rotational decay

[mdeland]

I have given some thought to what part rotational decay plays, if adequate to stabilization of flight path length, in the accuracy equation. Some think a bullet can be over stabilized as well as under which of course usually results in bullet tumble.
Another question I've had is , do patched bullets loose rotation slower than grease bullets, which is probably the case having less air drag but does that actually help bullet stability over the trajectory path ?
Does air drag play any part in bullet stability in flight or is it only a deficiency ? Kicking around some thoughts.

[rgchristensen]

I spent some time looking thru McCoy's "Modern Exterior Ballistics", and didn't find anything about decay of rotational velocity. I'm guessing that it is not an important factor.

CHRIS
RGChristensen

[mdeland]

Perhaps try a google search on the subject. I've never seen much about it probably because it would be so hard to accurately measure. We do know what a given bullet weight, length and diameter needs to start at (pitch) to be accurate over a given distance but what we don't know is how much that has to slow down before the moment over turns and stability is lost.
The over stabilization folks think that at some point over spinning increases yaw which hastens rotational decay (more drag).
Another question I have is, is drift influenced by the drag generated pressure wave surrounding a bullet or just the physical body of the bullet. Rotational velocity decay would probably effect this pressure envelope encasing a bullet in flight.

[desert deuce]

So, you have chosen to reopen that can of worms?

You may have to read this carefully more than once to understand what is being related.

None other than the late Dan Theodore agonized over this segment of bullet dynamics and elucidated various theorems ad nauseam on the subject in an effort to settle the question at least in his mind. And as was his want the question lead to experimentation in an effort to assuage his curiosity.

As I recall the question was not settled theoretically to his way of thinking.

It is sort of like to what is really important later in life when one is in need of a final resolution, Depends. What is needed at the moment matters.

Therefore, what is found to be successful in applying and employing different barrel twists, bullet weight, bullet length, alloy hardness, lube, velocity, design etc is resolved in applying the different variables to the task at hand and whichever combination produces the desired result at the highest level wins the argument. A sort of finding the right diameter and length of round peg to properly fit into a round hole without deforming or destroying either.

I contend that the solution arrived at was/is what worked best in application, so far, rather than developing the perfect combination in reality or theory.

The test results Dan reported seized my interest and at his instruction I obtained a 1-16 twist Kreiger .45 caliber blank, Dan designed the chamber reamer and the bullet mould. John King put the rifle together and Paul Jones made the bullet mould and it works as well or better than anything else I have tried particularly in less than ideal conditions at Long Range which was the purpose the rifle was built for.

IE. delivering a stable, accurate bullet on target at 1,000 yards under a range of conditions employing a Balanced Load.

[mdeland]

I sure miss reading Dan's thinking on stuff like this as well.
I think drift, drop and overall accuracy are directly influenced by the turbulence envelop/pressure wave around a projectile and not just the physical profile of it. Gravity is the only constant on the projectile body. All else is in a constant state of change. Temperature, humidity, barometric pressure, wind and even Coriolis effect.
The most efficient compromises built into a bullets flight stability should result in increased accuracy realization. Understanding cause and effect of these trade offs, I would think, should be a map of how to get there.
My guess is rotational decay may play a bigger part in this than we think.

[bruce m]

mike,
I am not sure of the exact deyails, but believe that Aberdeen proving ground measured spin decay in a 30 cal bullet as some % loss for a time of flight.
I remember this because it flies in the face of the old belief that spin decay is virtually non existant.
berger bullets have now been saying that it is virtually impossible in available barrels and bullets to overspin bullets enough to affect long range accuracy.
I have 1 1:8 twist 223 that groups 50 gn bullets all touching at 100 yds.
spinning bullets faster will give a little more spindrift, but not much.
the extra stability gained offers improved stability in wind shear situations, hence less unexplained misses.
the extra stability from faster spinning that causes spindrift comes about due to an increased yaw of repose, which slightly reduces b.c.
hence a faster loss of velocity and with it a potentially greater wind deflection.
this is outweighed by greater consistency.
air drag can play a big part on bullet stability.
the bullet has a centre of mass and a centre of (for this discussion) drag.
in all but minie bullets, the centre of drag is in front of the centre of mass.
the distance between these two points is called the overturning moment.
the longer the overturning moment, the faster the bullet needs to spin to be stable.
so reducing the drag on the nose shortens the overturning moment.
use a long spitzer shape to do this?
yes with jacketed supersonic flight bullets. it works well.
but then we come to the dreaded transonic zone!
firstly cast bullets cannot sustain a nose of that shape.
secondly, airflow at transonic speeds does some funny things that affect stability, moving the overturning moment backward and forward rapidly and repeatedly.
with grease grooves, there are some who believe the increased drag on them compared to a smooth sided bullet aids stability due to the drag being to the rear.
others believe that the disturbed airflow over them can be a cause of annoyance or worse when transonic.
who knows?
spin decay seems to be time related more than distance related.
thus, the shorter the time of flight, the less spin decay.
to achieve this higher muzzle velocities and b.c, are your friend.
the 2 nose shapes that seemed to evolve in the day were the ellipse, and the metford.
these are now ellipse and money, although brent uses a term for ellipse which describes it 3 dimensionally.
our alloys seem to hold up with noses of roughly 1.5 calibres length.
bruce.

[Distant Thunder]

I have often wondered just how much rotational speed can be lost in the 3 to 3 1/2 seconds our bullets are in flight to 1000 yards when they are spinning at 52,000 rpm when they exit the muzzle.

I would think that paper patch bullets would retain rotational speed better than grooved bullets because a higher percentage of their mass is at their full diameter.

Very little if any of the flight of a BPTR bullet is not within the transonic zone, 1340 to 890 fps. Any discussion of supersonic velocity in relation to bullet design or most anything else is more than useless, it's misleading (IMHO). We are in the twilight zone, "...a fifth dimension beyond that which is known to man, it is a dimension as vast as space and as timeless as infinity. It is the middle ground between light and shadow, between science and superstition, and it lies between the pit of a man's fears and the summit of his knowledge."

There are things we are not meant to fully understand, but that does not mean we can't use what we know to our best advantage. Or at least that which we think we know. When we are dealing with things such as this our experience is really all we have to go by, anything else is just someone else's opinion.

[gunlaker]

Bruce I remember when you posted that stuff regarding the spin decay a year or two ago. It was surprising! With respect to the stabilizing effect of grease groove drag, Dr. Richard Gunn wrote a paper on it that is in the ASSRA archives.

I expect that our larger diameter bullets should have slower spin decay than a .30 cal bullet. I believe this because the angular momentum increases as the square of the radius but the surface area increases linearly with the radius. I imagine that spin drag is proportional to the surface area and surface velocity, or maybe the square of the surface velocity.

Chris.

[semtav]

With the proper marked bullet base, half black andd half white, im sure with the proper instrument one could log the rotational speed of a bullet all the way to the target. No doubt the military has this type of technology. Doubt we'll see it for years .
Would be interesting

[desert deuce]

The dedicated shooter from Wisconsin's frozen northlands essentially said the same thing I did but in different words to which I will add: In target shooting it is the target that determines what works best under the conditions to which the combination is applied.

When we are dealing with things such as this our experience is really all we have to go by, anything else is just someone else's opinion.

I do recall Dan saying repeatedly it is important how the bullet in flight reacts during and after passing through the transition zone. On this my belief is observation of results at the target matter most.

[Distant Thunder]

Exactly my sun baked friend, we don't really need to understand, as if we could, why something works only that it does. Let your targets be your guide.

[Distant Thunder]

Exactly my sun baked friend, we don't really need to understand, as if we could, why something works only that it does. Let your targets be your guide.

[gunlaker]

To me there is as much joy in understanding how something works as there is in making it work.

It's certainly not necessary to have detailed knowledge to shoot these rifles very well, but it certainly isn't going to hurt anything.

Chris.

[bruce m]

jim,
your point about the transonic zone is central to this discussion.
chris,
I, like you was shocked to learn of spin decay, when the belief that there was none underpinned many things.
while I rely like to understand how things work, I am happy to accept a good result.
bruce.

[rgchristensen]

Harold Vaughn -- in his little book, "Rifle Accuracy Facts" (pp160 ff) describes how to measure rotational spin rate of bullets by embedding a small magnet in them and shooting them past/thru a wire winding. Easy to do near the muzzle, but longer ranges could pose problems.

CHRIS
RGChristensen