Does it make a difference?

[Larry Pletcher]

Larry, if you are really asking that question, then I am both disappointed and very enlightened about the results of some flintlock testing!

Runner,
It's really me. I don't know what your answer means. I DO take learning and teaching VERY seriously.
Pletch

 

[Rick Davis]

. . . After reading this, several folks need to do some reading on velocity, caliber, and twist. The statement that the most it can be doing is 1 turn in 66 inches about made me spew my coffee laughing.
You all have a nice day!

Hi Runner,
Here was my complete quote:
I believe Paul's facts are correct, but as in any discussion there is usually another perspective. Let's look at the velocity and twist from the deer tissue's point of view. Perhaps the ball was fired from a barrel with a typical round ball twist - say 1 turn in 66 inches. Regardless of velocity, the ball is, at best, making 1 turn in 66". If the deer's girth was 15 inches, the ball would make less than 1/4 turn inside the deer.

The twist of the ball measured in rpm increases with the velocity, but the rate of turn per inch of travel remains the same --- one turn in 66 inches whether the velocity is 1000 fps or 2000 fps.

I still stand by my statement, but I'm always willing to learn. If there is something you know that I don't please fill me in.

Regards,
Pletch

Me too. Always interested in learning something new.

 

[mykeal]

Yeah, Runner - instead of sitting there laughing at everyone, how about setting your coffee down and laying a little learning on us poor folks. Complete with references to this flintlock testing you refer to. I would like to add it to my database for sure. And helping folks out with a little education is certainly more in keeping with the objectives of the forum than laughing at people who would really like to know something. Please?

 

[paulvallandigham]

Okay. Here is something new to consider to explain why a slow twist rifle firing a ball at high velocity at a deer at 25 yards or under may just see the ball veer off the straight POA. Its called Torque. The ball may only be making One Turn in 66 inches, for our example, here, but when you send it into an animal at 2000 fps, knowing the soft lead is going to expand immediately on entering the body cavity, you have to deal with torque. Now, a faster Rate of Twist can control or overcome torgue cause by the high velocity simply like a high speed drill will cut through material better and " cleaner " at a high speed rather than at a low speed. The bit, or, here, lead ball, ' Slips through the material before energy can be diverted into surrounding tissues, creating all kinds of harmonic waves, depending on the relative density of the material being penetrated. Worse, in living tissues, you have fluid dynamics entering into the equation of torque, causing irregular resistance to that energy as the ball passes through the tissues. That produces an effect equal to driving over a bumpy road. Just as its harder to steer a car going over a bumpy road when its driven fast, than when its driven slower, the ball is going to have a harder time staying on course the faster it is passing through the tissues.

Increase the Rate of Spin to the ball, by shorting the Rate of twist of the rifling, and the spinning ball will resist deviations from its original path through a wider range of velocities, IMHO. Reduce the velocity of a ball spinning slow, and you also help it stabilize as it penetrates living tissues.Only when the speed of the ball drops to the point of stopping do you see the ball( or bullet) turn on skin, or a soft rib, or cartelage in a shoulder blade. And then, the distance the ball travels after it turns is measured in inches, at most. :hmm: :hatsoff:

 

[Runner]

A projectile fired by means of expanding gases in a rifled barrel starts at 0 rotations and it rapidly accelerated to whatever the barrel twist is in the time it takes it to leave the barrel. That is assuming it does not strip the rifling at all. After it leaves the barrel, the velocity drops off, but the spin increases as the velocity drops for a short time. Picture snapping a top between your fingers and watching it smooth out and accelerate in response to the impetus your fingers provided. It will spin faster than the impetus that started it. A bullet out of a rifled barrel does the same thing. As with a top, the spin holds up better than the velocity in most bullet types. As the bullet begins to slow down, the spin remains pretty close to stable inside any range we would find useful with our primitive weapons. Bullet shape effects this also. The longer bullets accelerate to higher speeds than roundballs do.
I once recovered a 180 '06 bullet from an 80 pound doe that the bullet hit in a rib broadside. I recovered it from the ham on the same side the bullet hit on! That is an extreme example of what can happen when a spinning bullet hits a target. It does demonstrate the spin on target thing tho. Out of that twist with the bullet mentioned, that would be impossible under the model you guys put forth. Most of the time the bullet will have slowed down a lot from our muzzleloading guns but the spin at target will be higher than twist. In other words, instead of doing 1-66, it will be spinning a lot faster than that, and it will be spinning faster per foot traveled than it was at the muzzle. Then it hits the target and the forward motion is rapidly being slowwed, but the spin is being destroyed. As the bullet begins to expand, the top looses balance. It does nasty things during that time period.
Every gun is different. That is why they all require a different load to achieve the needed accuracy.
Figure this one, which bullet is spinning the fastest at 50 yards? A copper jacketed bullet or a solid lead bullet of the same shape fired at the same speed from the same barrel? You should have learned this answer on the schoolyard merry-go-round.
I am sorry if I seemed rude. The final speed of the projectile it a mixture of projectile shape, degree of bullet balance, the length of the barrel dwell, type of rifling, speed of the twist, and how hard it was shoved!

 

[chapmans]

I think the only way to really settle the twist, torque, mushroom, ball direction path vs. velocity,deflection thing is for some good old fashioned testing. How about finding a .45 smooth bore, a .45 with a say 48" twist and a 66" twist and shoot something all with the same loads at the same distances and see if there is really any difference.I for one don't think the twist will make any difference but then I've never ridden a ball that was shot through a deer to really know what it's doing.
Personally I think once the ball flattens out the original twist of the bbl. has nothing to do with the path the ball takes,I think that all depends on the angle the ball hits it's first obstruction.
Longball 58

 

[Larry Pletcher]

. . . . Most of the time the bullet will have slowed down a lot from our muzzleloading guns but the spin at target will be higher than twist. In other words, instead of doing 1-66, it will be spinning a lot faster than that, and it will be spinning faster per foot traveled than it was at the muzzle.

Runner,
I reckon we'll have to agree to disagree about a couple of things. I spent most of the afternoon on different ballistics web sites including Sierra. I can find no evidence of a ball/ bullet/projectile that increased its rate of twist after it left the barrel. The velocity does in fact fall off faster than the twist because the drag on the bullet is greater than the drag on the rotation. But no one discusses an increase in twist after exiting the barrel.

If you know of a ballistics web site or book that will provide evidence for your statement, I'd appreciate reading it.

. . . . . . .Figure this one, which bullet is spinning the fastest at 50 yards? A copper jacketed bullet or a solid lead bullet of the same shape fired at the same speed from the same barrel?

From what I've studied, if both projectiles were fired from the same barrel at the same velocity, their twist rates after just leaving the muzzle would be the same. Fifty yards down range I'd speculate that the heavier projectile would have retained both velocity and twist better. Neither projectile would have a twist rate faster than the barrel's twist rate. Once the projectile is out from under the barrel's influence, I do not see a rate increase in twist. IMHO

Regards,
Pletch

 

[Runner]

Pletch, I studied ballistics a long time ago. If I could point you to a current source, I would, but I have not read anything on ballistics for many years. I have very little interest anymore. My days of trying to stabilize little bitty pills powered by big loads of powder are long past.
By the way, the answer is that the one with the most of it's weight closer to the center will spin much faster. The jacketed bullet will accelerate to a higher spin rate than the solid lead bullet.

 

[chapmans]

What makes the projectile (ball) accelerate after it leaves the rifling? Does a ball shot from a smoothbore start spinning after it leaves the bbl. or does it need rifling to get it started? It seems there are always more questions than answers.

longball 58

 

[Runner]

I never said any of the BS youjust posted. Welcome to the ignored group!

 

[chapmans]

That wasn't my BS I just read the quote in Pletch's post.

 

[Mike Vach]

Geez, guys. I just wanted to know if you thought I ought to buy a new .45 or a new .50!! :surrender:

 

[Dave K]

Seems to me, I remember that. :wink: So from what you read, what did you decide? If you are only ever going to shoot nothing larger than a whitetail and where you target shoot everyone is using a particular cal. and hunting with it. Just go with the flow. Either a 45 or a 50 will work fine. JMHO

 

[Larry Pletcher]

Pletch, I studied ballistics a long time ago. If I could point you to a current source, I would, but I have not read anything on ballistics for many years. I have very little interest anymore. My days of trying to stabilize little bitty pills powered by big loads of powder are long past. . . .

Runner,
I have been playing with spreadsheets since 4:30 this morning trying to make sense of this. I could not get past the idea that a ball can increase its twist rate. I still can find no ballistics that shows this. So I reworked the formula for calculating linear twist rates, and it finally hit me. It is not that the ball somehow picks up twist rate. There is no energy added.

In general terms to calculate twist, you divide distance traveled by the rpm. Since the distance traveled is decreased and the rpm (for practical terms) is not, then the distance per revolution is smaller - hence a faster linear twist. I had been trying to get my mind around an acceleration taking place, and all along it was simply a division with a gradually decreasing number.

I attached a spreadsheet that uses a .495 ball traveling at velocities provided by Lymans BP Handbook. I set up three examples with different starting velocities. The numbers in blue are from the book. The red "twist" numbers are calculated by dividing by the original barrel twist. I realize that I should be dividing by a slightly decreasing twist rate, but I could find no way to determine the rate at which the rpm falls off.

The relationship between the twist and rpm seems to be a direct relationship. If the velocity is decreased by one half, then the linear twist in inches is also cut by one half.

The faster we try to drive the ball the more pronounced this effect is, because the faster we drive a round ball the faster the velocity falls off. The chart at the bottom where the velocity is slower shows less of a change.

There are a number of interesting things that can be seen when making comparisons across the three examples. For instance:
ex. 1 ---125 yd ---velocity 1010 ---- 33" twist
ex. 2 ---75 yd ----velocity 1008 ---- 44" twist
ex. 3 ---muzzle ---velocity 1000 ---- 66" twist

Does this mean that there are stability issues with this ball at various distances away from the muzzle. Sometimes when you learn a new thing, you get more puzzled instead of less.

Thanks, Runner. Whether you meant to or not, you pushed me to study this. I knew that the decrease of bullet rpm was slower than the decrease in velocity. I just didn't connect that dividing a gradually decreasing number would lead to a faster linear twist. (BTW the chart is based on NO decrease in rpm at all.)

Regards,
Pletch

 

[paulvallandigham]

Larry: I think what you are seeing is that bullet or ball drag is greater, when driven over the speed of sound, than when the ball travels under the speed of sound. So,too, the drag on rotation of the ball or bullet is also greater when its driven over the speed of sound. Under 1100 fps, the amount of loss of rotation is proportionally less, along with the amount of loss of velocity over the yardage. :hmm: :thumbsup:

Paul

 

[Larry Pletcher]

Geez, guys. I just wanted to know if you thought I ought to buy a new .45 or a new .50!! :surrender:

Sorry about that. We do tend to get off on tangents sometimes. I have used and like both calibers. At the distances that I shoot, either work well. I think you would like either. If you have any doubts, go bigger rather than smaller.

Regards,
Pletch

 

[Runner]

A 45 for whitetail shooting roundball is a short range precise shot placement gun to be used in places where you can track your game. If there is another persons land you can't enter 40 yards away, the 45 with roundball just flat is the wrong gun. The fifty is better, but under those circumstances, I would load it with a 385 HBHP. There are way too many factors to give a simple straight forward answer. At 30 yards in ridge country woods, the 45 is fine. You put the ball thru his lungs and watch him run about 40 yards and pile up.

I described that part of it in my post. It is the acceleration part that you missed. That is ok. Borrow a top and paint a white line on it so you have a visable reference. It explains the phenomenon better than I can.
I really don't type all this because I have nothing better to do.
How does a pitcher throw a ball and have it drop sharply in the neighborhood of the plate?
How is it that his hand, the string, can impart less than one single rotation, and yet the ball spins way in excess of his hand speed?
At what speed of spin does a roundball become unstable, and at what velocity? Is there a predictable point at which a high rate of twist causes problems as the ball slows it's forward momentum?
Like I said, such questions don't really interest me much anymore, but they used to drive me nuts. Good luck!

 

[Mike Vach]

Well, I'm still looking. I am tending toward a .45, after considering a .36 for fun. But, I also saw a .54 that was intriguing. Went to a gun show over the weekend, but didn't see anything in my price range that caught my eye. I can't pay $5,000.

I going to check out TOW and we have a couple of antique arms shows coming up this month. Gawd knows what I'll end up with. But thanks for all the info. I learned more than I ever thought.

 

[Larry Pletcher]

. . . .At what speed of spin does a roundball become unstable, and at what velocity? Is there a predictable point at which a high rate of twist causes problems as the ball slows it's forward momentum?
Like I said, such questions don't really interest me much anymore, but they used to drive me nuts. Good luck!

Runner,
This has gotten more involved than I planned. I received a couple email responses from a number of ballistics sources. One from Sierra gave me a formula for calculating the spin decay. I'm still playing with that. I ran a trial model and emailed it back for him to pick apart. You comment, "such questions don't really interest me much anymore, but they used to drive me nuts" --- I'm beginning to relate to that.

Regards,
Pletch

 

[Dphar]

. . . .At what speed of spin does a roundball become unstable, and at what velocity? Is there a predictable point at which a high rate of twist causes problems as the ball slows it's forward momentum?
Like I said, such questions don't really interest me much anymore, but they used to drive me nuts. Good luck!

Runner,
This has gotten more involved than I planned. I received a couple email responses from a number of ballistics sources. One from Sierra gave me a formula for calculating the spin decay. I'm still playing with that. I ran a trial model and emailed it back for him to pick apart. You comment, "such questions don't really interest me much anymore, but they used to drive me nuts" --- I'm beginning to relate to that.

Regards,
Pletch

I think the round ball becoming "unstable" is a myth. Yes it is possible to have too slow a twist but it is not likely unless the customer specifies a very slow twist in a custom barrel. It takes very little to keep a RB from "tumbling". If it is stable at the muzzle it will likely stay that way since its rotational velocity does not really change over the distance/time a RB is in flight. It does not have to fly to 1000 yards or more as do BPCR bullets or other bullets used in long range shooting.
Bullets, however, are a different matter and can easily become unstable even if adequately stable at the muzzle for a host of reasons (departure angle when shooting at long range is one and afflicts everything from naval artillery to 45-70s to 308s) none or almost none apply to the RB. For example the RB is immune from the aerodynamic forces that effect long bullets fired at (relatively) high departure angles where the bullet nose is actually above the line of flight for the later parts of its trajectory. Which can induce gyroscopic precession and can actually cause it to become so unstable it will tumble even though rotational velocity is still "good".

Dan