Accuracy & Precision vs. Bullet Jump

[Skeptic13]

Eric Cortina is a champion F class shooter with a you tube channel. He has some good thoughts on this subject if you are interested.

 

[JFrank]

All yaw will damp out if the projectile is stable, simple physics.

You cannot see the yaw induced by in-bore cant, that is in the order of a 0.01" for a 3 inch gun. What causes increased dispersion from in-bore cant is the potential displacement of the CG off the bore axis. If the CG is not on the axis of rotation, there will be a sideways force on the projectile on bore exit.

This is a bullet traveling down the bore with a cant (much exaggerated). This shows how the CG gets shifted off the axis of rotation.
View attachment 974561

The yaw will on exit will damp out, as long as the projectile is stable, in addition to the off axis CG, there will be gyroscopic forces induced that produce side forces.

Well Sir'
I don't think you've squared the claim of bullet jump creating in bore cant leading to in bore yaw but that's just my opinion, im not saying your wrong' I just have a tough time wrapping my head around a .243 projectile going through a .243 barrel being engraved by riflings at .237 somehow canting and yawing while traveling down that barrel at 2920 fps.

But im just a farm boy so there's that.lol

 

[lysanderxiii]

The Kraut Korkscrew of post #34 is very exaggerated.
There is no restoring force that would let a bullet orbit around the line of flight.

There was a guy on a benchrest board who anticipated the Litz Challenge. He set an Oehler Acoustic Target at 100 yards so as to have no contact with the bullets, not even a Mann tissue paper target, and an aim point target at 335 yards. He said there was no instance in which the 335 yard group was smaller in angular measure than at 100 yards. If he said how many trials he ran, I don't recall it.
There were strenuous efforts to contradict him and debunk his observations but nobody had a comparable test where the same bullets were marked at two ranges.

That "cork-screw" is not the path of the bullet CG that is the yaw angle as a function of range, read the axis labels.

You are correct that there are no forces that push the CG closer the to the theoretical line of flight, but there are many forces that push it away from it.

An off-center CG of 0.00025" of a 150 grain bullet will produce about a .005 pound side load to bring the CG in line with the axis of rotation once the bullet leaves the barrel, that will cause a drift of about 0.05" after a second of flight (about to the 600 yard line).

 

[Nature Boy]

An off-center CG of 0.00025" of a 150 grain bullet will produce about a .005 pound side load to bring the CG in line with the axis of rotation once the bullet leaves the barrel, that will cause a drift of about 0.05" after a second of flight (about to the 600 yard line).

That's OK because an off-center CG of 0.00025" of a 150 grain bullet will be more than masked by my wind reading errors

Seriously though, I wonder how much of that CG variance is inside the manufacturer's tolerances?

 

[Nature Boy]

BTW, 0.00025" = 6.35 microns, or approximately the width of a single human red blood cell

 

[Jim Watson]

I couldn't read the fuzzy scale labels on the German corkscrew plot, but that makes sense. I wasn't the only one, people were interpreting it to show a bullet zooming around in a wide orbit and zeroing in on a straight trajectory.

Some guy name of David Tubbs "soft seats" his bullets. Just enough neck tension to keep the round together, loaded very long, he lets the lands seat the bullet the rest of the way.

 

[MCMXI]

Legend has it that Bryan Litz has a $10,000 bounty standing for anyone who can demonstrate this happening at his range.

I've either heard him or read something from him stating that all bullets are fully stable within 100 yards ... bullets capable of stability that is.

One thing I find interesting about seating depth is that I believe it’s related to neck tension. Shoot some rounds near zero neck tension without resizing using brass that is a bit loose where you can move the bullet back and forward with your finger pressure. When I’ve tested this i get fantastic groups and it doesn’t matter where I push the bullet forward or back.

also keep in mind the primer asplodes and pushes the bullet into the lands before the powder really ignites.

The way the problem is often stated causes our brains to focus on the distance between the bullet and the lands but what depth of seating is really changing is how much friction is between case and bullet

1911, I wonder if we could control brass metallurgy and sizing more granularly, if we would see accuracy oscillate corresponding to seating depth

@taliv, I remember a thread of yours a few years back where you were getting amazing groups and very low ES numbers with virtually zero neck tension. You and @lysanderxiii make a very interesting point about the relationship between seating depth and neck tension. So with virtually no neck tension I can see that the pressure/time curves for each load would be very consistent due to the lack of resistance until the bullet contacts the lands and starts to engrave. There's some ammunition on the market with a stainless steel case body and aluminum case head that supposedly offers very consistent neck tension due to the properties of the stainless steel.

 

[MCMXI]

Do all benchrest shooters load with virtually zero neck tension? Is that such common knowledge in benchrest that no one who's serious would consider jump or any neck I.D. less than the bullet diameter?

 

[Walkalong]

Do all benchrest shooters load with virtually zero neck tension?

When I was actively shooting registered matches it was very common to use very low neck tension seated into the lands so the marks left were square. Some used almost no neck tension, but you have to be very careful about having a round in the chamber and running out of time to shoot, as removing it could spill powder and cause issues.

It is easier to get consistent neck tension with light neck tension vs heavy. Being into the lands a bit helps start pressures and consistent burns.

 

[JFrank]

I can't speak for common knowledge the short range fellas or the score shooters as they probably forgot more than ill ever know I also can't think of a single mid or long range shooters that doesn't have a specific NT/ interference fit and seating depth measured in thousandths or pressure.
Example:
I have one barrel that currently shoots great at .026 jump with a .0015 undersized neck bushing
Another barrel with +25 jam
From my prospective Its whatever the barrel wants with minor tuning through the life of the barrel just no" soft seating" that I know of, now as soon as I post this someone will pop up or you find a barrel that likes something different and proves me wrong.

 

[lysanderxiii]

That's OK because an off-center CG of 0.00025" of a 150 grain bullet will be more than masked by my wind reading errors

Seriously though, I wonder how much of that CG variance is inside the manufacturer's tolerances?

About 2 to 3 times that for hunting. Maybe a full 0.001" for ball.

How much cant you get depends on the free-bore diameter, ogive, cylindrical length, and throat wear.

All the little things add (or subtract) up.

 

[Nature Boy]

All the little things add (or subtract) up.

Yes they do, as do the big things

 

[lysanderxiii]

I've either heard him or read something from him stating that all bullets are fully stable within 100 yards ... bullets capable of stability that is.

There are two types of stability: Gyroscopic and Dynamic

Gyroscopic stability means the nose is disturbed off axis, the various forces push to return the nose to the axis.

Dynamic stability means the yawing motion of nutation and precession is damped out with time, i.e., it goes to "sleep".

Not all bullets are dynamically stable, under certain conditions, both M193 and M80 are dynamically unstable.

 

[MCMXI]

So we still don't have a definitive explanation of how bullet jump affects accuracy and precision. One theory (thanks @taliv) is that bullet jump is irrelevant if neck tension is close to zero. Another theory is that being close to the lands decreases the contribution of bullet/bore misalignment but then explains away empirical data showing that significant jump can still result in excellent accuracy/precision by stating that neck tension or case pressure is more important than bullet jump and bullet/bore misalignment. So when neck tension is zero bullet jump doesn't matter, and when neck tension isn't zero bullet jump doesn't matter (at the nodes). Yep, this is as clear as mud.

 

[JDinFbg]

My schedule and weather conditions finally allowed me to get to the range to do some tests of bullet jump on accuracy. I somewhat did the Berger jump test that folks in this thread pointed me to. This was not an exhaustive test and was not done with starting powder charges as Berger suggested, but yielded some interesting, if not confusing, results. The rifle under test was my old 1917 Enfield 30-06 on which I had done some very extensive clean-up and which I documented in another thread in this forum. Needless to say, this rifle is far from pristine, has a heavily pitted barrel, and will never ever come close to being a match-grade rifle. But, nonetheless I wanted to see if I could find something that would consistently shoot halfway decent (1.5" or less) in this rifle.

My tests included two different powders, W-748 and IMR-3031 using the Sierra 125 gr. Spitzer, .311" diameter bullet. I heretofore had focused on shooting W-748 with this bullet since I had a goodly supply of the stuff and it was a powder potentially usable in several cartridges I have. Further, the 50th Edition Lyman manual lists W-748 as the potentially most accurate powder using a 130 gr. bullet, so I assumed it had to be a contender when using a 125 gr. bullet. I only tested two jump distances with 748, so the tests with this powder were not as extensive as they could, or probably should, have been. I also went back to my old 44th Edition Lyman manual and noted that it listed IMR-3031 as the potentially most accurate powder using a 130 gr. bullet. Since I had a supply of IMR-3031 on hand, I tried that powder, also, over a broader range of jumps.

My jump test results are shown in the attached PDF file, with all shots being at 100 yard targets. The rifle was under a shade canopy and not exposed to direct sun, the temperature was around 68 degrees, and there was very light wind during the tests. My tests started with first shooting 3 fouling/warmup rounds at a blank target. The sequence I then followed was to walk to/from the target board to retrieve/place targets, allowing the rifle sufficient time to cool down (but not get cold) between firing sequences. I messed up on the second jump tests with 748 since I ended up shooting at my fouling/warmup target that already had holes in it, so I couldn't distinguish what holes were from my intended test. I subsequently removed that target from the board so I had only one target in place at a time thereafter. My limited tests with the 748 loads were not very informative, and I found no discernible differences with the two jumps I tested. My tests with the 3031 loads were more revealing and definitely showed a trend toward achieving smaller groups as the jump got larger, but the results were highly inconsistent. For all the jumps I tested I would get the smallest 3-shot group for test #1, then things would blow up for test #2. I can't explain this since the time between each record test was about the same and I tried to observe and shoot all test shots under similar wind conditions.

It would appear that 'jump is my friend' with this rifle. More importantly, it would appear that IMR-3031 is a better performing powder in my rifle with this bullet than W-748. This seems to suggest that the Berger bullet jump test method leaves out the process of first establishing the best powder for ones rifle and presupposes that one instinctively knows the 'perfect' powder. Given that the Lyman 50th Edition manual lists 15 different powders potentially usable with the 125 gr. bullet, I think one should first shoot starting loads with as many different powders as one has to find out which one performs best before evaluating the best bullet jump. Given that I achieved my smallest group with a 0.125" jump with IMR-3031, I may test my 748 load with this jump also just to see what I get. The one thing I know is that I had never heretofore even come close to getting groups around 0.5" or 0.75" with this rifle, so I plan to do some further refining work. I just need to try to understand why my two tests at each bullet jump were so inconsistent.

 

[JFrank]

Good testing' thank you for posting.
Without wind flags I can never be sure of my test results nor can I be sure with too much time between shots.
With that said and for the sake of archives and new reloaders/tuners, a barrel is an osculating whip that goes in and out of nodes as powder charges increase or decrease" as exit timing becomes optimized rounds begin to impact targets near the same location and then ultimately overlap (positive compensation) those rounds impacting that same or overlapping point also create a group whose shape and size can be altered or fine tuned by seating depth ( in your case the Berger course seating test) bullet hold or referred to as neck tension can also make a significant difference a tuner may not notice at first or at relatively shorter distances.
The greater the distance the smaller the tune window becomes.
Just my $2.00
J

 

[Jim Watson]

Seems like a .31-06 with pitted barrel is kind of a coarse instrument for the sort of fine tuning that has been moving from target rifles into sporting guns, but you are seeing an effect. Might have something to do with a military rifle throat being designed for a long jump with service ammo.

My attempt at a Long Range .223 and a good .308 found generally better accuracy with extruded powders than Ball. Darned shame, especially AA2520 pretty much obviated the need for th electric dispenser I use with Varget, etc.

 

[JDinFbg]

Good testing' thank you for posting.
Without wind flags I can never be sure of my test results nor can I be sure with too much time between shots.
With that said and for the sake of archives and new reloaders/tuners, a barrel is an osculating whip that goes in and out of nodes as powder charges increase or decrease" as exit timing becomes optimized rounds begin to impact targets near the same location and then ultimately overlap (positive compensation) those rounds impacting that same or overlapping point also create a group whose shape and size can be altered or fine tuned by seating depth ( in your case the Berger course seating test) bullet hold or referred to as neck tension can also make a significant difference a tuner may not notice at first or at relatively shorter distances.
The greater the distance the smaller the tune window becomes.
Just my $2.00
J

At the range where I shoot, there are some cedar trees right next to the target board and around the bench, so I use those as 'wind flags' to gauge when to pull the trigger. However, a lot can be happening in the 100 yards in between for which I have no indication. With the IMR-3031 load I tried, I could be on the edge of the 'tune window' that would provide optimum performance. At some point in the future, my plan is vary that powder charge a few tenths of a grain either way to see if I can find the 'magic' spot, but my tests give strong evidence that my rifle likes the 0.125" bullet jump with that powder.

 

[JDinFbg]

Seems like a .31-06 with pitted barrel is kind of a coarse instrument for the sort of fine tuning that has been moving from target rifles into sporting guns, but you are seeing an effect. Might have something to do with a military rifle throat being designed for a long jump with service ammo.

My attempt at a Long Range .223 and a good .308 found generally better accuracy with extruded powders than Ball. Darned shame, especially AA2520 pretty much obviated the need for the electric dispenser I use with Varget, etc.

When I got back into reloading after many years hiatus, I had to develop all new loads since many of the powders I previously used are no longer made. My desire was to concentrate on using ball powders since they meter almost perfectly through a powder measure. So far I have not had much success with ball powders in any of my cartridges. It only took me 6 pounds of W-748 and a pound each of H-380 and H-414 in my various cartridges to find that out.

 

[JFrank]

When I got back into reloading after many years hiatus, I had to develop all new loads since many of the powders I previously used are no longer made. My desire was to concentrate on using ball powders since they meter almost perfectly through a powder measure. So far I have not had much success with ball powders in any of my cartridges. It only took me 6 pounds of W-748 and a pound each of H-380 and H-414 in my various cartridges to find that out.

Are you throwing powder charges for these tests you are performing?

 

[JDinFbg]

Are you throwing powder charges for these tests you are performing?

No, I ran everything across the powder scale and used a powder trickler to get them exact.

 

[film495]

How it effects dispersion, is in the name. When an unfired cartridge is in the chamber the bullet is held in reasonable axial alignment with the bore by the case neck. Normal case design has all or almost all of the cylindrical body of the bullet inside the case neck. In order to keep the bullet axially aligned in the bore, the cylindrical section of the bullet cannot get cocked in the free-bore (or leade) before engaging the rifling. When the pressure starts to push the bullet out of the case the friction is not equal all around the case neck, and the bullet will tilt toward the draggier side of the neck. The free-bore is not the same size as the bullet but has a small clearance, so it can tilt. The bullet must "jump" from the case neck to the start of the rifling and maintain axial alignment. The longer the distance from the ogive to the start of the rifling, the more opportunity for tilt. So, the tilted bullet goes down the bore with the long axis sweeping out a cone, due to the spin. When the bullet leave the constraints of the barrel, aerodynamic and gyroscopic forces will realign the bullet axis to some equilibrium yaw angle, and thus generate side forces on the bullet and cause the bullet to drift in that direction. Since the initial tilt direction is random, the resulting drift direction will also be random.

I subsribe to this theory, but also - that if the bullet is jammed or too close, it does not have any distance to accellerate enough, and I'm just guessing here, but - in theory, if it is going to slow - the impact can throw the tail off, as the obtruation or whatever it is called into the rifling is not fast enough, and can affect alignment, if the momentum of the rear causes the tail end to sway in a random direction. I can't take a picture of this happening, so - no idea how anyone would ever prove or disprove something like that.

 

[Bcwitt]

I've never been a fan of the "set the jump" method. I prefer to just have a nice tight crimp.

 

[Varminterror]

Eww.

 

[lysanderxiii]

I subsribe to this theory, but also - that if the bullet is jammed or too close, it does not have any distance to accellerate enough, and I'm just guessing here, but - in theory, if it is going to slow - the impact can throw the tail off, as the obtruation or whatever it is called into the rifling is not fast enough, and can affect alignment, if the momentum of the rear causes the tail end to sway in a random direction. I can't take a picture of this happening, so - no idea how anyone would ever prove or disprove something like that.

There are also variations due to pressure waves forming inside the case that will lead to velocity variations. These are dependent on the start pressure, the propellant burn specifics, exact case volume, and % case capacity.

So, certain propellants/loads might prefer more or less "jump".