M
mks
Guest
Yes, the same pressure acts on the lightweight neck and the heavy bullet. The lighter of the two is moved faster.
When does the case neck expand?
- Thread starter JerrySharrett
- Start date
Yes, the same pressure acts on the lightweight neck and the heavy bullet. The lighter of the two is moved faster.
M
mks
Guest
The pressure to plasticly expand the neck can be estimated as the yield stress of the brass times the thickness of the neck divided by the radius of the neck. For work-hardened brass in the typical 30BR, this gives about 2000 psi, or for dead soft brass about 600 psi. The body of the case is considerably thicker, so it takes more pressure to deform.
Can't remember who it is/was but a well known and trusted fellow has a computer program that says the case neck expands before the bullet leaves the case. I've not seen anything that disagrees with that. That said, with any clearance, the case essentially departs the bullet rather than the bullet departing the case.
There's nothing that can be done about this. On the other hand, I never really cared about neck tension but didn't have much luck with 133 powder. Folks say you need to increase neck tension to shoot well with 133. I don't know but it seems they were right. Keyword is "seems".
There's nothing that can be done about this. On the other hand, I never really cared about neck tension but didn't have much luck with 133 powder. Folks say you need to increase neck tension to shoot well with 133. I don't know but it seems they were right. Keyword is "seems".
Dusty Stevens
Hollow Point Dispenser
I wonder why long vld bullets being in the case neck the same amount as a ppc need more neck clearance to shoot without flyers? We can get away with .001 to .002 total clearance in a ppc but a dasher needs .004 to .005?
JerrySharrett
Senile Member
Does the Dasher really need 4-5 thou or is this for protection of sloppy reloading? Not to knock them but many 600-1k shooters don't have the necessity of ultra-precision reloading 100/200 BR shooters do.
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Dusty Stevens
Hollow Point Dispenser
Ive actually took a winning barrel, winning brass right from a big match and got flyers. This was around .0015 clearance. Turned them to .003 and it was gone. The solution was a bigger n.d. Reamer. I just build em. But with my br techniques i was able to isolate it. I cant explain it but i do go with it now myself. I havent seen any issues with .004 clearance but i have with .0015. That type shooting is so far on the back burner for me i dont try to explain it in my head i just go cause i cant find a downside
D
dk hunt 14
Guest
I disagree with this. I believe most do more to have better ammo. If the bullet doesn't leave first why does more neck tension tune a gun? Matt
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James Lederer
New member
This makes sense to me. In high speed photography of muzzle blasts and bullet exits we see gases escape in front of the bullet. My guess is these are gases that escape between the bullet and the neck before the bullet is fully engaged in the rifling and is still in the throat area which is larger than the bullet diameter..
M
mks
Guest
The 600-2000 psi is the pressure required to fully expand the neck. Obviously, if the neck is expanded away from the bullet, then there is no friction left on the bullet to keep it from accelerating forward.
Jerry -
With strain gauges on the barrels I have seen initial bullet engagement pressures as low as 2500-psi from less then 1/2-thousandths of neck-tension, to upwards of +8000-psi from 005-thousandths neck tension (6Dasher's with 105 class bullets).
As to neck clearance in my 6Dasher's, unlike Dusty's input, when I have tested clearances (at LR), I have seen very little to no accuracy gains or loss from tight clearance to excessive clearance. Same for back in my 243-Ackley days as well.
Also in closing, I strongly agree with Matt, and see more detailed preparations go into 600/1000-BR then I did back when I shot 100/200-BR. Two completely different animals is how I like to describe the difference between the two disciplines.
My 2-Cents
Donovan
With strain gauges on the barrels I have seen initial bullet engagement pressures as low as 2500-psi from less then 1/2-thousandths of neck-tension, to upwards of +8000-psi from 005-thousandths neck tension (6Dasher's with 105 class bullets).
As to neck clearance in my 6Dasher's, unlike Dusty's input, when I have tested clearances (at LR), I have seen very little to no accuracy gains or loss from tight clearance to excessive clearance. Same for back in my 243-Ackley days as well.
Also in closing, I strongly agree with Matt, and see more detailed preparations go into 600/1000-BR then I did back when I shot 100/200-BR. Two completely different animals is how I like to describe the difference between the two disciplines.
My 2-Cents
Donovan
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JerrySharrett
Senile Member
Hey Mr. M, I'm not talking about bullet movement and at what pressure. What i'm wondering is WHEN the neck expands where is the bullet base (if a FB) when this expansion tales place.
My thinking is that a tighter neck fit more accurately presents the bullet to the bore thus improving in-bore yaw. So, if that neck expansion takes place when or after the bullet exits the case mouth, theoretically a tighter neck fit would be better would it not?
I know the common response, let the target tell you but from what I've seen this variation of 0.001" through 0004" has such a finite effect it is mos-nix....after all this IS a Cabin Fever discussion.
Cold and snowy in the Upper Midwest? I'll be hookng to the bass boat in a few minutes!!
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M
mks
Guest
When a gap between the neck and bullet is created by rising pressure, but before the neck is pushed all the way to the chamber wall, combustion gases leak past the mouth of the case. When the gases leak at a sufficient rate to increase the pressure forward of the case, which is also helped by the bullet moving forward to seal the bore, the pressure compresses the (clean) air between the case and the chamber and pushes a limited amount of dirty combustion gases into that space. At the same time, internal pressure is expanding the case and pushing air out of the same space, thus the carbon rarely gets past the neck. It seems to be low pressure rounds that don't seal the body of the case to the chamber walls that exhibit carbon further down the case.
Why the waves of carbon on the neck? That is a tough question. I really don't know, other than to say that it would take very uniform conditions to NOT have inconsistencies in the encroachment of carbon into the gap. Gas finds the path of least resistance.
M
mks
Guest
The pressure acts on both at the same time. The bullet weighs 117 grains and the neck about 2 grains. Now, which do you think moves fastest?
Dusty Stevens
Hollow Point Dispenser
Im still trying to figure out why it matters? Its not like youre gonna be able to change it like with a bushing change
Had 5" or so thus far. But were having a heat wave (+32-F to 40's) so most of that has melted !.!.!
The bass boats have been put to storage and the ice-houses brought out to hit the waters (ice) !.!.!
But I'm not a fisherman, haven't did any in 30-years. Was always to busy building faster race cars, and now shooting smaller groups.
Back on topic: I see chamber finish as well as the brass exterior state, to have some effects to sealment.
Best Regards
Donovan