Round balls and obturation

[longcruise]

I'm in the mood to agitate so here goes.

We probably all have read about and understand the concept of bullets "obturating" the bore. It's a pretty simple concept. The charge ignites and the weight of the bullet column resists movement and consequently the pressure from the charge causes the bullet to collapse on itself and press against the sides of the barrel. So the expansion of the projectile obturates (plugs) the bore and holds (hopefully) the expanding gasses behind the bullet.

But then we have the round ball. We often speak of the ball obturating the bore just as a conical bullet does. I've always wondered about this and I've made a diagram of why I think the round ball may not expand and obturate the bore.

What I'm trying to illustrate here is the distribution of the pressure on the base (bottom half) of the ball. Since the bottom half is a half round shape, the pressures are not applied in direct opposition to the weight or inertia of of the whole ball. The pressure applied on the semi circle is actually opposing the collapse of the ball and keeping it round.

So, that's just what I'm thinking on this.

 

[cynthialee]

well the rifling marks I find on recovered roundballs tells the tale that the ball obturated

 

[SDSmlf]

What I'm trying to illustrate here is the distribution of the pressure on the base (bottom half) of the ball. Since the bottom half is a half round shape, the pressures are not applied in direct opposition to the weight or inertia of of the whole ball. The pressure applied on the semi circle is actually opposing the collapse of the ball and keeping it round.

So your diagram is of a roundball in a bore in a perfect vacuum, correct? You show no force on the opposite side from any atmosphere.

 

[FishDFly]

well the rifling marks I find on recovered roundballs tells the tale that the ball obturated

Nice to see you are still here. Have been wondering how you are.

 

[longcruise]

well the rifling marks I find on recovered roundballs tells the tale that the ball obturated

In a tightly patched ball, I think you can pull one and see the same rifling marks!

So your diagram is of a roundball in a bore in a perfect vacuum, correct? You show no force on the opposite side from any atmosphere.

No, it's just meant to be a simple diagram that helps illustrate what I'm thinking of the direction of force applied to the ball.

Thinking about your atmosphere reference. If we are going to apply atmosphere it would seem that it will possibly apply some force to the forward surface of the ball it would also be applied circularly would it not.

I just don't see the concepts of vacuum or atmosphere changing anything.

 

[The Crisco Kid]

You're in Colorado where there should be some snow banks to shoot into. Shoot into them and recover the balls in the spring. It's very interesting what you can learn from this since the snow has no perceivable effect on balls or bullets. They don't travel through the snow very far either. When I lived in Alaska we built an 8x8x4 foot box that we blew full of snow and shot into it all winter. Besides recovering all our lead we learned a lot. Problem is it can be a long time till spring but we got to sit around the wood stove and ponder questions like this all winter.

 

[Rock Home Isle]

I'm in the mood to agitate so here goes.

We probably all have read about and understand the concept of bullets "obturating" the bore. It's a pretty simple concept. The charge ignites and the weight of the bullet column resists movement and consequently the pressure from the charge causes the bullet to collapse on itself and press against the sides of the barrel. So the expansion of the projectile obturates (plugs) the bore and holds (hopefully) the expanding gasses behind the bullet.

But then we have the round ball. We often speak of the ball obturating the bore just as a conical bullet does. I've always wondered about this and I've made a diagram of why I think the round ball may not expand and obturate the bore.

What I'm trying to illustrate here is the distribution of the pressure on the base (bottom half) of the ball. Since the bottom half is a half round shape, the pressures are not applied in direct opposition to the weight or inertia of of the whole ball. The pressure applied on the semi circle is actually opposing the collapse of the ball and keeping it round.

So, that's just what I'm thinking on this.

View attachment 372025

The energy vectors in the diagram are not correct. The energy vectors will run parallel to the length of the bore. This is proven by the fact that in a slow motion video gases can be seen escaping past the projectile and exiting the bore, prior to the release of the projectile.

Obturation occurs due to differences in acceleration, within the projectile. In the instant of ignition pressure causes the rear of the projectile to instantaneously accelerate, while the front of the projectile hasn’t begun much forward movement. Essentially the projectile is squeezed, and flows sideways into the rifling.

 

[longcruise]

Problem is it can be a long time till spring but we got to sit around the wood stove and ponder questions like this all winter.

What did you all decide once the snow melted?

The energy vectors in the diagram are not correct. The energy vectors will run parallel to the length of the bore. This is proven by the fact that in a slow motion video gases can be seen escaping past the projectile and exiting the bore, prior to the release of the projectile.

Obturation occurs due to differences in acceleration, within the projectile. In the instant of ignition pressure causes the rear of the projectile to instantaneously accelerate, while the front of the projectile hasn’t begun much forward movement. Essentially the projectile is squeezed, and flows sideways into the rifling.

I think that's a good description of how an cylindrical projectile behaves but I'm still sticking with my concept of pressure surrounding the lower half of the ball. I agree that the forces pushing the ball are applied parallel to the walls of the bore but upon meeting the round surface they will be applying pressure toward the center of the ball.

I agree on the slow mo vids of the gas exiting the muzzle ahead of the ball but wouldn't that argue in favor of my concept of the ball not obturating the bore??

 

[The Crisco Kid]

Problem is it can be a long time till spring but we got to sit around the wood stove and ponder questions like this all winter.

What did you all decide once the snow melted?

I didn't shoot any round balls into the snow so I don't have anything for the OP's question. What I shot was mostly black powder cartridge loads. We did learn that 6 groove barrels shot better than 4 groove in some calibers since the long nosed bore riding bullets would tip into a groove and yaw. It showed up on the recovered bullets. It would be interesting to see if soft patched round balls would show more obturation than a ball that had been pulled.

 

[cynthialee]

Nice to see you are still here. Have been wondering how you are.

I am around most days, even reply to a post here or there. But I am not as involved as I was when I was new to long guns and casting.
Those old casting threads really brought my casting game up to par.

 

[cynthialee]

In a tightly patched ball, I think you can pull one and see the same rifling marks!

No, it's just meant to be a simple diagram that helps illustrate what I'm thinking of the direction of force applied to the ball.

Thinking about your atmosphere reference. If we are going to apply atmosphere it would seem that it will possibly apply some force to the forward surface of the ball it would also be applied circularly would it not.

I just don't see the concepts of vacuum or atmosphere changing anything.

the act of pulling the ball is not a fair measure as the act of screwing in the ball puller will expand the ball
also I tend to use a bit looser patch/ball combo than most of y'all do, due to my strength limitations and they still show obvious signs of obturation

 

[SDSmlf]

I agree that the forces pushing the ball are applied parallel to the walls of the bore but upon meeting the round surface they will be applying pressure toward the center of the ball.

Say you create a conical with one end round or spherical, and the other end flat. Two shots are taken with the same powder charge. First shot has the flat end to the powder, second shot has the round or spherical end towards the powder. Which conical will leave the bore at a higher velocity? Or will they both have the same velocity? And remember, the conical with the round end towards the powder will have pressure from the powder being applied towards the center of the of the round or spherical end, not parallel to the bore (nor adding to velocity increasing).

 

[SDSmlf]

I agree on the slow mo vids of the gas exiting the muzzle ahead of the ball but wouldn't that argue in favor of my concept of the ball not obturating the bore??

I’ve seen video of the gas exiting ahead of conicals. Does that mean conicals are not obturating?

 

[Rock Home Isle]

What did you all decide once the snow melted?


I think that's a good description of how an cylindrical projectile behaves but I'm still sticking with my concept of pressure surrounding the lower half of the ball. I agree that the forces pushing the ball are applied parallel to the walls of the bore but upon meeting the round surface they will be applying pressure toward the center of the ball.

I agree on the slow mo vids of the gas exiting the muzzle ahead of the ball but wouldn't that argue in favor of my concept of the ball not obturating the bore??

Well…then look at the vectors as you’ve placed them…what you are saying is that these forces, along the sides of the projectile, are not pushing the projectile out the bore, but are pushing the projectile into the opposite side of the bore.

Forces don’t don’t arbitrarily change direction. The force of the ignition of the powder will be directed out the muzzle, the vectors of energy move parallel to the bore. There is no outside acting force that will cause the energy to change direction, from parallel to the bore, to start angling into the projectile towards the center.

 

[morehops52]

Well…then look at the vectors as you’ve placed them…what you are saying is that these forces, along the sides of the projectile, are not pushing the projectile out the bore, but are pushing the projectile into the opposite side of the bore.

Forces don’t don’t arbitrarily change direction. The force of the ignition of the powder will be directed out the muzzle, the vectors of energy move parallel to the bore. There is no outside acting force that will cause the energy to change direction, from parallel to the bore, to start angling into the projectile towards the center.

I agree. The arrows should all be parallel to the bore pointing toward the muzzle. As far as obturation goes it's not just the ball but the combination of the ball and patch. For example a .495 ball with a .10 patch hasn't as much room to expand as a .480 / .25 combination. Aside from this discussion on something relatively meaningless (IMO) the deformation we are most concerned about is after it penetrates a deer hide. Happy New Year!

 

[Frontier's]

Out of all the tens of thousands of round balls, loosely patched and horribly tight patched, not a single one ever had rifling marks on it

 

[Frontier's]

I’ve seen video of the gas exiting ahead of conicals. Does that mean conicals are not obturating?

Depending on the conical and rifling depth, that could very well be. Conical like a maxi ball or a REAL bullet, would have a much more difficult time filling the grooves of barrel that's rifled deep. That's typically where a wad will come into play.

 

[JeeeperBillMI]

I'm in the mood to agitate so here goes.

We probably all have read about and understand the concept of bullets "obturating" the bore. It's a pretty simple concept. The charge ignites and the weight of the bullet column resists movement and consequently the pressure from the charge causes the bullet to collapse on itself and press against the sides of the barrel. So the expansion of the projectile obturates (plugs) the bore and holds (hopefully) the expanding gasses behind the bullet.

But then we have the round ball. We often speak of the ball obturating the bore just as a conical bullet does. I've always wondered about this and I've made a diagram of why I think the round ball may not expand and obturate the bore.

What I'm trying to illustrate here is the distribution of the pressure on the base (bottom half) of the ball. Since the bottom half is a half round shape, the pressures are not applied in direct opposition to the weight or inertia of of the whole ball. The pressure applied on the semi circle is actually opposing the collapse of the ball and keeping it round.

So, that's just what I'm thinking on this.

View attachment 372025

I believe the force diagram is missing a very important opposing force. That would be the force of friction between the patched ball and the side wall. If that ball is really deforming during ignition of the black powder it may even change the force of friction as it travels down the bore.

 

[#BPBReloadingFFL]

The patch is the gas seal on a round ball.

 

[Ditt44]

... and what about a patch that has had the ball move off to one side or the other in loading so that it is not equally spaced/fit around the ball? Or an over-bore patch, like me using a .50/.54 .018 patch in my .45 Seneca because I get the best groups that way?

How much, if any influence is there from patch size if it overlaps the front/muzzle facing of the ball? Does that not add resistance which could slow one side of a ball and accelerate the other?

I would honestly think the issue would be most evident in loads where the patch was too thin and potentially proven by the fact that balls fired with too thin a patch display one or more characteristics such as: torn patches, burned out patches, loose groups.... ?

Edit: JeeperBill sort of started that point before i could post ie: friction