Author Topic: Short barrels, fast twist, roundball projectile  (Read 15602 times)

Offline Dan

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Re: Short barrels, fast twist, roundball projectile
« Reply #25 on: May 07, 2009, 02:45:04 AM »
The relevance of twist rates to round ball stability notwithstanding, increasing velocity is a poor way to seek stability in any bullet, be it a round ball or conical.  I'd say the matter of spin stabilized projectiles is fairly complex for many reasons not worth putting forth here...let it be known that upping the velocity has little merit if the objective is to influence gyroscopic stability.  In part the reason is the influence of drag and overturning moments increases substantially in the transonic range and peaks at Mach 1.

Light target loads that do not stabilize a bullet at...oh....say 900 fps, will certainly not do so at 1160 fps or there abouts.

 

Offline Dphariss

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Re: Short barrels, fast twist, roundball projectile
« Reply #26 on: May 07, 2009, 07:52:36 AM »
The relevance of twist rates to round ball stability notwithstanding, increasing velocity is a poor way to seek stability in any bullet, be it a round ball or conical.  I'd say the matter of spin stabilized projectiles is fairly complex for many reasons not worth putting forth here...let it be known that upping the velocity has little merit if the objective is to influence gyroscopic stability.  In part the reason is the influence of drag and overturning moments increases substantially in the transonic range and peaks at Mach 1.

Light target loads that do not stabilize a bullet at...oh....say 900 fps, will certainly not do so at 1160 fps or there abouts.

 

This a whole "nother" can of worms.
Such as why to smoothbores often shoot best with heavy loads?
Why will some rifles not shoot light loads at all.
I have a 54 Douglas that is useless a 25-30 yards with 50-60 grains of FFFG.
So I shoot the HV load at everything.
I had a 72 twist 54 that was pretty harmless with 100 and did not tighten up till 120.
Now we can ask if this was some idiosyncrasy of the barrel or was it something to do with rotational speed.
Lots of people hear report heavy loads shooting better than the light ones. Like 60-70 gr in a 40 cal.
Then we have picket bullet guns in which the picket often requires more powder than the RB. The best load so far in my 40 cal with a 132 gr picket is 80 gr of FFG Swiss. I would have to go grab one but its about .7" long.

May not be the increased rotational velocity but something is going on.

Dan
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northmn

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Re: Short barrels, fast twist, roundball projectile
« Reply #27 on: May 07, 2009, 02:13:17 PM »
My 54 starts shooting for close range with about 70 grains of 3f which is not a heavy load.  At 25 and 50 yards 70 grains does all that I need it too.  I had a 58 Numrich whcih I think had a 72 twist.  It shot very well with 70grains and then jumped up to 110 of 3f to shoot well.  Most of the guns I went through followed a general pattern.  I do think the 40 I had, with a 1-48, needed a heavier charge as Daryl suggested as I never quite liked the lighter charges.  But then if you need to shoot heavy charges in a 40 why not shoot the same thing in a 45 and get better wind advantages?    Thats why I am not real fond of small bores as they seem to need too much to get them to shoot.  The 25 I am building should be small enough not to matter.  The 45s 50s and 54s seem to behave the way they should.  Go bigger and you have great hunting guns but they will thump the heck out of you for target matches.  I have felt for some time that twists are made too slow and faster twists could give some advantages.  The schutzen cartridge principle, use a smaller case and a heavier bullet for best accuracy as exemplified by the 38-55.  Why not in a large bore ML.  Say a 58 with a 1-48.

DP

Offline Dan

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Re: Short barrels, fast twist, roundball projectile
« Reply #28 on: May 07, 2009, 03:47:50 PM »
The relevance of twist rates to round ball stability notwithstanding, increasing velocity is a poor way to seek stability in any bullet, be it a round ball or conical.  I'd say the matter of spin stabilized projectiles is fairly complex for many reasons not worth putting forth here...let it be known that upping the velocity has little merit if the objective is to influence gyroscopic stability.  In part the reason is the influence of drag and overturning moments increases substantially in the transonic range and peaks at Mach 1.

Light target loads that do not stabilize a bullet at...oh....say 900 fps, will certainly not do so at 1160 fps or there abouts.

 

This a whole "nother" can of worms.
Such as why to smoothbores often shoot best with heavy loads?
Why will some rifles not shoot light loads at all.
I have a 54 Douglas that is useless a 25-30 yards with 50-60 grains of FFFG.
So I shoot the HV load at everything.
I had a 72 twist 54 that was pretty harmless with 100 and did not tighten up till 120.
Now we can ask if this was some idiosyncrasy of the barrel or was it something to do with rotational speed.
Lots of people hear report heavy loads shooting better than the light ones. Like 60-70 gr in a 40 cal.
Then we have picket bullet guns in which the picket often requires more powder than the RB. The best load so far in my 40 cal with a 132 gr picket is 80 gr of FFG Swiss. I would have to go grab one but its about .7" long.

May not be the increased rotational velocity but something is going on.

Dan

It is indeed....another can of worms.  Fortunately there be some well defined principles of physics which takes the necessity of in depth twist rate analysis out of the equation for round balls, so we can look to other causes as we speculate on the subject. ;D Round balls need a little spin...they either have it or don't; case closed. 

The "O" word comes to mind, but that is speculation.............

Round balls and exterior ballistics are as simple as the combination gets, whether it be smooth bores or rifles. I would ask you all to accept that, if for no other reason than you really don't want to read an in depth explanation about gyroscopic stability and so forth. I don't want to get into it anyway, because it really isn't pertinent to PRBs beyond what I've already stated. I will be happy to provide references to the subject if you want, but in the end all you'll be left with is a headache and the same understanding I have.  When you get into conicals it becomes a very complex study.  Just another reason to stay away from those bullets longer than they are wide. ;D

In simple terms:

1)  1:60" twist @ 1000 fps = 200 revolutions/second (rps) or 12,000 rpm
2)  1:48" twist @ 1000 fps = 250 rps (+25% of #1)
3)  1:36" twist @ 1000 fps = 333.3 rps (+33.3% of #2, or +66.7% of #1)
4)  1:60" twist @ 1250 fps = 250 rps

By the by, a child's top stabilizes as a much slower spin rate than 12,000 rpm.  So do round balls, just for very different reasons.  One does not need to over think this....
« Last Edit: May 07, 2009, 03:53:06 PM by Dan »

Offline Dphariss

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Re: Short barrels, fast twist, roundball projectile
« Reply #29 on: May 07, 2009, 08:09:46 PM »
Dan
I guess you could try spinning the kids top in a supersonic wind tunnel to see if the "relative wind" effected its stability.
Why is it that 72 twists are known for needing more powder in 50-54 cal than a 48 or a 66 to shoot with best accuracy? Its not that they can use it without stripping, they seem to NEED it in many cases.

It seems from my reading and experience that a significant number of rifles like quite a bit of powder.
I have read reports of 40 cal RB rifles shooting best with 60-70 grains of powder. Is it a requirement of the individual barrels, how they are keyed to the stock, internal stress or is it initial rotational velocity?
Look at the heavy benchrest RB shooters. Why do many of them use so much powder? Why are they doing this?

How about velocity bleed off? Most RBs are subsonic at 100 yards regardless of initial velocity.
But does the higher initial velocity, giving a higher rotational velocity, (which falls off far more slowly than the linear velocity) make the RBs more stable as they pass through the high drag transonic range?
Is this why faster twists shoot better, usually, with smaller powder charges than slow twists?? Is it the initial rational velocity keeping them more stable at the high drag velocities?
Is this why some bench rest shooters use large charges of powder to shoot 150-200 yards with a PRB?
How does ball size effect the need for more or less twist?
Forsythe tells us that a 69 cal. (14 bore) can use a 8' 8" twist and will shoot at well as he can to 200-250 yards.  But that a 12 twist will only shoot to 150 with the same accuracy. Why is this? If the twist is irrelevant so long as it has "some" then the 12 ft twist should shoot as well as a 8'8" to 200-250 yards. Is Forsythe just pulling our legs?

There has not been much interest in the scientific community, such as  gov't ballistics labs, in studies of spin stabilized RB ballisitics that I know of. In fact, at least until a few years ago, many of these folks did not think a 50-100 Sharps would shoot 1500 yards. So the round ball as a rifle bullet is somewhat unknown aside from Lyman's book which, my copy at least, does not delve into accuracy of the loads at all.

Dan
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Offline Acer Saccharum

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Re: Short barrels, fast twist, roundball projectile
« Reply #30 on: May 07, 2009, 08:36:14 PM »
I talked with Ernie Stallman from Badger barrels some years ago about long range BP cartridge shooting.

He said they developed loads and spins to keep the bullet BELOW the speed of sound, 1200 ft/sec.
The bullet breaking the sound barrier would disturb the accurate flight.

I don't know if this has any effect on patched roundballs, but some of it might translate. If you are above the speed of sound upon leaving the muzzle, you might want to have it stay above the speed of sound until the bullet is done. ie, passed thru it's target at the intended range.

Just thinkin out loud.

T
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Offline Dphariss

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Re: Short barrels, fast twist, roundball projectile
« Reply #31 on: May 07, 2009, 09:04:59 PM »
I talked with Ernie Stallman from Badger barrels some years ago about long range BP cartridge shooting.

He said they developed loads and spins to keep the bullet BELOW the speed of sound, 1200 ft/sec.
The bullet breaking the sound barrier would disturb the accurate flight.

I don't know if this has any effect on patched roundballs, but some of it might translate. If you are above the speed of sound upon leaving the muzzle, you might want to have it stay above the speed of sound until the bullet is done. ie, passed thru it's target at the intended range.

Just thinkin out loud.

T

This depends on how far you are shooting. In BPCR silhouette the velocity is not so important. And the reduced velocity can actually reduce wind drift. But the flatter trajectory is an aid as well and my best silhouette rifle shot a 380 gr at 1425. Using too much elevation tends to move the shooters face off the stock. Trade off..
The launch angle of a low velocity load, 1100 fps vs 1350+ can be a factor at 1000 yards.
But there are numerous schools of thought on this too. All have valid points. The old time LR shooters shot 100-105 grains of powder in a 44-45 caliber with 520-550 gr or heavier PP bullet.
I am not sure everything would translate to the PRB but there are some interesting questions. Like does  the balls rotational centerline travel at an angle to the "relative wind" at longer ranges as the bullets do.
Few RBs will stay supersonic past 90-100 yards but they will shoot with hunting accuracy to 200 or so, at least the 54s and above.. I think having sufficient rotational velocity at the trans-sonic velocities in important and this can be achieved by velocity better than by a fast twist at low velocity. Since the fast twist low velocity greatly raises the trajectory.

Dan
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Offline T*O*F

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Re: Short barrels, fast twist, roundball projectile
« Reply #32 on: May 07, 2009, 09:54:37 PM »
Quote
I am not sure everything would translate to the PRB but there are some interesting questions.

Because of its unique shape, a round ball's ballistic coefficient does not change during its flight.  A long bullet does.  This change determines the bullet's stability and accurate bullets need to be designed for the range at which they will be shot.  A certain bullet may not stabilize for a hundred or more yards and beyond a certain distance it will start to tumble.  Each distance will require a different length bullet.

To make a round ball shoot at longer distances one need only to slow the twist down and add more powder.
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Offline Dan

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Re: Short barrels, fast twist, roundball projectile
« Reply #33 on: May 07, 2009, 10:56:11 PM »
Dan, if there's not a booby trap somewhere in all those questions I'm Chinese! ;D ;D

There is an old saying about ballistics:  "If all else is equal."  Simple concept, but very difficult to put to practical use.  More of a conceptual presentation than anything else actually and it says simply that if things are not the same your derived data is subject to question.  EX: Same gun, different barrel...not the same.  I'm sure you understand that variables are bountiful in this discussion.  With the addition of each variable, the conclusions become less valid for broad application.  The many comments made to this point about the necessity of heavy charges versus light charges needed to make a gun shoot is but one example.

To the issue of research on PRB ballistics. No, there isn't much out there and there probably won't be until the likes of Mr. Pletcher or like minded types set to and do it themselves.  Ballistic research is geared in very much the opposite direction these days.   The technology exists but the motive is absent.

Going back in time to Forsythe's day  there is nothing but the highest regard for his work and that of others in the field. He was not pulling our legs and in fact it amazes me that the pragmatic approach to ballistic research and their intuition revealed so much to them. And there is the IQ thing at work.  Forsythe was fairly brilliant in my opinion. I think his observation about 8-1/2' versus 12' twists are consistent.  There is a practical minimum twist rate for a given caliber PRB. I don't know what it is but would think it caliber influenced to some degree. See my comment below about squares and cubes. On the other hand and this goes to the issue of variables, recall the success had by Mr. Crowley with his slow twist rate .50 caliber. Somewhere between the measures Forsythe commented on if I did my math correctly.

I would say that in very general terms, the exterior ballistics of PRB and conical bullets share but a few things. Gravity and drag are the big ones I'd say. Even though both are stabilized by rotation from rifled bores, there are distinctly different reasons for doing so and I think I touched on the primary issue early in this thread. 

On the part of conical bullets, the CG and CP are not co-located and as a result gyroscopic stability is required to stabilize the bullet.  CG and CP are co-located in round balls, thus there is no prevailing influence of overturning or pitching moments to influence its stability.  Like conicals, PRBs are influenced by the magnus force to the extent that balls with random rotation typical of a smooth bore musket will disperse randomly.  The slow spin of a RB rifle presents to consistent rotation on a single axis and does away with this adverse factor.

Conical bullets are affected by transition through the speed of sound when the CP shifts and a nutation or wobble is induced as it tries to stabilize from the influence of this upset.  From my understanding of the circumstances, not all bullets experience this but in broad terms the CP shift is forward, or destabilizing.  Since round balls do not have dislocated CG/CP, I have no reason to think this influences their flight path. I've certainly never seen such representation made in writing. Many folks seem to think the transition through the speed of sound is turbulent.  It isn't.  Regardless of twist rate or velocity from a PRB rifle, the spin rate is very low.  I doubt it has anything to do with stability in the transonic range, either positive or negative.  I say this simply because the reasons for spinning a round ball are different than for a conical and PRBs do not carry the same baggage in an aerodynamic sense.  Gyroscopic stability in conical bullets presents a lot of penalties for the benefits gained...it is not a free lunch.  If you peruse the Nennstiel web site linked below you'll begin to see that if you don't already.

Comment made about the ball orienting itself to the flight path:  I don't think it would and such absence would, to very small degree, increase the influence of the magnus force. Understand that the magnus force is very small and not worthy of deep consideration in this discussion. It is present however.
http://www.nennstiel-ruprecht.de/bullfly/magnusf.htm#header
To the question, the act of a bullet's spin axis conforming to the flight path is known as the tractability condition.  It occurs because the bullet is 1) gyroscopically stabilized and 2) is pitched up relative to the flow field.  This results in nutations of small order and if properly stabilized the bullet will precess to a new pitch axis which reduces the upsetting forces.  Again, round balls have co-located CG/CP and there is no upsetting force which might cause the precession. Even if the ball is slightly oblate in shape due to obturation, there is no significant displacement of CP to promote the tractability condition.
http://www.nennstiel-ruprecht.de/bullfly/tractf.htm#header


Random thoughts on variables which may have influence on the observations made by many of you.

1) Obturation is a variable influenced by charge, alloy and probably another half dozen things to small degree.  A ball increases cross sectional area by the radius squared as caliber increases. It increases mass by the radius cubed. Raise the caliber, and you raise the BC, SD and a few other things.

2) Different rifles, each with a family tree of variables.  Barrel pinning, breech, caliber, lock, patch...it's a long list I think. Each is its own animal.


I dunno that covered all the questions, but it's margarita time. ;)

Daryl

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Re: Short barrels, fast twist, roundball projectile
« Reply #34 on: May 08, 2009, 03:02:09 AM »
From a short barrel, fast twist, this thread took some turns. Certianly not pointing fingers as I'm the worse thread-drifter here. ;D

roundball

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Re: Short barrels, fast twist, roundball projectile
« Reply #35 on: May 08, 2009, 05:34:18 AM »
It seems to me that the 1:48" twist is the ideal round ball twist for the .40cal...and IMO the .45cal is not all that much different than a .40 so it should follow that the .45 would do well with a 1:48".

I've owned / used multiple T/C standard .45cal x 1:48" barrels over the years, all with max loads and never had anything but great accuracy out of them...same thing in T/Cs .50/.54cal x 1:48" barrels.
I always used snug PRB combinations requiring a short starter and never had any strange fliers or indications that rifling was stripped.

My typical hunting loads:
90grns Goex 3F'
Oxyoke OP wad
.018" pillow ticking
Hornady .440/.490/.530 balls

Typical 100 yard benched groups ranged from 1+7/8" to 2+3/4", iron sights and tired eyes...personally, unless someone is heavy into competition I think 1:48" twists are fine for .45/.50/.54cals

Offline Dphariss

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Re: Short barrels, fast twist, roundball projectile
« Reply #36 on: May 08, 2009, 07:13:24 AM »
I like 48" in 50 cal and would not worry about one in a 54.
But there are a host of variables.
How tight is the fit, what is the land groove ratio, what is the groove depth.
I had a 48" twist 50 cal douglas that was probably the best shooting RB rifle I ever owned with 1/2 ball weight of powder.

As I tried to point out in my question post there are a host of variables.
The Europeans who developed the short barrel 1 turn in a barrel length idea were right. It shoots great from all accounts. BUT ITS NOT A HUNTING RIFLE past 30-50 yards.
The slower twist will shoot as well and will provide a flat trajectory or in the case of Forsythe a flat trajectory and enough drive to penetrate large game.
While 5 drams of powder in a slow twist 14 ga  barrel 26" long would drive an hard ball through an Indian Elephants head a charge of 50-60 grains as was often used in the fast twist rifles would not.
If you could HIT a deer at unknown ranges with it it would kill. But its still a PITA to use and would be limited to short ranges due to trajectory concerns..
Dan
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Offline Dan

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Re: Short barrels, fast twist, roundball projectile
« Reply #37 on: May 08, 2009, 02:29:45 PM »
From a short barrel, fast twist, this thread took some turns. Certianly not pointing fingers as I'm the worse thread-drifter here. ;D

Well, I'm a frequent offender too. ;D  I thought myself well behaved on this discussion though.  My bottom line for PRB guns remains enjoyment.  They are fun and I enjoy the simplicity.  I save all my anal retentive behavior for the other side of the fence with white powder and conicals...mostly.

erdillonjr

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Re: Short barrels, fast twist, roundball projectile
« Reply #38 on: May 09, 2009, 07:28:29 PM »
I prefer a 1/66 teist on a 45 it is much less forgiving as to what it likes to eat