Bullet Jump/Jammed - Is it the Bullet design or the rifle

[Doom2]

Let's say you have a pet load for one caliber. Let say it is 180 grns secant type bullet. Let's say it likes to jump 0.040". If you load a 180 grn "tangent" type bullet, same rifle, same everything else, will it be the same 0.040" jump?

The answer is maybe. It has little to do with ballistics. The variables that seating depth primarily effect are bullet alignment and neck tension.

Bullet alignment is a function of both case runout and chamber dimensions. It will very from rifle to rifle.

Neck tension will affect combustion to a some extent but the primary effect is on time in the barrel. This gets gets into the optimum barrel time theory.

For your case it's likely the depth will be different because the bearing surface of the bullets will be different so velocity and barrel time will be different.

 

[Laguna Freak]

There are a couple of articles by him actually and I have reference them before. I am trying to understand the mechanics and is it the bullet, the rifle, the cartridge..
I am not questioning the methods...I just always try to get the why

I wish I could help with that but the dynamics of the equations exceed my grasp. I'm left with processes of elimination.

Please post up when you figure it out.

 

[asd9055]

I understand barrel harmonics, maybe I should ask the people that do the FEA for some modelling

 

[asd9055]

I wish I could help with that but the dynamics of the equations exceed my grasp. I'm left with processes of elimination.

Please post up when you figure it out.

I don't think I CAN figure it out, but I know we have a lot of intelligent and experienced handloaders/marksmen here can help shed light
I learned so much here over the years...

 

[sedancowboy]

I wonder if when we change seating depth that we are actually changing the timing of the release of the bullet from the muzzle. We are seeking the bullet to be released under a certain point in the harmonic wave at the muzzle for consistent hits on target so we adjust seating depth in an attempt to control the release of the bullet from the muzzle.
I have been reloading for 51 years and the farther along I go the more important neck tension and seating depth become for consistent results. I might add that a good muzzle crown is very important to good accuracy.
However I don't think any of this matters if you don't have a consistent chamber and throat with the correct leade. Hybrid ogives seem to be more consistent and less seating depth sensitive than the other two.
While I have stated the obvious above to try and add to the question that we may never know the answer to. Is it the rifle or the bullet? It is both different styles of bullets might need different leade angles and different free bore lengths.
Weatherby learned early on that a long freebore would lengthen the pressure curve or lower the peak pressure curve to acceptable levels without decreasing acceptable accuracy. Sometime read the Huston warehouse testing which has been published here in the last few months. Their results were bullets always jammed for best accuracy. My experience is that most modern bullets capable of the level of accuracy we have come to expect do best with some freebore and a 1 1/2 degree of leade angle.

 

[Dragoon300]

Weatherby magnums, and Remington ultra magnums need long jumps to deal with the pressure of their ammunition which is generally high velocity. Shorter jumps are generally more accurate because the projectile has a shorter distance to the rifling and stability. This means less chance of entering the rifling crooked.

 

[Tripodmvr7]

Getting close to the lands makes for pressure spikes and seating to a specific COL is very difficult as thousands of an inch can mean large differences in pressure and speed. For hunting loads my normal advice to rookies are to obtain a jump of at least 1mm with cup and core bullets and 2mm with monolithic bullets. This ensures chambering of the ammo - much more of a criteria than not being able to fire a shot. The proviso is that enough shaft contacts the case neck (minimum of 80% of calibre = 7,82x0.8 for .308 bullet). The other determinant is of course the magazine length.

American rifles normally have small freebore, but with European makes a jump of 6 to 10mm is normal. Even with these large jumps my 7x64 gives better than MOA without going to great lengths. The video below relates to the distance from the lands and is a practical approach to the problem posed.

 

[Dragoon300]

Jump can be greater if the projectile is long enough to still be in the case neck when entering the rifling. If it is not long enough it is space walking to the rifling. Mechanicaly the longer the jump, the more chance of error creeping in when the projectile enters the rifling. This is not to say long jumps are not accurate, they can be. However, they are not generally more accurate than seating closer to the rifling.

 

[asd9055]

Getting close to the lands makes for pressure spikes and seating to a specific COL is very difficult as thousands of an inch can mean large differences in pressure and speed. For hunting loads my normal advice to rookies are to obtain a jump of at least 1mm with cup and core bullets and 2mm with monolithic bullets. This ensures chambering of the ammo - much more of a criteria than not being able to fire a shot. The proviso is that enough shaft contacts the case neck (minimum of 80% of calibre = 7,82x0.8 for .308 bullet). The other determinant is of course the magazine length.

American rifles normally have small freebore, but with European makes a jump of 6 to 10mm is normal. Even with these large jumps my 7x64 gives better than MOA without going to great lengths. The video below relates to the distance from the lands and is a practical approach to the problem posed.

I appreciate the video. I am not asking how to perform the depth seating. The question is more about what factors determine the jump distance, bullet design? rifle throat/lead? barrel? cartridge? Has anyone put more thought in it and has some conglusions?

 

[misterc01]

Cant find that, but I can find "Handloading for Competition Making the Target Bigger" by same author.
I am not looking on "how to", I am looking on what/why, the physics. Is it the bullet design, the rifle, the cartridge...i.e., Weatherby mag vs win mag vs prc or Creedmoor. We know Weatherby designed his with extremely long jump.

Oops = we are talking about the same book - memory hiccup! I took alot of what he said as an explanation of why as well.

 

[asd9055]

Oops = we are talking about the same book - memory hiccup! I took alot of what he said as an explanation of why as well.

Thanks. I am just wondering if in his vast experience with various calibers and rifles, he has reached any other conclusions.

 

[redleg1013]

Let's say you have a pet load for one caliber. Let say it is 180 grns secant type bullet. Let's say it likes to jump 0.040". If you load a 180 grn "tangent" type bullet, same rifle, same everything else, will it be the same 0.040" jump?

Scienfic method my friend, grab up some Spitzers and some VLDs, a comparator and test your hypothesis.

 

[misterc01]

Have you contacted one of the Master bullet smiths from a manufacturer? They might have the answer for you.

 

[asd9055]

Scienfic method my friend, grab up some Spitzers and some VLDs, a comparator and test your hypothesis.

Great idea, but to have a statistical large enough sample to make conclusions it would take time. Plus, I don't have to re-invent the wheel if someone has already invented it...