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[HRM Johnny]

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[Taylorbok]

@Rflshootr your definition of jump is not congruent with the rest of the shooting community

Jump is the distance your bullet has to move forward to reach the lands.

in your described situation the addition of .100" FB has taken the bullet from touch to .100 jump. Freebore is not always equal to jump but the way you've presented this makes it so.

 

[59FLH]

"Jump on the other hand is when the base of the bullet has left contact with the mouth of the case and is no longer supported by the neck."

Does that not occur upon case expansion?

 

[Rflshootr]

Again, another case of bad information being passed on until people believe it to be so. Kind of like the government and the news media telling you black guns are bad.

 

[Taylorbok]

The way most chamber are set up the case mouth loses contact with the projectile almost instantaneously as it expands to the chamber wall. If you needed the base of the bullet to clear the case neck before it was considered jump basically no chambers would have jump, the projectile is still "inside" the case neck as it engraves the rifling.

 

[Rflshootr]

"Jump on the other hand is when the base of the bullet has left contact with the mouth of the case and is no longer supported by the neck."

Does that not occur upon case expansion?

If it happened the way you're thinking, then why does neck tension play a role? So, what you're tell me, is the neck immediately expands and lets the gases go past the bullet before it even gets to the rifling? To have expansion of the brass, there has to be enough pressure to overcome the constraint of the brass. Before that happens, the bullet has the least resistance of the 2 and moves forward. Then the engraving forces take over.

 

[Taylorbok]

Again, another case of bad information being passed on until people believe it to be so. Kind of like the government telling you black guns are bad.

If you want to make up your own definition for things I guess you can but don't expect others to recognize that.

If it happened the way you're thinking was so, then why does neck tension play a role?

Neck tension has very little affect on anything (Other than holding the bullet)

 

[59FLH]

If it happened the way you're thinking, then why does neck tension play a role?

So the bullet wont fall out of or unwanted movement in the case?

 

[Tiny Tim]

Let me rephrase so you'll understand where I am going with this. If you chambered a barrel with a .000 free bore reamer, and then found the length of where a loaded round just touched the rifling and you had that measurement, then you pulled the barrel and throated it for .100 free bore and reinstalled it, the bullet should have to move .100 further out to touch the rifling, correct? If you now fired that round that was seated at the original measurement, is that jump or free bore that the bullet is moving?

The barrel/chamber now has .100" free bore. In your scenario, the loaded bullet will also have .100" jump to engage the rifling. In this example, the two numbers just happen to be equal.

Missed some of the posts before sending. Now I understand your definitions of these terms. Not certain which is technically correct, but i am open to learning.

 

[Tiny Tim]

So when I tool my OAL guage I assume I was measuring where the bullet touches the lands. So would Free-bore begin after a space that rifling is taken away? Or am I measuring wrong?

Darrin

You are measuring correctly. As others are hashing out the terminology. Free bore is generally accepted as the length of rifling removed ahead of the chamber neck (approx .0005" over bullet diameter). This does not include the leade (tapered transition from the free bore to full land height). The two together form the throat.

Free bore is fixed, unless you modify it or it erodes. Your seating depth adjusts the bullet "jump" to engaging the rifling.

 

[Rflshootr]

So the bullet wont fall out of or unwanted movement in the case?

So you can fine tune the powder burn by controlling how long and how much pressure it takes to get the bullet moving before it touches the rifling and before the neck expands.

 

[cape cove]

Neck tension has very little affect on anything (Other than holding the bullet)

Most certainly not the case in my testing. NT plays a big role in cartridges like my Weatherby ones or if your are jumping .030 or more when it comes to accuracy. Just ask the benchrest guys. My PPC likes .004 NT much better than say .0015 when using N-133. When jamming the bullet it plays somewhat less of a role concerning accuracy but still has an effect. JME

 

[Taylorbok]

Most certainly not the case in my testing. NT plays a big role in cartridges like the Weatherby ones or if your are jumping .030 or more when it comes to accuracy. Just ask the benchrest guys. My PPC likes .004 NT much better than say .0015 when using N-133. When jamming the bullet it plays somewhat less of a role concerning accuracy but still has an effect. JME

IME it has little affect but it is good practice to keep it consistent.
Almost everyone that thinks they have proof of these small changes making a big difference presents 3-5 shot groups. I've been fooled by it also. If you open up to 10 or larger shot groups the lines start to blur and the difference seems to go away.

 

[Treeslug]

Again, another case of bad information being passed on until people believe it to be so. Kind of like the government and the news media telling you black guns are bad.

Black guns matter ! Black trucks matter ! Black birds taste like dove !