Good morning fellas
Stormrider51 , welcome to the TMA . I think you will find that things are much different here then in other places . While you may find disagreements , those disagreements WILL be done in an adult manor and with respect . We simply do not allow what so many other places allow .
As to your subject .
I would also agree with Dave’s points but in this case and most times for these type of discussions , its understood that obturation of RB is the expansion or deformation caused by ignition
There has for a very long time been this division of opinion as to if a RB actually obturate’s or not ..
On one side you have those who feel that a ball starts moving before pressure builds to the point that it can expand the ball
Then on the other you have those that believe the resistance of the ball itself is the key to the expansion. .
As to the first . This is I believe based on the theory that the ball is part if the total load , there for it isn’t hit by pressure but accelerated by the pressure . As such they poo poo the tests that fedela and others did concern short starting barrels . Which as mention documented obturation happening .
However we should understand that in those tests , the reason for that obturation was indeed the effect of a pressure wave hitting the ball . Not the result of the ball traveling on the leading edge of that pressure wave .
Thus IMO , their reason is understandable , but for the fact that it doesn’t take into account resistance of the projectile , nor does it account for the resistance of the opposing air pressure while the ball travels on that pressure wave .
The problem in disproving or proving the theory is that regardless of what you shoot into the projectile is going to encounter resistance . That resistance is going to expand the ball and thus yield inaccurate results . Especially concerning ballistics jell . Even water is going to provide a given amount of resistance . As such your most likely still going to see a .002+ expansion.
Many years ago I read an article where a fella was trying to prove the theory by placing a paper target in front of the muzzle , just out of the muzzle blast area . . He then measured the resulting hole.
Supposedly this showed that indeed the ball had expanded.
Problem was , folks suspected that the patch had not yet separated . Thus creating the larger hole . So he again tested his theory at a longer distance . The results still showed a larger projectile then was loaded . But could this have been a natural result from air resistance or was it a result of his loading
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Your results as of yet , are showing deformation by loading , not really obturation cased by the ignition process .
Myself I don’t see how you will be able to physically , accurately prove the theory without having a long enough barrel to contain the projectile to its outer limit of its movement .As such I think we must resort to math and physics to prove obturation.
Remington as I recall actually did this back in the 40’s . not on only did they do it mathematically but also physically.
If we accept that lead is a physical plastic and get over the idea that a projectile is a solid , just because it feels hard . We then can understand that there is a given point where all materials , when subject to pressure , begin to act like liquids, even though they may remain somewhat solid . If we understand that ,then we can understand that when the pressure exerted against it reaches the plastic deformation “ all materials have this point , “the ball then will expand.
Remember in school they taught us that for every reaction , there is an equal and opposite reaction . With plastic deformation , while its given that plastic deformation irreversible an object in the plastic deformation range will first have undergone elastic deformation, which is reversible, so the object will return part way to its original shape. IE, equal and opposite reaction
think of a rubber ball . But with lead the plastic deformation is greater then its Elastic deformation is less . But that has not change the fact that it has or will expand even greater . Physics say its has to .
Again Remington proved this long ago . Even today with cased bullets , slow motion x-ray photos show the bullets expanding within the bore . Those same photos show an even greater increase in the center of the body when viewed after leaving the bore and at the targets.
This is caused by the resistance of the bullet traveling through air . IE weight encountering drag at velocity and thus the effects of the projectiles plastic deformation value ..
IE the forces on the front of the projectile are causing that section of the projectile to encounter drag . while the back section of the projectile isn’t encountering the same amount of drag . as such the back is trying to push past and through the front , thus causing the projectile to expand in the center . This effect increases when the front comes in contact with even more resistance . IE a target .
A good example of this is a big lead conical after mushrooming from hitting a target . Often times you can still clearly see the base . But the forward section is enlarged . This enlargement isn’t the front being forced out . it’s the center of the projectile expanding out and around the front of the projectile which contacted the resistant first .. As the total mass slows to a stop , it leaves the back end of the projectile setting center of the mass . Again Physics
So once we understand the dynamics of whats going on . We then have to accept the fact that a RB upon ignition has to expand within the bore even if from nothing else then the effects of acceleration on it physical properties . The faster that acceleration , the greater the expansion .
Anyways , sorry to be so long winded . Its a very interesting subject that has been proven time and time again . Both in actual application and scientifically .
Of course there will always be those who don’t believe it . Thus we will always have the fun of proving it over and over again .
