Round balls and obturation

[flatcreek]

What do you think this tells us?

That tells me the ball is expanding into the rifling when fired, it also expanded when it hit the target.

 

[Patched]

That tells me the ball is expanding into the rifling when fired, it also expanded when it hit the target.

Looks a little like evidence of obturation upon firing…fatter at the “equator” and closer together at the “poles” no matter how slight, is obturation.

 

[flatcreek]

Looks a little like evidence of obturation upon firing…fatter at the “equator” and closer together at the “poles” no matter how slight, is obturation.

It can only expand the depth of the rifling .008 .010. I agree.

 

[Freedom]

That tells me the ball is expanding into the rifling when fired, it also expanded when it hit the target.

Thank you flatcreek.

The ball was shot into a big fallen aspen tree. There was a woodpecker hole on the back side. The ball was laying in the hole.

 

[45man]

The best way to catch balls and bullets is either snow or a plastic 55 gal drum full of rubber mulch. shoot into one end.

 

[PUK]

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 Lyman black powder handbook has photos of round balls in flight which clearly shows obturation.

 

[Patched]

The Lyman black powder handbook has photos of round balls in flight which clearly shows obturation.

I saw that too…
My thinking is, if any soft pure lead projectile obturates in the bore with the force of the powder burn, then they all do regardless of the shape of the base…the walls of the barrel restrict it but the softer patch material must compress slightly, tightening the seal, providing the needed expansion room for the obturation…

 

[Doug Fay]

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

Being an old engineer and physics enthusiast, I LOVE those kinds of questions! I'm intuitively convinced the ball still obturates, because there is still a lot of force pushing the ball forward, and just as much pushing the ball back, due to the inertia of the ball, thus making the ball fatter. I suspect the sideways pressure squeezing the ball thinner to counter the obturation (never heard that word before!) is minor compared to the forces trying to squeeze it "fatter". But like I said, that's just my intuition. I'll think about it some more and see if I can come up with some sort of formula that better describes what's going on.

 

[Barry Strickland]

Try this as a simple experiment. Take a lubed swatch of your patching material and pop the ball into the muzzle with your peg starter. Now, instead of
cutting off the material pull the ball back out of the bore. Notice the patch material has made a pattern around the ball at the equator and the lands
of the rifling.
The lead ball had to deform into the patch and rifling to make this indention pattern. This is just from the pressure of your hand pushing the ball
into the bore. What happens when thousands of pounds of pressure is exerted on the ball?
If you are using a combination loose enough to thumb start the ball I doubt you would see the pattern of the patch material. That is fine for
hunting where a fast reload is required.
For serious target competition a tight combination is commonly used.
Why do we insist on using dead soft lead for the balls instead of hard lead? I think the answer is obvious.
Barry

 

[stephenprops1]

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.

You must have a lot of free time on your hands. LOL

 

[longcruise]

Also, try loading a loose patch and ball that wouldn't be tight enough to swage lands upon loading. Shoot it and check for rifling.

 

[45man]

The question I have is whan a loose patch is used, why do holes get burned in it if the ball sealed? Seems to me that gas has escaped around the ball.