long range than short range. It's all caused by positive compensation of bullet exit angles at different muzzle velocities. All rounds in a given lot of ammo don't have exactly the same pressure curves, barrel time and muzzle velocity.

The SMLE's performed better at longer ranges had to do with bullet stabilization, not so much barrel movement.

Generally, with service ammunition, variations in muzzle velocity, variations in bullet CG concentricity and variations in base geometery will give a fairly wide group. However, with the SMLE, the barrel was made rather light and thin and they did have a bad whip problem and had two nodes in its vibration, one on each end. This is why these have a mid-barrel tensioning strap, to damp the central bowing. One of the major improvements made during the No. 4 development was stiffening the barrel, so damping the barrel at only one point was enough.

The amplitude or frequency of vibration itself matters very much. Along with barrel time, that's what controls the actual bullet exit angle relative to the line of sight. The barrel will be at its resting place (relative to the LOS) every 180 degrees of its whip cycle. If a given load has a 1.2 millisecond barrel time (about average), that barrel has to have the right resonant frequency and harmonic thereof to do that. It'll be on the up swing half the time; down swing the other half. Most barrels have a resonant frequency of under 100 Hz. When fitted to the action, it'll change a little bit. One cycle every 100 milliseconds.

That will only affect the point of impact. if the bullet always leaves at the same point of the barrel whip, it will go the same place as before.

You are confusing accuracy with precision.....

 

[Doc7]

How can a bullet which has any deviation in velocity whatsoever leave a barrel at the same point in a 100 Hz oscillation?

 

[Bart B.]

Doc7, the harmonic frequency the barrel's muzzle end vibrates at is several times its resonant frequency. One of Varmint Al's pages below lists a barrels resonant frequency (mode 1) and higher frequency harmonics (modes 2, 3, 4, ......). You can see about how many milliseconds one cycle takes; the reciprocal of cycle time is the frequency. If a cycle is .0012 second, frequency is 833 Hz.

An insight about when bullets leave the muzzle. . . .

http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/barrel_vibrations.htm

The one above can be used to show how barrels whip horizintally; use the center of mass to represent how much to the side it is. Kolbe's software is for a rifle fired in free recoil. Amounts and directions for human held rifles will be different. But the concepts are the same. Use a barrel time of 1.2 millisecond to show where a given load examples bullet exits the muzzle.

http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/tuning_a_barrel.htm

Click on the "PDF" link in this one if you want to save it or read easier:

https://archive.org/details/philtrans05900167

When the Brits switched their service round to 7.62 NATO, it's much lower muzzle velocity spread didn't have much positive compensation. It's improved accuracy over the .303 round showed horizontal shot stringing with slightly out of square case heads on new ammo. So a rifle smith made some 4-lug actions that minimized that problem.

http://www.varmintal.com/a22lr.htm

http://www.varmintal.com/aeste.htm

http://www.varmintal.com/alite.htm

http://www.varmintal.com/amode.htm

http://www.varmintal.com/atune.htm

http://www.varmintal.com/apres.htm

Lysander,
Not all bullets leaving at the same place in the muzzle whip cycle have the same velocity. Those with a higher average pressure during barrel time will leave faster. And with humans not holding the rifle the same for each shot increasing muzzle velocity spread and changing the amplitude of the barrel vibration cycles which means more spread in bullet departure angles. All of which causes several people to have different zero settings on sights with the same rifle and ammo. I've coached long range team matches with 4 people shooting the same stuff. Zeros across them varied both in windage and elevation by 1 MOA or more.

Good marksmen hand holding their rifles rested atop something on a bench will have a different zero for a given rifle and ammo than the same stuff shot from field positions; standing, kneeling, sitting or prone. That's 'cause the barrel whips differently depending on how much mass their is in what's holding it as well as where its center is relative to the bore axis. Hand held bench rested rifles typically zero a MOA or so horizontally from what they do in field positions; to the right for right handed people.

Oft times, I've given my Sweany Site-A-Line optical collimator to people to show them their rifle barrel's bore axis does not point to some place on target above the aiming point equal to sight height plus bullet drop. That's where most folks think it points. But it's always been somewhere else depending on several things that end up putting the muzzle axis pointing there when the bullet leaves the barrel; both in elevation and windage. Try that with your own rifles.

 

[Trent]

The SMLE's performed better at longer ranges had to do with bullet stabilization, not so much barrel movement.

This was also observed (but not very well understood) as far back as the 1909 F.W. Mann's volume, The Bullets Flight. I have a first edition printing of that volume in my library and there's at least one point where he observed this in testing (although his conclusions were incorrect at the time).

So as long as the barrel's business end is always in the exact same point in relation to it's start, at the instant the bullet leaves, do you mean when the barrel's muzzle axis is at the same place as when it fired; pretty much straight out? And either on the down swing or up swing?

If so, that's the worst place possible. That's Browning's theory on how their BOSS works. The muzzle axis rate of change is maximum there and has the highest MOA per millisecond change at that point. The normal spread of the bullet's barrel time (case mouth to muzzle exit) will cause the greatest spread of departure angles at that point. If it happens on the muzzle axis down swing, that's counter productive to compensate for muzzle velocity spread. It'll string bullet impact vertically over a greater distance.

The argument presented about the "better of two points" being the extreme edge of whip vs. the neutral position in the bore assumes that the oscillation curve is weakest at the peaks and that the fastest momentum of the muzzle is at the center (it's resting place); but is that really the case on a tube? It is on a wire, but we're not talking about a wire; we're talking about a thick tube of metal that has the greatest natural resistance to movement at it's resting point.

Your point also assumes that there is a constant oscillation; or a constantly aggravated oscillation that progresses to it's worst point as the projectile travels down the barrel - but that is not true - the vibration affecting the muzzle of a barrel changes the entire time the projectile is traveling down the bore and is affected by stresses inherent to the metal. If you slow a projectile's launch down or speed it up, there is not a direct correlation to the change in vibration - we're talking about chaos mathematics and bifurcating algorithms at this point in time, where you cannot directly predict what a change in the amplitude of launch will do to the vibration characteristics (e.g. it cannot be predicted, only measured when it's tested, and it is subject to change each time). This is the juncture where conventional non-linear mathematics trying to model interior and exterior ballistics fall apart, because of the inherent random factors involved.

Changing the contact point or tension on a barrel to dampen the effect changes the entire equation of vibration and oscillation and can cause an otherwise stable barrel (at time of projectile exit) to no longer be stable, or vice versa. As you mentioned in your observations about different shooters holding a rifle differently, and affecting the point of aim; what is not as evident is if this is due to additional vibration or a complete neutralization of vibration.

If that happens, one could easily argue the barrel was not bedded with sufficient barrel tension on the contact spots, OR the barrel itself is not mated to the receiver properly, as light contact forces on the foregrip by holding the firearm should not affect vibration to that degree, IF everything was done properly to set the gun up to begin with!

A barrel that changes point of impact merely by shifting your hand on the forestock forward or backwards is in dire need of being rebedded or having the barrel trued & torqued to the action again - as the bedding doesn't have tension anymore, or the nut is not tight, or the barrel is not trued to the receiver. (e.g.... That gun needs service.) Merely resting the forestock on a bag or holding it different should not ever change the ballistic impact unless the bedding was done improperly in the first place (insufficient tension) or there is a loose component somewhere along the line.

(Resting the *barrel* on something is bad, and that should never be done, as that *will* change the harmonics, period).

The point to make here is that, generally speaking, given different launching conditions you will find that there is a point of neutral harmony where the barrel is simply NOT vibrating or oscillating at all; it is in between episodes of vibration.

This is what I was talking about with neutral harmony - finding the right conditions under which the barrel is stable at the point in time the projectile exits. Oscillation / vibration is a non-event at that juncture because there are no nodes, anti-nodes, or movement - it is a window in time where nothing is really moving (or perhaps phrased better, everything is at such a high frequency / low amplitude that it's effect on trajectory is inconsequential)

For those trying to catch up;

A bullet which leaves the muzzle with an induced yaw - caused by crown damage on the muzzle, base damage on the projectile, where uneven gas pressure exists on the base of the projectile as it leaves the muzzle, OR by lateral throwoff - the barrel itself is moving), will cause a particular effect due to magnus force / moment where the projectile tracks in a corkscrew pattern until drag straightens the projectile and dampens the swerve motion.

F.W. Mann documented (but inadequately explained) this effect as early as 1909 in his book The Bullet's Flight. (I've got a first edition of that volume in my library). He documented the corkscrew effect in his testing but there was not sufficient understanding of what was actually going on to pin down the issue and he certainly made no attempts to mathematically model it.

Eventually drag brings the projectile pointing straight again and the attack angle decreases, dampening the trajectory instability. Some time after all of this after the advent of radar, the US Army tests were conducted to pin down a true 6 DOF model for the flight of uncontrolled projectiles.

D.E. Carlucci and S.S. Jacobson's book Ballistics cover the concept with some attempt to model it. Chapter 6 devotes a great deal of discussion and calculation to Magnus force and moment, pitch damping, etc, while chapters 11-13 cover lateral throw off, swerve motion, and nonlinear aeroballistics.

The British SMLE tests with "bifurcating groups" (two distinct groups at longer ranges) demonstrates what happens when there is excessive barrel whip, one has to argue - is that due to an induced yaw on the projectile at the time the muzzle leaves (the effect of which is dampened out over time), OR is that due to the MUZZLE POINTING TWO COMPLETELY DIFFERENT PLACES at the time of exit?

The case you make about "the best time for a bullet to leave the muzzle is when the muzzle is at it's extreme whip" is woefully incorrect as the precession due to any induced yaw at the natural point on exit will be dampened out over time whereas no external force exists to correct the lateral throwoff of the trajectory when the muzzle is pointing the wrong way!

 

[lysanderxiii]

...where the projectile tracks in a corkscrew pattern until drag straightens the projectile and dampens the swerve motion....[e]ventually drag brings the projectile pointing straight again and the attack angle decreases, dampening the trajectory instability...

Not always. Some projectiles, (the M193 most notably) do not show a reduction in yaw over the entire flight. And drag is one but not the largest cause of the reduction of yaw. It has to do with the location of the CG and center of pressure.

...The British SMLE tests with "bifurcating groups" (two distinct groups at longer ranges) demonstrates what happens when there is excessive barrel whip, one has to argue - is that due to an induced yaw on the projectile at the time the muzzle leaves (the effect of which is dampened out over time), OR is that due to the MUZZLE POINTING TWO COMPLETELY DIFFERENT PLACES at the time of exit?

As I stated earlier, the SMLE had a two node vibration. Depending on whether the initial motion of the barrel was this way or that way, would determine which of the two groups the bullets would go into....

 

[Trent]

Not always. Some projectiles, (the M193 most notably) do not show a reduction in yaw over the entire flight. And drag is one but not the largest cause of the reduction of yaw. It has to do with the location of the CG and center of pressure.

True enough, if you consider different / multiple core combinations or projectile shapes that fall outside of the norm, or (as mentioned earlier) projectiles which suffer from a CG defect (dynamic imbalance, or a static imbalance from a mass asymmetry).

It takes a pretty precise balance for a projectile to *stay* out of alignment and continue with precession for a long duration of time with all of the external forces at work on it. Once the motions decouple and fall out of phase the projectile stabilizes; having them stay in precise balance for the entire duration of flight would be quite a feat (again, a mass asymmetry would cause that). CG stays constant but as spin slows and the velocity slows that center of pressure will shift; either it will tumble (if spin rate is too slow to stabilize the bad CG vs. the new center of pressure), or it will stabilize (if it is not).

But that really isn't the case when we are talking about precision on long range match shooting; we're using projectiles which should easily dampen themselves during flight.

As I stated earlier, the SMLE had a two node vibration. Depending on whether the initial motion of the barrel was this way or that way, would determine which of the two groups the bullets would go into....

Right, which is why I was alluding to chaos mathematics earlier (bifucating algorithms - you can't predict which way the algorithm swings last, before it becomes unstable; the same way with those SMLE barrels; those are actually a valid real world example of what you'd see on paper working out a bifurcating equation.)

We know that the root cause of bifurcating algorithms which ultimately become unstable, is rounding errors introduced (the algorithms remain stable longer as you increase the precision of mathematics; on a cheap calculator they grow unstable after X iterations; on a computer that can do 1,000 decimal places of precision they grow unstable after Y iterations..); translated to a physical level, we're talking about nature "rounding off" forces on the atomic level. This could drive the distinct shape of a maple leaf forming as cells divide, the distribution of two fluids being mixed in a container, the precise moment in time a liquid transitions to a gas, or the initial direction a barrel moves when a rapid gas expansion happens inside of it.

Given "A" specific loading that SMLE could shoot two distinct groups - if the bullet is leaving at the extreme of the above-average "whip" on either end - or somewhere in between (vertical stringing) if it does not. Not every thin barreled SMLE shot two groups, and not every load developed for it does so, either - although the load they wanted to use surely did with that barrel profile (more often than not)!

 

[Bart B.]

.....whatever.

However, if you really think the barrel's pointing at its natural point of aim above the down range aiming point to compensate for bullet drop and sight height after it's zeroed in, prove it with a good collimator. That's better proof than any thing else; it's reality.

I've put mine in dozens of match winning rifles; none of them do that for a zero at any range.

How would you explain several people needing different zeros on the sight when shooting the same rifle and ammo?