Hi Molly,
>You may be failing to distinguish between deformation and obturation. Obturation is a special case of deformation, wherein the base of the bullet is expanded by the pressure of the round, to the limits of the chamber and bore. Deformation is ANY change in shape of the bullet.
this is the basis of my point. i understand the defined differences between "obturation" and deformation. however, I am trying to say that, for whatever reason lead deforms when pushed through the barrel. it seems to me, that this being due to obturation or other "unknows" is moot. why would it make a dramatic difference? again I am oversimplifying and don't want to lead you to believe I don't understand the differences. none the less, the boolit deforms to fit the bore.
>Actually, none of these properties are as important to a bullet's performance as its density. The high density of lead enables it to maintain velocity and flatten trajectory better than most other metals, all else being equal. There are better metals from a performance perspective though: Gold is more dense, and some aristocrats actually had gold bullets for their muzzle loading arms. However, gold is not an optimal choice from other perspectives. (VBG)
well I had hoped that my key point of "ability to retain weight" would follow suit with what you are saying. I do understand that density (hence weight in an appropriate size) is a primary asset of Pb.
>I am not sure how obturation can even be a debate. The fact that we cast boolits is simple proof.
Of what? That lead will melt and pour? That's not in question.
>The fact that we swage boolits in sizing dies, is simple proof.
Of what? That lead is maleable? Again, that's not in question.
>The fact that we match alloy to pressure is simple proof.
Again, of what? That if we soften an alloy enough, it can be made to obturate? Still not in question.
>The fact that we can recover the projectiles and see etched rifling is simple proof.
>Seems to me that any etching remaining on a recovered bullet drive band is ample proof that obturation or expansion did NOT take place, as it would have wiped the etching away.
your above assumptions, about my statements (hate to be rude, but you did say something about assumptions earlier in this thread) are partially correct. except for the last one.
if obturation occurs then the projectile would take the shape of the bore. lands and grooves. it would be my inclination to believe, that obturation continues to happen even after the boolit leaves the muzzle. that being my theory/analysis the boolit would be etched with the rifling pattern at the very end of the barrel (muzzle). this recovered boolit would show these signs. UNLESS, gas blow-by / flame cutting stripped them off. does this sound logical. i am not trying to post a rebuttal or argument, simply trying to give you an idea of where I am coming from....and hopefully, understand where you are coming from.
Richard Lee wrote (modern reloading 2) that he witnessed lead streaking from a boolit, that was inappropriate for the pressure curve encountered. he goes on to say that he further tested this by charging even higher. the boolit exploded in flight and "broke the windshield of his new barracuda". I think this kind of proves that obturation and deformation continue to occur during the boolits entire travel. my only guess would be that this is due to a jet of hot gasses that follow the boolit, or are generated by a high/low pressure zone in front of and behind the boolit. it all seems to be much more complex than we are all making it out to be.
I personally believe that obturation and deformation are due to specific pressure curve effects on specific alloys. i.e. too little pressure for the boolits hardness, and the boolit will not bump up and seal the bore and will fail in the barrel (at some point). too much pressure and the boolit with bump up, but be followed by a trail of gasses. when the boolit no longer has a surface (harder than it) to expand against (leaves the muzzle) it will also fail (due to gasses).
it is my opinion, based on testing and limited experience, that the alloy MUST match the pressure curve of the charge and all of the associated variables (some of which are unknowns). in this scenario, the boolit bumps to seal the bore. after all we can't really cast boolits with the rifling in them already...(well Lee does make the REAL minnie ball) it's not practical, or worth the time. therefore the boolit (properly sized or not) does obturate, as does a boolit of imperfect size. in any event obturation is critical to stop failure of the projectile (in the ways I described above). I hope this is cogent....
so it seems that no one, has ever offered conclusive proof that any of these phenomenon occur. only speculation based on observation. my observations and understanding of flow dynamics and the effects of pressure on metals (family has been in metalworking for 2 generations, certainly not saying I am an expert) would lead me to believe that obturation DOES occur. that obturation is a form of deformation. it is hard for us to argue that the boolit does in fact deform. from casting, all the way to the target. with MANY points in between.
maybe we are arguing the same point, and it's an issue of terminology. I am still not quite sure what your conclusion is. you believe that obturation does not occur, yet you emphatically state that deformation occurs during the powder expansion/ignition and all along the bore, with many points to illustrate why this happens. ok, fair enough. if a smooth or rough bore cause deformation of some kind (which you agree to) and the projectiles travel to that point of the bore is under the power/pressure of a powder charge and it's associated gasses, wouldn't that be obturation??
P.S. I think that Richard Lee's chapters on this subject (modern reloading 2) are the best explanation of these phenomenon. I don't want to make assumptions, but you have read these chapters (7-10 I believe)? He breaks this down, with limited scientific analysis (i agree that it is not complete) and with countless hours and years of experience. also a lifetime of access to the industry, far beyond what any of us have. I think it's not possible to overlook his findings. He simply is more of an authority on this subject, than all of us combined (IMO). His findings have gone a long way to change some of my opinions of cast boolits and pressure curves. I hope you can offer me proof, beyond Mr. Lee's that he is incorrect. I am eager for you to change my opinions, in the same way Mr. Lee has....let's keep the dialogue going and keep it friendly. We are all on that same team
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P.S.S. this is grossly oversimplified, but it's a definition, none the less (see below)
http://en.wikipedia.org/wiki/Obturate