On the matter of obturation

[Daryl]

Dan- I see your points, as well as the Cast Bullet Shooter guys have a formula for showing at what pressure THAT particular brinel hardness bullet will obturate. Has to do with elasticity of the bullet material.  We've mentioned this here at ALR before - I can't find the formula.

In further thinking on the matter, I feel that if the pressures were capable of being bumped up to around 14,000PSI, obturation might take place with a round ball.  Since most shooters today are not interested in approaching those pressures with a round ball, yet they are easily exceeded with a slug, even a picket, we will not see obturation in round balls. This is especially evident in the larger bored guns.  This 'type' of thinking also shows that obturation may happen in the little calibres a lot easier than in the big ones. It is easy to get a .36 or .40 cal. ball running way over 2,000fps than a 500gr. 14 bore ball.  As noted in Lyman's book, pressure and the velocity obtained in rifles with round balls are very closely tied.  ie: 1,500fps in a .50 cal. is produced by the same pressure (very close) as needed to push a .54 ball to the same velocity, and a .58 and a .75, etc.  While it takes only a weak piddling load of around 75 to 80gr.2F in a 42" barrel to get a .50 up to 1,500fps, it takes 90gr. in a .54 to get the same speed and yet takes 175gr. in the .75 to get that same 1,500fps, yet all produce the same pressure - well below what is needed for obturation to take place, by the formula I referred to.
I'm getting aheadache again - think I'll go shoot some now - oops. too early, meeting Hatchet Jack at the trail to 'obturate' some steel targets with my 14 bore rifle.

[Leatherbelly]

 As long as those roundballs leave a round hole in the center of the paper,WHO CARES!

[Daryl]

A 15 bore ball, impacting a steel plate, obturates enough to leave a 1 1/2" disk of 'silver' colouring on the black plate - COOL!

[Leatherbelly]

  Yep, I like the way a pure lead ball obturates thru the boiler room of a deer.

[rich pierce]

I don't have any training in physics and have not studied ballistics so can't offer theory.  I don't think they obturate to any degree based on balls I've found and that it's so easy to get blow by and burnt patches.  A crazy idea I have is that the presence of rifling could actually work against obturation as you have gasses shooting through the grooves if the patch is too loose.  Feel free to shoot that one down (a little pun intended there, lol).

As an uninformed bystander I'll ask a question.  Does the fact that it is a round ball affect it's ability to obturate?  In other words would a flat bottomed bullet be more likely to obturate (wadcutter)?

[Daryl]

I think you are  "Spot-On" Rich.

[Dphariss]

I don't have any training in physics and have not studied ballistics so can't offer theory.  I don't think they obturate to any degree based on balls I've found and that it's so easy to get blow by and burnt patches.  A crazy idea I have is that the presence of rifling could actually work against obturation as you have gasses shooting through the grooves if the patch is too loose.  Feel free to shoot that one down (a little pun intended there, lol).

As an uninformed bystander I'll ask a question.  Does the fact that it is a round ball affect it's ability to obturate?  In other words would a flat bottomed bullet be more likely to obturate (wadcutter)?

Upset, if it takes place, occurs before the ball ever moves.
At least it does in BPCRs with heavier bullets. Heavy bullets such as 360-380 gr 40 calibers and 500+ grain 45s will easily upset .010" or more and will shoot well to very well is slightly under BORE diameter.
They will upset enough even if not pure lead, to fill small gaps between the chamber and the cartridge case. Leaving either paper or lead rings. Recovered bullets will show that the "ring" is taken out of the bullet or patch at or within a few thousandths of where the bullet was seated to in the case.
Smokeless powder, at least at moderate pressure levels (for smokeless) will not upset even lead bullets before they gas cut if undersized unless they have a hollow base they will not seal the bore. The initial acceleration is not sufficient for solid base bullets. BP will work fine with undersized solid base bullets at the same velocity.
Even fast powders like Unique will not upset bullets where they are seated.
I found this in shooting a SAA colt with the barrel removed. My normal load on Unique in a 44-40 would bounce off  a 3/4" particle board. The same bullet loaded with black would punch through the 1st and nearly the second board.
Now does the patch inhibit upset? Even fairly light paper patched bullets 45 caliber 280-300 grains will still upset nicely if not too hard.
But a RB is not a bullet. I suspect that it may upset some, at least in the larger bore sizes, I am not sure its enough to be measurable since it make take large charges of powder and catching a RB undamaged at the velocities involved may be difficult.
I am not sure this is resolvable. To me it is a theoretical exercise. The PRB rifle does not have to rely of bullet upset or careful bullet design to assure uniform upset to assure accuracy as some BPCR loads require.
The bottom line is that at the pressures generated lead in not very "solid" is it has much inertia. Fredrick Mann called lead bullets "putty plug" bullets since they were so easily changed by pressure.
Dan

[Daryl]

Dan- I agree with everything said - especially as to the 'theoretical exercise' - however, there are a lot fo shotoers thinking they can use light patches thinking they are getting a seal due to obturation, where we know this doesn't happen due to the gas cutting that happens along with burnt up patches and poor accuracy, not to mention, OK, I will, increasing fouling, shot to shot.