rotational decay

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bruce m
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Re: rotational decay

Post by bruce m »

mdeland wrote: Tue Mar 24, 2020 2:28 pm Well, some of us are interested in the investigation of how and why things work and others not so much. Neither is less then the other. If one is drawn in then there must have been and interest to one degree or another. I certainly have gained knowledge from the discussion and appreciate all who have contributed to that advance.
What happens in discussions like these ,which increase our thought perimeters, is very often applicable in divergent direction,which intern leads to other discovery. We often think the Old Dead Guys knew it all and all we have to do is rediscover what they found out but that is never true about anything. There is always more to be learned.
The more one can learn about any subject is beneficial because it provides a broader foundation to sort out chaos.
Just as more experience on a spotting scope enhances ones ability to see what is happening with the wind and make the correct sight knob twist so does increased understanding of bullet flight.
mike,
the odg knew a lot about wind reading.
they had to because their first shot could not be a sighter, rather it was for score.
today some guys take a number of shots just to get on target before going for score.
they also knew about elevation changes due to barometric pressure changes and wind direction.
first they had to hit the target first shot, then keep on target.
barometers were common on ranges like creedmoor and sea girt.
the other thing they had to deal with is they were not string shooting.
everybody fored their shot before you shot again,so you had little feedback from your previous shot.
you just had to read the wind every time.
if I talk to fullbore shooters, they think that is like every shot being a first sighter.
bruce.
ventum est amicus meus
mdeland
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Re: rotational decay

Post by mdeland »

They did have the advantage of a larger target to make the first shot count though.
bruce m
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Location: australia

Re: rotational decay

Post by bruce m »

it was indeed bigger, but could still be missed.
1000 yds is a long way and there is many a slip twixt hit and miss.
bruce.
ventum est amicus meus
Aviator
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Re: rotational decay

Post by Aviator »

Well, I have really enjoyed this thread, even all the twists and turns.

I have spent a lot of time in the last year trying to get better at casting bullets, loading ammunition, load development, wind condition reading (guessing?), sight alignment, trigger operation, and follow through. And have made some progress, I think. At least 3000 bullets fired from my 45-70 Shiloh in the last 12 months,, more likely around 4000. I appreciate those who have helped me along this journey.

But I am in the camp of also enjoying trying to understand how and why things work.
I do not believe that exploring theories, and seeing if they correlate to what actually happens is a negative trait. :shock:
But of course your mileage may vary... 8)
buffalocannon
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Re: rotational decay

Post by buffalocannon »

Dan T (RIP) and I had our moments but no one was better at explaining this type of stuff to the layman. Miss him.
Kenny Wasserburger
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Re: rotational decay

Post by Kenny Wasserburger »

I learned the lesson of spin drift at my range, at the ranch with my 45-70 at 800 yards one fine early evening. Range faces south, it was dead calm, a rarity here in Wyoming but a fine summer evening. My dad was spotting for me and I had a dead nuts no wind zero. Dad comments that I am missing just off the right edge of my target, 8 feet wide 5-1/2 feet tall, 36 inch steel bull. 2.5 MOA of left resulted in bullseyes yet on the right half of the bull. 3.5 MOA give nearly perfect center bulls, I believe I shot 17 consecutive bulls, before the light quit on me. I remembered learning about precession in my Highpower shooting days, or spin drift as some call it. Further reading back in 1996 on this was an unknown in BP Shooting. Today we have a much larger pool of knowledge to draw upon. I understand the concept and the practical applications of this in shooting our slow and heavy guns. My long range setups have about a 3MOA left built into my mechanical no-wind Zero. I don’t loose any sleep over it, once that lesson was learned and applied to my zeros, I went on about learning and studying the subtleties of wind and mirage reading. There my friends is where the real education begins, and how to come back to your no wind zero and correct for a total reversal.

Something way more important to study on and practice, than to worry over spin drift,in my opinion
Others may disagree, all well and good but I think that’s chasing a fruitless quest. Understanding what’s happening, is necessary for certain, yet there much more important aspects to study, as others have mentioned. A good balanced load, wind shear, mirage, temp bands, humidity. Light is another.

Kenny Wasserburger
We'll raise up our Glasses against Evil Forces, Singing, Whiskey for my men, Beer for my horses.

Wyoming Territory Sharps Shooter
mdeland
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Re: rotational decay

Post by mdeland »

Yeah, I think everyone on here misses Dan Theodore. He was sort of the "Townsend Whelen " of BPCR.
Longrange
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Re: rotational decay

Post by Longrange »

OK think of a bullet as a flywheel, larger the dia the less rotational decay at a set r.p.m. to a smaller dia at the same r.p.m. whitch one will stop first on freewheel .? Vic
mdeland
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Re: rotational decay

Post by mdeland »

Smaller diameter, all else being environmentally equal.
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desert deuce
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Re: rotational decay

Post by desert deuce »

Thinking about rotational decay.

A little Kenny Wasserburger story from Raton daze from 1,000 yards as I remember it. Jimbo spotting, Kenny Shooting his scoped rifle and a down spotting scope by his mat.

Anyone that has shot with Kenny knows two things, first...his ammo is too pretty to shoot, second...Kenny is capable of amplifying sound

My partner and I are pretty much the other end of the range and of course using hearing protection. Nevertheless we could hear Kenny perfectly. Occasionally we could hear a bit of Jimbo's voice even though the wind was from us to them.

A thunderstorm was rapidly moving in from about 8 o'clock.

Kenny got his sighters before the blow started and it was after his first record shot on the score card as the 9 o'clock winds began to hum and increase incrementally

Kenny, "twenty minutes left it is Jimbo.

Kenny, "Good call, good call Jimbo."

Kenny, "Jimbo, did you say another ten left Jimbo?" "OK"

Kenny, "Good call Jimbo."

Kenny, "Did you say go five right and hold on the target to the left?" "OK."

Kenny, "Now where to you think that one went Jimbo?"

Jimbo, "I don't know Kenny! I did not see a thing."
Sometimes you get the chicken, and sometimes you get the feathers!
bruce m
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Re: rotational decay

Post by bruce m »

ah the old fire your sighters and a change happens syndrome.
who can you turn to at a time like that?
far better to get 2 sighters in on the 2 extremes if you can, but that is not always possible.
now to precession and nutation, particularly in relation to wind shear and spindrift.
any bullet fired from a rifled barrel spinning undergoes precession.
what this means is that while the axis of the bullet at its base is stable, the nose is going in circles around the axis.
as the bullet flies, the circles become smaller and smaller, until they all but disappear.
however it is not quite that simple due to nutation.
nutation is the nose of the bullet going in smaller circles around to circle described by precession.
the line described looks a little like a multi leaved clover.
at the end, usually about 200 yds of flight, nutation disappears altogether, and precession is virtually non existent.
this is the point at which we say the bullet has gone to sleep.
it is why many want to test loads at 200.
fclass guys are now starting to test at 220 to be sure.
the faster the twist, the greater time of flight for a bullet to go to sleep.
benchrest shooters use slow twists and short bullets for this reason at 100 and 200 yds, and it is possibly why slow twist 40s are good at mid range.
but we long range shooters need longer bullets and faster twists.
when the bullet goes to sleep, all the forces on it, drag, gravity, etc are in balance.
these forces cause the axis of the bullet to settle into a position pointing above the arc of flight, and to the right of it for a r.h. twist.
the faster the twist, the greater is this angle.
because it is at an angle to line of flight it is described as yawing, and because it is asleep, it is the yaw of repose.
the angle it is at in relation to line of flight is called the angle of attack.
the lower the angle of attack, the less drag, because there is less frontal area.
the yaw if repose has a tendancy to steer the bullet, and thus a flying bullet will not hit the ground as soon as if it were just dropped, because it is steering itself upward a little, being nose high.
it will also steer a little to the right (again for a r.h. twist).
this steering is called spindrift, and the faster the twist, the greater the angle of attack, and therefore the greater the spindrift.
this can be calculated mathematically.
Kenny's 16 twist will have a little more spindrift than my 18 twist 45, as does my 13 twist 40 cal.
what is going on to a bullet during precession and nutation will make it very susceptible to any disruption, as another force suddenly comes into the equation that is not yet calculated.
this is partly why we fear problems at closer ranges more than long.
strong windshear in this period of flight will upset not just the yaw of repose, but precession and nutation which must be gone through to achieve yaw of repose.
during precession and nutation, the bullet is actually travelling in a minor corkscrew rather than an arc, purely because it is steering itself that way.
getting back to dan theodore.
when he was around, we long range shooters did not have to think about much, because he helped us.
one of my inspirations for pp shooting was Kenny w.
he never made any secret of failures, and kept hanging on for grim death to achieve his goal.
I recall him talking about finding one of his fired bullets in the carpark.
dan helped him and he gave dan credit.
now he shoots seriously good waterlines like palma shooter would hope for.
by the time I started pp, the new breed of pioneers had made it easy for me.
brent was another one. and there were others too.
not only was it the what to do, but more so the results achieved that inspire.
bruce.
ventum est amicus meus
mdeland
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Re: rotational decay

Post by mdeland »

It sort of looks like the .45 caliber projectile diameter,lengths and weights are one of the better balance/ convergence points to best accommodate the various dynamics involved and influencing the stable flight of a projectile.
Dan felt the .38 calibers should really shine as well but they seem much harder to get to behave as easily as do the .45's at long range.
bruce m
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Re: rotational decay

Post by bruce m »

mike,
dan was even experimenting with 8mm for long range.
but 32, 38, and 40 cals need quite fast twists to stablive their long range bullets.
few people are prepared to spend the money on that type of experiment.
45 cal is a lovely balance between those calibres and 50 cal whe talking recoil.
50 cal is king of the mentioned heap, but kicks too much in all but a very heavy rifle.
bruce.
ventum est amicus meus
mdeland
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Re: rotational decay

Post by mdeland »

I made a 38-70 using a 10 pitch Douglas barrel on a Uberti Hywall and Dan helped me design a 380 grain Mini groove bullet that I have not actually ever gotten around to working up a good load for. Might get some time this summer to wring it out. To much work both gun and glass to get much range time any more.
Would like to get up to Talkeetna to use there 1k range.
VectorMan
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Re: rotational decay

Post by VectorMan »

bruce m wrote: Wed Mar 25, 2020 9:10 pm 50 cal is king of the mentioned heap, but kicks too much in all but a very heavy rifle.
bruce.
What twist would be necessary for 1000 yard target competition in a heavy 50-90 rifle?
"keep adding powder til it bloodies your nose and blacks your eyes, then back it off bout 5 grains."
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