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Cartridge Firing Dynamics

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303Guy View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 303Guy Quote  Post ReplyReply Direct Link To This Post Posted: October/30/2007 at 00:51
Thanks for that.  I had that problem with my humble mini-14.  It was more accurate with slow powders (possibly barrel harmonics?) but they slammed that bolt back hard enough to bounce the cases into a jam against the scope mount - it was a spring eject.  I eventually installed a smaller gas port insert.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 303Guy Quote  Post ReplyReply Direct Link To This Post Posted: October/30/2007 at 01:03

Originally posted by Graysteel Graysteel wrote:

........
To be honest I have no idea why neck crimping is used in that round. But, I wouldn't think it is related to pressure curve.

 

OK.  I have thought about this one and based on what you folks have explained, I would hazard a guess that what is happening here is that a slower igniting powder is being used and to get consistent ignition, a lower power primer together with more neck grip (crimping), the bullet is not dislodged by the primer, thus allowing the pressure curve to develop.  Does that sound probable? 

Another, less likely scenario is that crimping actually bulges the case at the shoulder/body junction or at the base of the neck, which aligns the case better. 

A third (and slightly more likely than the second) scenario is that the crimp does not expand much, acting as a bullet guide as it enters the bore.  After all, I am using a waxed, paper cup as a bullet guide.

 



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Post Options Post Options   Thanks (0) Thanks(0)   Quote 303Guy Quote  Post ReplyReply Direct Link To This Post Posted: October/30/2007 at 01:53

 

This picture shows the rough chamber impressions on the lower two cases very clearly.  The top case shows a bright ring closer to the neck.  This ring has a corresponding depression ring on the inside of the case, visible only in bright sunlight.  Neck scratches are also visible.  This is damage from another rifle.  This particular case has been reloaded by me, quite a few times now and has just produced my most powerful load - A 56gr cast bullet in front of an undisclosed amount of powder.  The case shows less pressure than the two beneath it and has zero elongation.  The centre case has 'sliding' marks near the rear but these are not visible in the photo.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 303Guy Quote  Post ReplyReply Direct Link To This Post Posted: October/30/2007 at 02:24

 

A 55gr Hornady Spire Point and a 50gr Sierra Blitz Med Vel recovered from the firing tube.  They had been fired through a medium consisting of wet wool in a beer can, then captured in wool and cloth.  This being from the rust damaged, hornet bore.



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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graysteel Quote  Post ReplyReply Direct Link To This Post Posted: October/30/2007 at 08:27

I think Dale commented on the reason for the crimping...

 

"some early hornets were at one time chambered in lever actions and the crimping would keep the bullet from set back"

 

A slower powder is not specifically more consistant, it simply reacts more slowly giving a smoother (less peaked) pressure curve. Generally, the driving force behind powder selection has to do with tuning around peak pressure.

 

I have to say I am curious about your bullet catcher.  :)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 303Guy Quote  Post ReplyReply Direct Link To This Post Posted: October/31/2007 at 11:50

These folks who crimp are doing it for accuracy.  Some say they get best results with a heavy crimp and pistol primers, a few say just the crimp and others say no crimp with pistol primers while still others say no crimp and small rifle primers.  But they are using Lil'Gun.  Or most of them I think.....?

 

I shall post a picture of the bullet catcher.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 303Guy Quote  Post ReplyReply Direct Link To This Post Posted: November/01/2007 at 03:20

 

The first picture shows the 'test tube' or catcher.  It gets a can filled with water soaked wool stuff from inside a chair.  The second picture is the firing tube chamber with the 'test tube' in place with the test can.  Under the can is more, dry wool stuff followed by rags.  At the bottom of the firing tube is sand over a heavy 4140 steel disc.  The firing tube is 'padded' with rolled carpet off-cuts.  The third picture shows the firing tube with its lid on.  The rifle muzzle is held onto the red rubber cap thing and fired vertically down into the assembly.  The whole thing is made up in mating segments from carbon steel tube.  It is 'bullet proof'.  It is due to get a 'slight' addition in the way of a heavy piston in a barrel in the lid, to absorb the gas volume of the 303.  Otherwise the lid section lifts off.  This thing absorbs all the noise from firing.

 

The last picture is the result of a hornet bullet (one of those shown in the previous post) through the test medium.  This medium seems to give a reasonable flesh simulation for testing and comparing.  It helps explain how that little bullet can blow a feral goat's shoulder off the body and still penetrate the chest to reach the heart.  The can acts as a kind of recording of the temporary cavity.  The bullet travels another can length before being stopped in a ball of heavy cloth.  Without the can of wet wool medium, that bullet would travel right down to the sand below.



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