Obturate??

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Gooddaytoya!

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I read in a post here that a ball and patch "obturate" as it's being loaded, or as it's being started. What exactly does this mean? The definition of obturate is "block, or close off". I think I know what it means in this context but I'd like to hear it from you guys.
 

Notchy Bob

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I know the technical definition of "obturate," but in shooting, I think it more generally applies to the lead ball deforming in response to the rapid expansion of gases as the powder detonates. The inertia of the lead ball has to be overcome before it moves, and it is my understanding that the rear end of the ball flattens slightly and the radial diameter increases. Most people say "the ball bumps up" in size. I think the jury is out with regard to the amount a lead ball actually obturates. I doubt it is much, and I expect harder alloys would bump up even less. There is no question that the "skirt" of a hollow-base bullet blows out to better fill the grooves, though. Minie bullets were designed to do that.

As for obturation during loading, that's a new one to me.

Notchy Bob
 

Ponderosaman

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Could it refer to the loading jig to cut bullet on a muzzleloader?
 

Phil Coffins

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Obturation happens as the projectile is accelerated from a stopped position. This even happens to a jacketed bullet as was proven by Dr. Mann at the turn of the last century. His book is “The Bullets Flight” covers this and more, good reading for any serious shooter. If a ball is pounded into the muzzle rather then pushed it would expand as well.
 

Gooddaytoya!

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Could it refer to the loading jig to cut bullet on a muzzleloader?
Obturation happens as the projectile is accelerated from a stopped position. This even happens to a jacketed bullet as was proven by Dr. Mann at the turn of the last century. His book is “The Bullets Flight” covers this and more, good reading for any serious shooter. If a ball is pounded into the muzzle rather then pushed it would expand as well.
So it's not about loading or patches or ball size, strictly speaking.
 

Gooddaytoya!

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The reason I asked this question is because I've never been convinced that my ball and patch combination fills (blocks, obturates) the spaces between the lands and grooves. I think my ball is too big and my patch is way too thin. A gunsmith set it up for me that way, along with some other minor work, but I think he blew it on the ball and patch sizes. I want to avoid gases jetting past the load just after detonation. Unfortunately I've never found one of my patches down-range for examination. Maybe I can find a muzzleloading - savvy gunsmith within driving range.
 

Bad Karma

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The reason I asked this question is because I've never been convinced that my ball and patch combination fills (blocks, obturates) the spaces between the lands and grooves. I think my ball is too big and my patch is way too thin. A gunsmith set it up for me that way, along with some other minor work, but I think he blew it on the ball and patch sizes. I want to avoid gases jetting past the load just after detonation. Unfortunately I've never found one of my patches down-range for examination. Maybe I can find a muzzleloading - savvy gunsmith within driving range.
I’m a big fan of a wad of some sort under the ball or bullet. Are you getting the kind of accuracy you ecpect or want from the load? If not then yes, I’d experiment with smaller ball/thicker patch combinatio.
 

longcruise

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As said, obturate means to plug. The projectile itself does not obturate. A projectile that obturates the bore upon firing is caused by the sudden acceleration at the base creating a pressure from the weight of the top of the projectile resisting movement. So the projectile "bumps up" from it's own weight. In the case of a bullet, it bumps up more at the base and progressively less towards the nose.

There is disagreement on whether or not a ball bumps up.

I would say that we depend on the patch ànd ball to obturate the bore upon loading. Maybe not completely but at least adequately?? Patch/ball combos that are loose tend to be inaccurate.
 

Grenadier1758

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The reason I asked this question is because I've never been convinced that my ball and patch combination fills (blocks, obturates) the spaces between the lands and grooves. I think my ball is too big and my patch is way too thin. A gunsmith set it up for me that way, along with some other minor work, but I think he blew it on the ball and patch sizes. I want to avoid gases jetting past the load just after detonation. Unfortunately I've never found one of my patches down-range for examination. Maybe I can find a muzzleloading - savvy gunsmith within driving range.
We want to avoid the gases jetting past the patch and ball as the rifle is fired. In truth there is so much pressure from firing that is is impossible to stop all gas blow by. The best we can do is keep it under control as best we can. If you have a rifle with an even number of lands, then you need to have a digital Vernier Caliper in your tool kit. The calipers will give a reading of the inside diameter of the lands and the grooves. It can be used to measure the diameter of the ball and the thickness of the patch and provide an indication of the compression.

When you have the measurements of land to land diameter and groove depth, we can begin to make some starting selections of ball diameter and patch thickness.

The ball diameter can be 0.010" less than the land to land diameter. The patch thickness then is selected to be the compressed thickness equal to the groove depth +0.005". Going thicker is a better choice than thinner. You will need a short starter (straight starter) to load the ball, but accuracy will improve. Of course, patch lubrication is necessary. I use a mix of one part water soluble oil and seven parts of water. Dish soap can be substituted. Olive oil, actually most cheap cooking oils can be used and of course spit is the classic.

Do try to find your patches. With a new rifle I would expect to see some cutting from the lands and maybe some cutting from the crown. It took a few hundred rounds through my rifle before cotton patches would hold up. There is just a little bit of obturation on firing to firmly set the ball and patch into the rifling.
 

necchi

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Now that they've told you what it means,,
I have seen the "Obturation" issue un-resolved and under contention here on the forum since I became a member.
I'm in the school that it does happen, much like a baseball being hit with a bat,,(look it up)

Unfortunately I've never found one of my patches down-range for examination.
Well, you need to fix that huh? Patches can and do get shredded sometimes. But they do leave remnants that are expelled from the barrel, and are found.
Always, they don't magically disappear.
Go shoot your gun, pointed at a proper berm, instead of sighting or trying to hit the bullseye, watch in a closer proximity, look through the smoke,, the "patch" drops off pretty quickly. You can actually see it.
 

SamTex1949

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in regards to patch findings, if you have a friend with you have them watch for the patch s it leaves barrel. its gotta be out there in front somewhere.
 

Billy Boy

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I read in a post here that a ball and patch "obturate" as it's being loaded, or as it's being started. What exactly does this mean? The definition of obturate is "block, or close off". I think I know what it means in this context but I'd like to hear it from you guys.
The ball ‘slugs up’ under firing press and fits bore tighter.
 

springfield art

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I read in a post here that a ball and patch "obturate" as it's being loaded, or as it's being started. What exactly does this mean? The definition of obturate is "block, or close off". I think I know what it means in this context but I'd like to hear it from you guys.
I think they just mean it goes a little crooked going in; tilted, if you will.
 

Barry Strickland

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Grenadier1758 gave you an excellent answer but I would like to add to it.
Take a section of your cotton fabric, say it is .020" compressed thickness. Using your ball say .010 under land to land bore size and start them into the barrel. Pull the combination out of the barrel by the remaining fabric. The ball should have a definite pattern of the fabric imbedded in its surface where it was being compressed by the lands. The combined thickness of the patch, 0.040" will force the ball to deform slightly into the grooves and also to slightly lengthen the ball. This would not "obturate" the bore unless it had very shallow groves. This type of loading produces the best accuracy and this tight of a load is not necessary for hunting loads. If you were to go to NMLRA Nationals at Friendship and explore the bench rest shooters you will notice many using over bore size balls plus false muzzles for a very tight combination.
There is some confusion about obturation but it certainly does "bump up" on the explosive force of ignition. The Navy has a article on the ships guns that speaks of the obturation of the projectile upon firing.
 

beardedhorse

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Tight fitting balls and patch combinations such as in some Jaeger rifles where a mallet is used to drive the ball down to the powder can upset the sphere. Also there are shooters who don't swab between shots and when the fouling keeps the ball and patch from seating on top of the powder they throw their ramrod down the bore two or three times to seat it. I don't think this does anything positive for accuracy. Besides obturating the ball, it compresses the powder. But then the bench rifle shooters are doing something similar in tight ball-bore combinations. Gooddaytoya might try a smaller ball and thicker patch combination so the patch doesn't get burned up every time you fire.
 

fcderosa

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As a former Army Master Gunner (retired) I can tell you without a shadow of a doubt that obturation is the sealing of propellant gasses. This sealing is done so that at the time of ignition, the expansion of propellant gasses will establish the point of least resistance. In most cases it will be the projectile, causing the projectile to exit the barrel thus releasing propellant gasses. Should forward obturation not be achieved it will either allow the gasses to escape around the projectile, or if partial obturation is achieved, push the projectile out the barrel with that part not sealing being scored. This is what causes scoring. Scoring in the long term causes keyholing. A cartridge firearm is pretty much the same, except you have forward and rearward obturation, where on a muzzleloader you should only have forward obturation. The rule of thumb here is; energy will always seek the path of least resistance.
 

Silent Walker

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I had to google this topic, as I've never seen a round ball obturate from being shot, I have however seen recovered round ball like these in the photo with the rifling marks.

 

russellshaffer

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Gooddaytoya! said:
Unfortunately I've never found one of my patches down-range for examination.

It is quite possible that your fired patch is a charred,black stringy thing that is unrecognisable. A friend had some patches that were totally destroyed but were very easy to find when fired over snow. He couldn't hit anything with them. An observer staning in a safe possition to the side might be able to see the patch drop. It worked for me and my buddy.
 

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