Obturation of a patched round ball...real or imagined?

[Billnpatti]

I read in many of the posts on this and other forums about how a PRB obturates in the bore due to the pressure of the burning gasses. I know a hollow based bullet with a skirt, such as a mini ball, does, in fact, obturate in the bore. It is specifically designed to do so. However, I have seen no actual studies done to determine if there is any real obturation of a PRB. I know that PRBs have been recovered from game and show signs of flattening but that is to be expected due to it striking the game or to have been hammered into the bore upon loading as in the case of a very tight fitting patch/ball combination. I would love to see an actual scientific study done to see if there is any sign of obturation of a PRB inside the bore of a rifle due solely to the effect of the burning gasses and sudden acceleration when the gun is fired. :idunno: My present inclination is to believe that it is not true. :shake: But I have been wrong before. So, if you prove me wrong, you ain't gettin' a virgin. :haha: :haha:

 

[mooman76]

I have heard bot that it does or can and have also heard they it is not possible. I'm not sure it can be either proven or disproven because of load methods. Who is to say the ball was or wasn't deformed due to the amount of pressure someone used to load the ball vs shooting pressure.

 

[Rifleman1776]

Yer on a roll today, B&P. :wink:
You said the word

obturate

. :shocked2: Now yer really :stir:

I have no opinion on this. Was originally taught by mentors (some of the greatest shooters ever at Friendship) that, yes, the ball does obturate.
But, over the years I have never seen any evidence that it actually happens.
My 'gut' believes it happens but, really, I'm not sure. And I rarely doubt wat my gut tells me. :hmm:

 

[Vaino]

A RB obturates the bore because of the patch thickness...the amount of obturation depends on how completely the bore is sealed due to a sufficiently sized RB/patch thickness combination.

As far as the RB compressing and thereby increasing its diameter and adding to the total bore seal...don't think there's enough resistance to compress the RB even when caused by inertia which is evident because of the pressure "spike" and the resistance becomes a lot less as the PRB proceeds down the bbl.

Minie ball skirts are flared out to seal the bore and do this when inertia causes a pressure spike which is the highest pressure that will be evident during the Minie ball's trip down the bbl.

The TC Maxiball is supposed to obturate the bore more tightly because of the inertial pressure spike causing the smaller dias in between the bore dias to compress thereby increasing the dias of the larger flats that ride in the bore. TC says this happens and maybe it does or doesn't.

Black powder w/ its much lower pressures isn't capable of compressing a lead RB from a practical standpoint in my opinion....even at the inertial pressure spike......Fred

 

[coloradoclyde]

Pure lead will obturate at black powder pressures, but if your using wheel weights it may likely will not.

Heavy handed loading of a soft ball can cause some obturation.

what does a ball look like when you pull it?

 

[azmntman]

it has a hole started in it

 

[George]

Here's a picture from the first Lyman Black Powder Handbook which you might find interesting. It is of two .560 lead balls fired over 120 grains of Gearhart-Owen's 2F powder.



Spence

 

[Vaino]

When a RB is pulled the outside dia is larger and this is due to the screw displacing lead. Don't know what this has to do w/ bore obturation.

From a practical standpoint, don't think the minor compression, if there in fact is any, of a RB even w/ a proper patch thickness would cause more obturation because the RB isn't responsible for the obturation....the patch is....Fred

 

[coloradoclyde]

This is a good read. chapter 7
http://www.lasc.us/Fryxell_Book_Contents.htm

This is the one I should have posted.
http://www.lasc.us/FryxellCBAlloyObturation.htm

 

[coloradoclyde]

When a RB is pulled the outside dia is larger and this is due to the screw displacing lead

Good point Fred.

 

[Grumpa]

If I'm interpreting this picture right (?) it looks like two balls that were fired over the same charge of powder...in which case the obturation/flattening observed would seem to have been caused by the lower ball having to overcome the inertia of the upper ball.

This would show that there is enough pressure with BP to cause obturation, but the circumstance of the double ball would add a variable not present in a normal load.

Whatever, it is a cool picture.

The opposing flats might also explain why two balls loaded in this fashion strike at somewhat different spots on the target.

 

[coloradoclyde]

I have recovered many round balls with flat equatorial bands on them ...proof of obturation.
If you look closely at them they will be imprinted with the patch weave pattern and the rifling.

 

[Grumpa]

Spence's picture shows evidence of the band you speak of.

These posts get me thinking about things I might not otherwise consider. :hmm:

If you load a smoothbore with a lead ball that just barely clears the bore wall, will the ball obturate to fill the bore, and give you greater accuracy? Just wondering...I have no experience with smoothbores.

 

[Vaino]

W/ a tightly patched RB, an imprint from the patch is sometimes present before shooting.

Shooting 2 balls w/ a heavy powder charge and also references to cast lead bullets shot in CFs aren't pertinent to the topic.

The amount if any that a RB increases its dia due to pressure induced compression, is from a practical standpoint, a minor factor in bore obturation.....the patch does the work of sealing the bore.

That's why it's important to use a sufficient patch thickness that will seal the grooves....RB dia w/ even a minimally sized patch thickness will always seal the lands, but that's not where gas leakage will occur.

Just a guess...many who shoot MLers don't use sufficient patch thickness to seal the grooves....perhaps hard loading is one reason?

So in essence, even if a RB increased it's dia slightly, this increase would bear on the lands which it normally would do anyways, but the possible gas escape routes are the grooves.

Nice discussion although an academic one.....Fred

 

[roundball]

Imagined.

 

[Rifleman1776]

I have recovered many round balls with flat equatorial bands on them ...proof of obturation.
If you look closely at them they will be imprinted with the patch weave pattern and the rifling.

That happens on loading, not shooting.

 

[roundball]

That happens on loading, not shooting.

Exactly.

 

[coloradoclyde]

Well I don't shoot a tight combination, but we can easily test the theory by examining a ball pushed through a loading block.

 

[coloradoclyde]

whether we obturate the ball manually by loading it or by firing... it still obturates.

 

[roundball]

whether we obturate the ball manually by loading it or by firing... it still obturates.

That's been proven longer than you've been alive...however long muzzleloaders and PRBs have been in existence.