seating depth 102nd post I think

[ClassCat]

thank you

You're Welcome

 

[denobravo]

May I ask if your testing a VLD Secant ogive projectile?

Ya know , I dont know> I am shooting 115 dtacs I think that would be tangent

 

[mensajd]

I am not the fan of sticking the bullet into the lands. I want my bullets to have that running start. When my bullets hit the rifling I want the bullet to have 'that jump'. I do not want my bullet to slow down. if the bullet slows down it has to get started again.
F. Guffey

In the bullet firing from the jammed-in-the-lands case, the bullet does not "slow down." In that case, the static friction (also referred to as "stiction") is higher and requires more pressure to start the initial bullet movement.

 

[mike a]

In the bullet firing from the jammed-in-the-lands case, the bullet does not "slow down." In that case, the static friction (also referred to as "stiction") is higher and requires more pressure to start the initial bullet movement.

Stiction? Aka Stickytion or Grabbtion? Just haven fun guys.

 

[dcali]

I don’t think anyone fully understands the science behind seating depth. I’ve thought about it a bit and this is the best I can come up with, which is admittedly speculative.

We know and can prove that starting a bullet into the rifling as straight as possible makes for good accuracy.

We also “know” that pointier bullets are more sensitive to seating depth. Or at the very least we know they are different.

What is also true is that a fixed diameter datum ring (like the rifling) on a shallower nose cone (like a VLD) is harder to align than a steeper slope like a tangent ogive.

So my speculation is that seating depth in some way impacts our ability to keep the bullet straight. The fact that some have observed it to be cyclical in nature (that there can be more than one good depth) makes me wonder if there isn’t some oscillation that happens as the bullet jumps towards the lands. Seating depth may just be a way to time the bullet’s impact with the rifling when it is at its straightest.

What I definitely know is that it’s a very important part of load development and it’s not completely independent of charge weight.

 

[denobravo]

I don’t think anyone fully understands the science behind seating depth. I’ve thought about it a bit and this is the best I can come up with, which is admittedly speculative.

We know and can prove that starting a bullet into the rifling as straight as possible makes for good accuracy.

We also “know” that pointier bullets are more sensitive to seating depth. Or at the very least we know they are different.

What is also true is that a fixed diameter datum ring (like the rifling) on a shallower nose cone (like a VLD) is harder to align than a steeper slope like a tangent ogive.

So my speculation is that seating depth in some way impacts our ability to keep the bullet straight. The fact that some have observed it to be cyclical in nature (that there can be more than one good depth) makes me wonder if there isn’t some oscillation that happens as the bullet jumps towards the lands. Seating depth may just be a way to time the bullet’s impact with the rifling when it is at its straightest.

What I definitely know is that it’s a very important part of load development and it’s not completely independent of charge weight.

very well said and I am sure your right
I did notice that as my dpth decreased as the bto grew so did the velocity, all groups had single digit sd some single es
It did shoot best at kissing the lands .000 jump lol
So I thought also about that speed increease resonates a good harmonic ...found "A sweet spot" if not the
so my ladder test will be looked at very differently now
thanx Again

 

[lpreddick]

I had a good and respected friend convince me to doo a seating depth test before my ladder test
I usually test "in the lands"
So I tried it and was very surprised
Can anyone explain the science behind the test as it will help me "believe" as I go forward
my old way was ladder and then an OCW
or just ocw
How does this test work..seems like speed makes the harmonics as the depth changes pressure
but I would really like to hear (read) the science of it
thanks guys

The best read I've found on this topic is Chris Long's paper ...shock wave theory. This paper is slightly technical but after reading it several times, you can start to understand why seating depth changes affect point of impact and especially group size...accuracy. My understanding of Chris's theory is that when propellant sublimates( explodes) a shock wave( vibration) is created and this wave goes down the barrel to the muzzle and returns to the breech, then repeats until it is damped out( barrel harmonics ). This wave makes 4-6 trips before the bullet exits the muzzle. The best accuracy occurs when the bullet exits when the wave is at the breech, ie the muzzle is vibrating the least. This make a lot of sense. Seating depth positions the bullet to hopefully exit when the shock wave is at the breech. A bullet .020 into the lands is .040 closer to the muzzle than a bullet seated .020 off the lands. The shock wave varies little with changes in propellant charge, but group size can change significantly with .005 change in seating depth.

 

[denobravo]

The best read I've found on this topic is Chris Long's paper ...shock wave theory. This paper is slightly technical but after reading it several times, you can start to understand why seating depth changes affect point of impact and especially group size...accuracy. My understanding of Chris's theory is that when propellant sublimates( explodes) a shock wave( vibration) is created and this wave goes down the barrel to the muzzle and returns to the breech, then repeats until it is damped out( barrel harmonics ). This wave makes 4-6 trips before the bullet exits the muzzle. The best accuracy occurs when the bullet exits when the wave is at the breech, ie the muzzle is vibrating the least. This make a lot of sense. Seating depth positions the bullet to hopefully exit when the shock wave is at the breech. A bullet .020 into the lands is .040 closer to the muzzle than a bullet seated .020 off the lands. The shock wave varies little with changes in propellant charge, but group size can change significantly with .005 change in seating depth.

excellent and thank you