Round Ball Behavior ??

[roundball]

I need the expertise of the Forum brain trust to explain round ball flight characteristics.

A recent situation has occurred which I don't understand:

A .490 ball gives almost minute-of-angle accuracy to and through a 75yd target, but then when fired at 100yds, the same ball becomes a wild knuckle ball in the 24yds between 76 and 100.

Yet a .495 ball is fine all the way out to at least 125yds.

What is the explanation for the difference in the above round ball behavior?

 

[Legionair]

I'm going to go out on a limb here (not being a mathematician or physicist), and say friction.

It would seem to me that the spinning ball creates a certain amount of friction against the air as it flies. Over the distance, this friction builds up until it begins to alter the balls flight. A larger or smaller ball will generate different amounts of friction before altering its flight.

Also add temperature, humidity, wind, patch thickness variation, grease volume, powder volume fluctuations and the cumulative effect could be great.

Of course this is just me thinking out loud and could be a load of horses$*t. (Not intentional, though).

Anybody smarter than me want to chime in??

Legion

 

[Hamourkiller]

This is me thinking out loud.

Super sonic Pistol cartridges can show the same behaviour as the round drops below the speed of sound. The bullet is buffeted by riding the shock wave. If my theory is correct (your round ball is falling under the speed of sound around 100yds or so) that would explain why the larger thus heavier ball is getting to 125 yds before going sub sonic.

Hope this helps and good luck with the problem.

 

[Zonie]

Durned if Ah knows.
I ran a quickie Ballistics calc for both the .490 and .495 balls at 1600 FPS muzzle velocity and a 5 MPH crosswind.

It is true that at that velocity, at about 40 yards, the velocity drops to a subsonic value but the difference between the .490 and the .495 diameter balls is only about 2 FPS.

The numbers my freebie program gave, the .490 ball has these velocities and drift:
0=1600 fps, 0.0 inch
40=1210 fps, 1.1 inch
50=1141 fps, 1.3 inch
60=1090 fps, 1.7 inch
100=952 fps, 6.2 inch

Do you suppose the .495 diameter ball holds on to it's Bore Butter better?? :grin: :grin: :rotf:

 

[spitfire]

Thy patch thickness may come into play with the respected RB diameters to bore maybe? :hmm:

 

[Protrucker]

I'm going to go out on a limb here (not being a mathematician or physicist), and say friction.

No offense, but I think that you're all wet with the friction idea. The larger ball has greater surface area which would add to the friction created. If friction was the culprit you would think that the ball with more surface area/friction would be the one that lost accuracy.

But then again......I may be the one that's all wet! :haha:

 

[musketman]

I need the expertise of the Forum brain trust to explain round ball flight characteristics.

A recent situation has occurred which I don't understand:

A .490 ball gives almost minute-of-angle accuracy to and through a 75yd target, but then when fired at 100yds, the same ball becomes a wild knuckle ball in the 24yds between 76 and 100.

Yet a .495 ball is fine all the way out to at least 125yds.

What is the explanation for the difference in the above round ball behavior?

Are you using the same patch thickness for both sizes of roundball?

If so, this is one variable, the .495 will have a tighter patch fit and seal the barrel better, thus giving a stronger hold on the rifling with less slippage, resulting in greater stability...

The larger ball will be less prone to wind deviation than a smaller diameter ball, doesn't seem like much, but with distance the discrepancies between the two are more noticeable...

 

[Legionair]

ALL WET?!? :cursing: (kidding)

That's o.k. I been all wet lots of times. :haha:


It seems that there may be to much variance from shot to shot. With lube, patches, cleaness of barrel, ball itself. :hmm:

Still sounds like a "SPITBALL" to me... :grin:

Legion

 

[BS]

I need the expertise of the Forum brain trust to explain round ball flight characteristics.

A recent situation has occurred which I don't understand:

A .490 ball gives almost minute-of-angle accuracy to and through a 75yd target, but then when fired at 100yds, the same ball becomes a wild knuckle ball in the 24yds between 76 and 100.

Yet a .495 ball is fine all the way out to at least 125yds.

What is the explanation for the difference in the above round ball behavior?

Did you shoot the 490s and the 495s on the same day?

Cause wind will affect the RB in strange ways. :shocked2:

 

[mmahlock]

Being a member of the military weapons industry, I believe that the stability of a round ball is based on three things.

1) Mass of the ball
2) Leading edge shape
3) Crosssectional area
4) Forward velocity
5) Rotational speed

There may be some other factors but these are the most relevent that I can remember sitting here.

I would conjecture that your problem with the .490 ball is one in which, given the fixed nature of items 1-3, a combination of 4&5 at that range results in the ball becoming unstable. The .495 ball factors 1-3 combined with 4&5 appear to result in a stable ball out to 125 yds possibly more.

To fix the problem with the .490 ball either its forward velocity or rotaional speed must be increased. Since th rotational speed is a function of the rifling, I'd say you need to up the powder charge a tad (this is a highly sophisticated technical unit of measure) until the group closes up.

Do this off a bench with the rifle as solidly fixed as possible to eliminate any other accuracy factors.

But then I could be all wet. :shocked2:

 

[roundball]

Actually, I'm personally having fine accuracy with .490's, always have...I'm just trying to understand how/why this would be as it's been experienced by another shooter.

 

[BS]

You got me thinking about stability! I have been shooting paper patched balls with my .73 cal. I can shoot under 1 1/2 at fifty yards. My best is 5/8s for a 3 shot group...............now move out to 100 and all kinds of .....stuff ....happens.

On a windy day i will get a nice horizonal string of shots....as expected, then a high rigt, then a low left, this opens the group up to over 8"s.

I wonder if my huge sprue [Lyman mold] is catching the wind? I am going to load some sprue down and file some others off, trying to blend them smooth.

Any Ideas?

 

[Wehrmacht]

I don't know,either 'cause I'm just a hard working ole country boy,but I'm going to hang with Hamourkiller on this one. I think it is tied to the ball dropping below the speed of sound.

Objects crossing above or dropping below the sound barrier do weird and mysterious things!Remember, when Chuck Yaeger first broke the sound barrier he said the same thing the billy goat said when it ran into the hornet's nest :haha:.

 

[jaybeach]

My best guess would be that it has to do with the rotation of the ball. Like a nice spiral football pass that looks real good for a while, then starts to wobble the further it goes.
I suspect that this is caused by the combination of slowing down, and the amount of spin that was put on the ball at the start.
In the case of the roundball, the smaller ball may not be getting the full benefit of the rifling - or at least as much as the .495 ball is.
Perhaps a slightly thicker patch for the smaller ball would help more than a higher charge.. but maybe more of both . . . Maybe.

IM jaybe :thumbsup:

 

[guncobbler]

It would be interesting to compare the velocity of a 490 and 495 ball. Maybe the 495 has more velocity because of the greater intial friction creating a hotter poweder burn. Does anyone have a chronograph to do that test?

Another thought is that your lot of 490 balls might be less consistanct than the 495 lot and that the diferrence is just beginning to show at those ranges. Are you using swaged or cast balls? Have you tried a different lot? GC

 

[JOHN L. HINNANT]

Greetings All,

Roundball, I am NOT a ballistics engineer, so I cannot explain with precise detail and facts why the problem you outline happens, but I will tell you my thoughts on the subject. After that all of you gentlemen can critique my post, and maybe collectively, we can figure this out.

Through experience, I am of the opinion that the .490 ball is too small to be properly gripped by the rifling and is on the thin edge of stability.

Contrary to popular belief, the greased patch does not prevent the ball from being engraved into the rifling. Simple mathematics will tell you that. In 50 caliber for example:

Bore groove diameter is ........... .500"

If groove depth is .006", then
land to land diameter is........... .488"

Therefore it stands to follow that .001" of each side of the ball is engraved into the rifling. Even with .016" to .020 " thick patch material, the ball does not have enough grip on the rifling to properly stablize.

At 25 yards, the instability is not enough to be seen on the target. There is probably enough irregularity in shot placement on the target at 50 and 75 yards to see a breaking down of accuracy if you know what you are looking for. The groups may be small (depending on the individual's preconceived accuracy standards), but not too well formed.

Proportionally, I would hazard to guess the difference in group size between 50 and 75 yards is much greater than between 25 to 50 yards.

Exactly where the train comes of the tracks after 75 yards could be an interesting shooting experiment. Targets could be set up at 80, 85. 90, and 95 yards.

On top of the suspected instability, add a varying mirage factor caused by even mild wind or wind eddies, and suddenly the ball just gives up. Poor thing it never had a chance.

As you know, Mossie had this exact same problem and improved his accuracy by changing to .495 size ball. His rifle barrel has .010" deep rifling grooves with a land to land diameter of .480".

This would mean that .0075" of each side of the ball is being engraved into the rifling. The ball along with the patch is better gripped by the rifling, thus increasing the external ballistic stability of the ball.

As you know, the ball is not a good projectile design, and not much of anything is required to disturb its flight path.

Increasing patch thickness to compensate for a too small diameter ball does not solve the accuracy problem.

There are some more points to make,but my two finger typing method has reached its limit for now. I will revisit this subject later.

Best regards and good shooting,

John L. Hinnant

If you are not an NRA Member, why not? I an carrying you load.

 

[Slamfire]

Bore diameter would be .500, per definition. Then the rifling grooves are deeper by whatever amount. In supository barrels you can see this in the English and U. S. nomenclature. A .256 Rigby is actually a 6.5mm or .264 in bullet diameter.

 

[JOHN L. HINNANT]

Greetings Slamfire,

Just spent the last hour out in the shop slugging and miking four different new/old never used barrels. two from the same maker.

None of the four complied with what either you or I stated.

Very, very odd indeed. I have to think this one over. Will re-slug and mike again in the morning, when I am more rested and maybe have a more clear mind.

However, let us use your defination for this discussion. That would only make the .490 size ball an even more loose fit in the bore.

Your information is indeed appreciated.

Best regards and good shooting.

John L. Hinnant

If you are not an NRA Member, why not? I am carrying your load.

 

[JOHN L. HINNANT]

Greetings Slamfire and All,

If the forum will allow me a bit of latitude, I will respond to Slamfire's reference to the 256/6.5MM/264 centerfire bore and the relation to my posting about the 50 caliber bore size/rifling groove diameter.

Slamfire, you are correct about the the 256 Rigby being 6.5mm caliber and shooting a .264 inch diameter bullet. This is true of any 6.5 mm caliber: they all shoot bullets with a .264 inch diameter.

A similar catridge is the the 270 Winchester cartridge. This round shoots a.277 diameter bullet through a .270 bore with a rifling groove diameter of .277 inch.

Two other examples are the 30-06 and 308 Winchester. Each one shoots .308 inch diameter bullet through a .300 inch diameter bore with a rifling groove diameter of .308 inch diameter.

In the case of the 256 Rigby cartridge, it does indeed shoot a .264 diameter. But the bore size (top of land to top of land diameter) is .256 inch diameter with a .264 inch diameter rifling groove diameter.

For those members who wish to check the validity and/or accuracy of the above, I refer you to the late Roy F. Dunlap's monumental classic, GUNSMITHING, pages 193, 197, 199, 201, 202, 203, and 204.

When one thinks about the above dimensions, it becomes quite clear that upon discharge of the cartridge, the top of the rifling lands are IMPRESSED INTO THE BULLET, while the bullet diameter fills out into the bottom of the rifling grooves.

With all of this in mind, perhaps my original staement with given dimensions will make more senses. And I do stand by those dimensions within a certain paremeter or standard.

These dimensions work only if a standard of .500 inch is used for the rifling groove diameter. If deeper grooves are desired AND the .500 inch groove diameter is maintained, then it stands to reason that the land-to-land diameter is DECREASED. I do believe there is a point at which the land-to-land diameter should not be decreased. What that is that is I do do not have the technical knowledge to determine. BUT, I once asked a custom barrel maker if he could make a barrel that was custom bored for a .495 ball. He did and it is super accurate with the .495 ball, but definately does not like the .500 ball. When I slugged the bore, the rifling groove diameter was less than the .500 standard, but had .011 inch deep rifling. It is my only 50 caliber rifle that shoots a .490 ball with good accuracy. Good enough to use it for hunting.

Again, this means more of the soft lead ball diameter is impressed into the rifling grooves. Over the years, only pure soft lead has proven to forgiving enough in this respect. A harder lead alloy will not do this or at least to the point to give us the same level of accuracy as pure lead.

Now if the .500 inch diameter rifling standard is not maintained, then deeper rifling can be cut without decreasing the land-to-land diameter to a point that is deterimental to accuracy or safety.

Of course, increasing the groove depth by increasing the the overall groove diameter usually means a thicker patch is required. This is not always the best way to obtain best accuracy.

In my experience, as well as some of my friends, barrels with grooves wider than the lands produce top accuracy with bore size balls. Example .500 diameter balls in 50 caliber barrel.

Barrels with this type of rifling would enclude the Green Mountain, the older Hoppy Hopkins H&H, and the Tru-Bore.

While I do not know for sure, I do highly suspect the wide groove rifling is a way to shoot a tight patch/ball combination and maintain the .500 inch rifling groove diameter standard, and at the same time minimize deformation of the soft lead ball while loading it.

Well.., with all of that said, it is just another way of repeating my earlier post.

And now Roundball, I am not sure if this is any part of an answer to your question.

Best regards and good shooting,

John L. Hinnant

If you are not an NRA Member, why not? I am carrying your load.

 

[Big John]

I don't understand why everyone is concerned with your behavior Roundball.