@lysanderxiii, the problem with your explanation is that there are multiple bullet jump nodes at varying jumps. If your theory were correct, you'd only find good accuracy/precision very close to the lands and very bad accuracy/precision the further you get away from the lands but this simply isn't the case.
Accuracy & Precision vs. Bullet Jump
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Nature Boy
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Here’s an example of a seating depth test I did. I performed a second test from these results for -0.020 off and +0.005 in. I found that -0.022 and +0.007 produced the tightest groups. That has remained consistent over several hundred rounds.
Edit: As a note on my nomenclature, I determine where the bullet touches the lands as my 0.000. Anything minus is off the lands, anything plus is into the lands
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JDinFbg
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I found that interesting, also. They recommend that process for all bullet types whether VLD, tangent, or hybrid. It is a different approach than I have always followed, where I started with some minimum jump (say 0.020") then varied the powder charge until I found what shot well. If I didn't find anything, I either varied the jump a little or tried a different powder until I found something that shot well. The Berger process leaves me wondering if the jump "sweet spot" band width remains constant regardless of what powder one uses? For example, if IMR-3031 and H-4895 were options for your particular cartridge and bullet weight and one employed the Berger process with starting loads of each, would one find the jump "sweet spot" at the same place for both powders? This may just be one of those things that goes in the black box that Nature Boy referred to in Post #19 above.
JDinFbg
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All the groups you showed would result in a dead prairie dog at any reasonable distance. I'd be tickled pink to have a rifle that shoots that well over those jump variations. I'd be hard pressed to determine if the differences were a function of seating depth or just the slight variations one encounters in shooting (e.g., sight picture, rifle hold and trigger squeeze, wind, etc.). Also, how far can one really jam a bullet into the lands before the process of closing the bolt just causes the bullet to be shoved back into the case?
JFrank
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Nature Boy is a very good tuner, I've personally adopted his style of seating as well as primer testing several times on my way down the rabbit hole.
Thx Bill
J
lysanderxiii
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You are assuming that changes in combustion volume due to seating depth and changes in neck tension due to grip length have no influence on dispersion. Which is not a good assumption.
There are many factors that influence dispersion, and it is almost impossible to change just one without changing several others as well. So, yeah, you're always going to see "nodes" where the differing influencing factors effects are minimized.
BigAlShooter
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Yeah, I can't explain as good as lysanderxiii just explained it. The closer to the lands, the less chance for inconsistencies of alignment and starting pressure build up. From my experience, the closer to the lands, the more accurate, but not by a whole lot. You can always find a good load that is far enough off the land so that you can at least use your magazine. That's where all of the other parameters of reloading come into play. If you keep everything consistent, then the OAL is less of a factor, but still one of many factors. That is reloading - trying to make every factor as consistent as you can. That is why it is a great hobby.
JFrank
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@lysanderxiii
Am I hearing excess jump creates in bore yaw that somehow corrects it's self once exiting the muzzle ?
Or just my old guys comprehensive skills fading.
Am I hearing excess jump creates in bore yaw that somehow corrects it's self once exiting the muzzle ?
Or just my old guys comprehensive skills fading.
lysanderxiii
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Take a child's top and spin it on a large flat table with a large tilt (the axis of rotation is off vertical) and watch it as it spins. It will eventually standup straight. The faster you spin it, the faster it will assume a vertical position.
Here is a trace of the yaw angle of a highly spun .357 bullet. You can see the angle damps out as range increases. (The x-axis is in calibers, and the launch twist was standard 1-18-3/4, so the bullet makes two revolutions every 100 calibers of travel.)
At 6:11:00 you can see this phenomenon in video with bullets big enough to see.
JDinFbg
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That is a very interesting video. It demonstrates that bullets will actually straighten themselves out in flight. It would be interesting to know if a faster rate of spin relative to caliber would cause this to happen quicker. It looked like the projectile for the large Russian gun was not seated axially aligned in the case and it still performed well.
GooseGestapo
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It’s actually an M4 Sherman. Not Russian.
I’ve seen the nutation or “wobble” shooting a friends Sharps .45-90. It shoots tighter groups at 300yds than at 100 or 200yds due to stabilization or the bullet “going to sleep” as it used to be called.
Sierra Bullets had a section in their 1970’s manual regarding this phenomenon.
Shooting NRA PPC, I’ve often watched shooters shoot at 50yds and seen the bullets from revolvers look like streaking strobe lights due to bullets “wobbling” from various causes. Add in a cross wind and the results are like throwing knuckle balls.
The same phenomenon kicks in, in reverse as the bullets slow down and go transonic at extreme long range. Hence the current trends in F-class and such.
Interesting stuff!
I’ve seen the nutation or “wobble” shooting a friends Sharps .45-90. It shoots tighter groups at 300yds than at 100 or 200yds due to stabilization or the bullet “going to sleep” as it used to be called.
Sierra Bullets had a section in their 1970’s manual regarding this phenomenon.
Shooting NRA PPC, I’ve often watched shooters shoot at 50yds and seen the bullets from revolvers look like streaking strobe lights due to bullets “wobbling” from various causes. Add in a cross wind and the results are like throwing knuckle balls.
The same phenomenon kicks in, in reverse as the bullets slow down and go transonic at extreme long range. Hence the current trends in F-class and such.
Interesting stuff!
JDinFbg
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You have to watch the entire video. They start with the Sherman, but then shoot a 176mm Russian artillery gun in the last part of the video. Their target is a row of watermelons, and you can actually see the watermelons deflect the projectile.
JFrank
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Entertaining yes , it shows out of bore yaw, not in bore yaw.
Also not sure how you tie bullet jump together with this in bore yaw or out of bore yaw
Also not sure how you tie bullet jump together with this in bore yaw or out of bore yaw
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JDinFbg
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Yes, and it raises a whole lot of questions. I suspect a projectile could start out in the chamber perfectly aligned with the bore and still exhibit yaw once it exits the bore. I can see a whole host of factors that could cause this.
Going to sleep is what we used to call it.
JFrank
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Or just plain Lazy
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
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
Varminterror
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Legend has it that Bryan Litz has a $10,000 bounty standing for anyone who can demonstrate this happening at his range.
Legend has it, despite countless claims, it’s never yet been demonstrated, and his money remains safe.
Nature Boy
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Speaking of legend, if you haven’t read this it’s worth the time......
The Legend of the Houston Warehouse
https://www.angelfire.com/ma3/max357/houston.html
The Legend of the Houston Warehouse
https://www.angelfire.com/ma3/max357/houston.html
Benchrest shooters tend to do this, mine weren’t quite that loose. It goes hand in hand with being seated into the lands for good start pressure.
Varminterror
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Spin stabilization reducing yaw (which is the entire point of spin stabilization) is a very different thing than a bullet shooting smaller groups at longer distance than at close range. As evidence of that, track the center of mass of the bullets in the tank video above - they aren’t following a decaying helical path, despite considerable axial instability at the outset. Nobody is spiraling bullets in curved helices which decay into tiny little groups. Tip yaw and flight are very different things.
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lysanderxiii
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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.
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.
Nature Boy
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Considering we are all talking theoretical here, my though is the bore has a better chance of straightening out an off axis bullet than the forces of physics at play after it leaves the barrel.
Regardless, put me in the Brian Litz camp that says bullets suffer from a chronic case of incurable insomnia and never get any sleep.
Regardless, put me in the Brian Litz camp that says bullets suffer from a chronic case of incurable insomnia and never get any sleep.
Jim Watson
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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.
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.
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