On the matter of obturation

[Dan]

Recall the recent discussion on barrel length and twist rate for PRB shooting?   http://americanlongrifles.org/forum/index.php?topic=4675.0

One element of that I thought worth looking at was how to source the data.  Well, that may be a puzzle in itself.  I just returned from the root cellar after toying with balls, patches and a ball puller and have the following observations to pass along.  And no, I don't reach many conclusions from this, just passing it along.

A .440" round ball from Hornady was given a reference datum by marking pen then measured around its circumference in four segments.  The range of diameters resulting was .438-.441", averaging .439" diameter.

I seated the ball in the muzzle of my rifle with a strip patch of .010" thickness, using a scrap of persimmon carried in the field and range for that purpose.  Ball was pulled and no weave marks or change in dimensions were observed.  Not altogether a surprise as the bore is .450" as measured by a pin gauge. Given the combined ball and patch nominal diameter of .459", I assume that the compressibility of the patch material was greater than the softness of the ball alloy.  The latter has not been evaluated but it behaves as pure lead or close enough.

Next I seated the ball again with a round patch of .020" thickness.  This requires a very firm rap with the seater....and a ball puller to remove. Total diameter before seating was average, .479".  What I found was a puzzle:

The ball had weave impressions on the land segments but none on the groove. The impressions were circular in shape and they were quite clear as this lubed patch is of very course weave. The ball puller was screwed in to only about 1/3 the ball diameter.  The measurements that followed were taken on the same plane as the impressions after each had been marked above the major diameter with a marking pen. I did so because the flat portion of my caliper is wider than the impression and I think the knife edge segment at the tip would impress itself into the alloy and queer the data.  The minor diameter (weave impressions) of the ball averaged .4375" and the major diameter was .440".

So what I'm left with is this question. Did the force of seating the ball swage it sufficiently to modify the major diameter or was it the ball screw that did so?

[T*O*F]

Did the force of seating the ball swage it sufficiently to modify the major diameter or was it the ball screw that did so?

Yes, either or both could have done it.

Pletch and I were going to do the obturation testing several years back.  When I get the time, I will do the write-up for our rationale back then.  I know he's planning on completing the testing and perhaps wthis will serve as a reminder to him.

[fdf]

My speculation would be that the ball puller would cause expansion as the displaced lead from the metal screw on the puller must go some place.

You might consider the same test and use a CO2 discharger to push the ball out of the barrel in to some sort of soft medium, maybe an old feather pillow or couch cushion.

Just my thoughts.

FDF

[doug]

[quote
Pletch and I were going to do the obturation testing several years back.  When I get the time, I will do the write-up for our rationale back then.  I know he's planning on completing the testing and perhaps wthis will serve as a reminder to him.

    I tried testing obturation a different way; I tried shooting a round ball out of a 2 groove rifle.  When shot with belted balls, it leaves distinctive holes in the target sort of like Mickey Mouse heads :>)    My logic was that if a round ball obturated to any significant degree,  it would grow lugs (or ears) where it swelled into the grooves and that this would show up on the target.  Unfortunately that did not happen.
     I think that what I need to do next is to fire several shots with the same combination, into snow next year and recover the balls.  That should show much lesser degrees of swelling if such should occur.

cheers Doug

[ironwolf]

  Use a barrel with no plug and push the ball through.

[Dan]

TOF, thanks, I'd like to see that.  I had suspicions it could be a combination of the two. On the other hand, if a brisk slap with a starter can do that, would not a powder charge be more of an influence?  Well, we will see.  I've a good source of sawdust...a summer project looms.

ironwolf,  not having a spare barrel and wishing to keep the variables at a minimum, I'll have to pass on that suggestion. Point taken, but I don't have the resources and won't take my flinter apart for this.  I'm thinking the proof will come from the small balls and thin patch.  They will or won't obturate upon firing.

[roundball]

Just as FYI...I've shot my .45cal deer hunting loads into lined up gallon jugs of water and found zero patch weave marks or any indications of obturation on any of the recovered balls...same with the occasional ball recovered from a deer.

90grns Goex 3F
Oxyoke OP wad
.018 pillow ticking
Hornady .440

[Daryl]

RB - Due to the ball/patch/barrel combinations I use, I get patch weave marks on the sides of the ball, just pushing a ball through a barrel. Recovered balls show the impression of the cloth weave on the base.

Forsyth says there is no obturation of the round ball, only conicals. That's good enough for me.
Obturation must take place at or very near the ball's (or bullet's) seat in the barrel as the charge starts to manifest itself.  Once underway, the very physics of the matter eliminates any further obturation as it is the resistance to movement due to the projectile's mass that causes the obturation in the first place.  Was there obturation with round balls, thinner patches should not show burnt up strips of cloth on the ground after firing, which happens even in the .69 (484gr. ball weight) with 82gr. 3f or 160gr. 2F if the patch isn't substantial enough. This much mass should certainly cause obturation if it happened with a round ball.  Forsyth says it is due to the ease with which the ball is set into motion, compared to a slug which creates more friction in the barrel where it sits, or to it's easily expanded base.

I checked the bore with a bore light and it's bright and shiny all the way to the bottom.  Rifle was built in 1986.  edited - now I think I was charged 10 'pounds' for the light. That's an expensive bore light.

[Dphariss]

These are pure lead .662 balls, one unfired, the other struck a deer at about 40 yards. Ball entered beside the windpipe.
Was patched in linen about .015 thick. 140 gr of FFG Swiss.
Dan

[Daryl]

Good photo, Dan - like the shiny ball - must be the 'pack-rat' in me. I also noticed the very large seam around the ball showing a poor casting or wide seam lock-up -  - HA! - just kidding.  What does a ball look like if pushing into the bore, then extracted?  I suspect that 140gr. charge will produce enough pressure to imprint the patch weave into the base of the ball in any case.

[T*O*F]

TOF, thanks, I'd like to see that.

OK.  When I lived in Illinois, the largest barrelmaker in the USA offered us the use of a pressure gun and commercial bullet catch media to test roundball obturation.  The pressure gun was given to him by the US Government for a contract he did the development for.  He also offered to make us barrels to fit it and the molds to accompany them.  He also stated that Remington had already done this research back in the 40's and he had read such research that proved round balls do obturate.  Pletch and I were going to do it again for current publication.  I was supposed to take the pressure gun home, but it weighed more than my truck could carry and I had no way of unloading or storing it, and I moved south in the interim.

Our premise at the time was as follows:
Lead is a plastic solid.  If you place a pure lead roundball on a sheet of glass and let it set, eventually it will develop a flat spot on the base because of this plasticity.  If you drop a roundball from 3 ft the same thing will occur.  Likewise, if you bounce your ramrod on a loaded ball, it will flatten and foreshorten.  This is obvious if you eject the ball after doing so.  Clearly, none of these events come close to the pressure exerted on a ball during firing.

The next step was to eliminate any variables.  A cloth patch is softer than lead and will resist obturation thru compression.  Patching was to be eliminated.  Likewise, rifling served no purpose in proving the premise.  We proposed to have smoothbored barrels in various calibers made with molds that would throw balls near exact bore diameter which could be started without any force and had little or no windage past the projectile when fired.

Starting with the smallest loads and working up, we could determine at what pressure the charge overcame the plasticity of the lead and foreshortened the roundball.  This would be evidenced by a flattened ring at the bearing surface of the ball where it contacted the barrel.  The crushers of the pressure gun would enable us to measure the pressure at which this occurred.  It was postulated that the larger the ball, the less the presssure required for obturation, because of the physics involved.

Sadly, we were never able to reaffirm Remington's findings for the reasons stated above and without a pressure gun, I see no other way to do so.

[JCKelly]

Question, really for the Long Range guys, if one shoots a .45 round ball from a hexagonal bored Whitworth barrel, will it make round or hexagonal holes in the target???

[T*O*F]

Question, really for the Long Range guys, if one shoots a .45 round ball from a hexagonal bored Whitworth barrel, will it make round or hexagonal holes in the target???

Neither, it wouldn't even hit the target.  Round balls aren't for shooting 1000 yards.

In reality, there is too much windage around the hex corners which would result in massive blowby which couldn't be taken up with a paper patch, and using a tight cloth patch would only results in regular .45 cal roundball performance.

However, as you probably know hex Whitworth bullets were originally only available directly from Whitworth.  Others routinely shot round bullets which did bump up to hexagonal when fired.  The same is true today.

[Daryl]

Yer sharp, TOF.

Are you saying the patch will help prevent a round ball from obturating due to it's "taking up the slack room"?

[Dphariss]

Good photo, Dan - like the shiny ball - must be the 'pack-rat' in me. I also noticed the very large seam around the ball showing a poor casting or wide seam lock-up -  - HA! - just kidding.  What does a ball look like if pushing into the bore, then extracted?  I suspect that 140gr. charge will produce enough pressure to imprint the patch weave into the base of the ball in any case.

Cameras will show the darnedest things.
Will have to go out and look at the box of balls in the shop now to see if the "ring" is really there.
Moulds that are not heat treated before the final machine work tend to warp the first time they are heated so it could be. Its a Lyman .662.

Dan

[Daryl]

Good photo, Dan - like the shiny ball - must be the 'pack-rat' in me. I also noticed the very large seam around the ball showing a poor casting or wide seam lock-up -  - HA! - just kidding.  What does a ball look like if pushing into the bore, then extracted?  I suspect that 140gr. charge will produce enough pressure to imprint the patch weave into the base of the ball in any case.

Cameras will show the darnedest things.
Will have to go out and look at the box of balls in the shop now to see if the "ring" is really there.
Moulds that are not heat treated before the final machine work tend to warp the first time they are heated so it could be. Its a Lyman .662.

Dan

Lyman, eh - my Lyman .662" mould casts perfect balls, of course.

[Dan]

He also stated that Remington had already done this research back in the 40's and he had read such research that proved round balls do obturate.

Frankly, I do not see how they would not if a pure lead ball and fair charge were used. It is a very soft material.  No disrespect intended to those of differing opinion, but this one brings out the Missouri Mule in me.....

Proving it one way or another will obviously be a challenge...

By the way, Hornady represents that their round balls are swagged from pure lead.

[Daryl]

Hornady says they use pure lead for boxed round balls in 100 piece boxes and they also advertise 3 or 5% antimony in the 5 pound boxes of buck sizes. Is it also possible they make pure lead buck sizes for the ML market?

Considering the patches ability to hold, is it possible the patch will limit or prevent a round ball from obturating?

Forsyth didn't seem to think RB's obturated and I kinda think they don't either. If they did, a .017"-020" patch and pure lead .684" ball should seal in a .690" bore with .012" rifling - it doesn't, even with light loads . The patches burn out along the rifling lines. Thicker patches giving more than .004" compression per side are re-usable.

[T*O*F]

Yer sharp, TOF.
Are you saying the patch will help prevent a round ball from obturating due to it's "taking up the slack room"?

Daryl,
I eschew obfuscation and am saying no such thing.
Being an obdurate cynic, I reduce everything down to its simplest form.
What I said was that in an experiment, all variables must be removed before prodeeding.  If one's premise is proven, then the variable can be added back in to test for other results.

[Dan]

Daryl, the only thing of Forsyth's I've read (in part) is "The Sporting Rifle and its Projectiles". I have not read it all and do not question your words about his opinion, but am left to wonder how he would have determined this one way or another.  At the time it was written (1862) there was precious little precision measuring equipment afoot.  Do you recall how he made the determination on this subject?

[Dphariss]

Daryl, the only thing of Forsyth's I've read (in part) is "The Sporting Rifle and its Projectiles". I have not read it all and do not question your words about his opinion, but am left to wonder how he would have determined this one way or another.  At the time it was written (1862) there was precious little precision measuring equipment afoot.  Do you recall how he made the determination on this subject?

I do not know of any study that has been made or how it would be made. I doubt that there is significant upset in a RB small enough to use in small arms and cannon balls are generally hard.
I figure that if there is any upset its just enough to help tighten the fit on the lands a little. This might be why some guns do better with FFFG. I just do not know. Its seems plausible that the pure lead ball could shorten slightly under acceleration but proving this would be nearly impossible since the upset if going to be slight if its takes place and catching the ball without deformation is also very difficult.
I do know that I have never seen a RB that looked as though it had upset into the grooves at all.
This is a question more for theoretical discussion than real practical use.

Dan

[Daryl]

Daryl, the only thing of Forsyth's I've read (in part) is "The Sporting Rifle and its Projectiles". I have not read it all and do not question your words about his opinion, but am left to wonder how he would have determined this one way or another.  At the time it was written (1862) there was precious little precision measuring equipment afoot.  Do you recall how he made the determination on this subject?

I'd think probably in a similar manner to what one might fudge through this today - ie: if the ball obturated, why is a 'substantial' patch needed? Today, we (some) tend to think in terms of ten's of thousandths of an inch as being important and proving points of argument. Certainly, they probably didn't have this degree in accuracy of measuring - but - this takes us back to what is important or useful - a truly obturating ball would certainly make loading easy for us today. Simply load a greased but thin patch, undersized ball and let the obturating ball seal the powder's expanding gasses below it - load with 2 fingers and shoot all day without cleaning - just doesn't happen. Thick, lubed patches creating tight combinations allow this - why? - no obturation, that's why - tight is necessary.  Yeah, simplistic, but fairly accurate, I think, as was Forsyth - yeah, he made some boo-boos, but few and very far between.  Just finished rereading the book for perhaps the umpteenth time - he was way ahead of his time.

[Dan]

Ok.  For the record, that part of his book I've read held nothing I'd call a mistake. He was far ahead of his time.

If we start with the first part, use of a thin patch and finger pressure the load we are at the doorstep of finding out the truth in my eye.  If the assertion that RBs do not obturate is correct, the obvious question arises then, at what point does it begin?  I'll suggest a short list of factors that would reasonably be considered to influence this in any gun.  Sectional density, friction, pressure, alloy hardness....a start anyway.  At what point to these variables join to precipitate obturation?  One could easily work this starting with a Lewis style belted picket bullet with a round base of approximate sectional density as a corresponding caliber round ball, then add bore rider length to up the SD I suppose.  It would isolate one of the variables to some degree.

It would be helpful if anyone here could work some acceleration calculations to determine what gee force a .45 cal. RB experiences when charged to reach about 2,000 fps or so in a 42" barrel, versus a .45-70 bullet of about 500 grains leaving a Sharps barrel of 30" at 1300 fps.  That particular math world is not my forte'.

[Dphariss]

Dan
The G force is really not going to matter the ball is far easier to move.
Its not the G force so much as the inertia of the front of the bullet not moving while the back of the bullet is under pressure and needing to move. I suspect the 45 ball has a higher G applied since it will be going faster in less time.

Dan

[Dan]

Well, maybe we're looking at opposite ends of the football on this. Dunno.  My perspective comes from Mr. Newton.  F=MA   

It takes Force to overcome Inertia and Accelerate things. Gee is the common measure of Acceleration as I understand it.  My guess is that when Force exceeds the plastic limits of lead it obturates.  Gee would be a handy reference to ascertain how much force is applied to the ball.....unless I've lost my mind.

http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion

I'm off to Cody for the week.  With any luck at all they have a good bar and I can accelerate my obdurate ways.