Why does 'jump' matter?

[longlowdog]

Why does bullet jump to the lands make a difference to accuracy? If the bullet travels say 28" down a barrel to exit at 2850fps regardless of how it starts its journey,, in the lands, small jump or large jump), what effect has the start of the journey made on the end of the journey?
Further more if some bullets like to bridge the gap and others don't how does the "concentric entering of the rifling" arguement hold up?

 

[DrJeckyl]

If you so much as scratch the surface of the bullet, especially at the base, you will very likely have a flier from the gasses using the raised portion as more leverage to send the bullet askew.

Take a basketball that doesn't enter the ring just right, what happens...it bounces around till it drops, this is an extreme way of making this clear.

If you have the bullet enter at an angle, just ever so slight, it will dig into the side of the jacket until it straightens out in the bore, now you have a mark on one side of the bullet and since we are spinning in the neighborhood of 200,000 RPM's or more, and traveling at over 3 times the speed of sound, aerodynamics play a part as well as balance.

The rotating bullet now might have a larger spiral with the blemish but if the bullet were supported at the lands and the case, it would be guided into the barrel more smoothly, or unless your neck thickness is uniform all the way around, and your chamber is trued and the bolt face is square with the bore..yadda, yadda, yadda.... jumping a greater distance will likely be less problematic since everything is more lined up from the start.

Different strokes for different folks, some like to touch, some like to jump, watch the pressure if touching but some bullets like it in the lands and some don't, just have to experiment.

HTH

I think that's the way it works

 

[Twud]

Are you saying that jumping a bullet causes this, or jamming?

 

[LHSmith]

Not an expert, but it is my understanding, a round loaded with the bullet "cocked" in relation to the case will enter the rifling and exit the muzzle "cattywonkers" when either jumped or jammed ...don't matter.....and this will show up downrange with less than optimum results. This is not from a ballistician, but a mere mortal who pounds on nails for a living.

 

[tenring]

Theoretically, jumping a bullet produces less pressure than jamming it into the lands. If the chamber is cut properly, a bullet will self center slightly when jammed. A bullet which begins its journey straight should exit the muzzle without yaw. But, jumping a bullet into the lands sometimes produces very good accuracy. You won't know unless you experiment. In general, VLD's shoot best when jammed .005 - .020"

 

[shootingsight]

I agree that I'm not sure I understand the theory of jumping, or not.

If it were purely a case of making sure the bullet were fully straight in the bore, I'd make the case to start the barrel as a smooth bore, and only have the rifling start after the entire bullet's bearing surface were in the smooth bore. Trying to justify that a bullet pushed into the rifling by a case, or pushed into the rifling by an explosion will result in one being straighter than the other does not make intuitive sense.

The other rationale that's been thrown around is the difficulty of pushing a bullet into the rifling if it is already started, versus standing off, so you can get a 'running start' and have some momentum to get you going, which might cause more or less pressure spikes. I'm not sure I follow this logic either. Making the math easy by assuming a 30" barrel and a 3,000 fps projectile, you gather 100 fps speed for every 1" of barrel length. After only a 0.020" jump, you will only have 2 fps of 'running start'. Even if you make the case that acceleration is not linear, it's still a small number.

Clearly, there are measurable differences in accuracy depending on standoff, so there is something to it, I'm just not sure if anyone understands exactly what it is.

Art

 

[BoydAllen]

What we have to work with are mostly theories, except the part about differences in pressure. If you look in the front of some reloading manuals, in the general instructions on reloading, you will many times find information, determined by actual testing, of the differences in pressure of loads that have their bullets set to jump as compared to touching or into the rifling. The difference is often given as 5-6,000 PSI, I believe. Older manuals will give pressure as Copper Units of Pressure,CUP), which are close to PSI but not identical, due to the difference in equipment used to take the readings. This difference in pressure may not work the same way in some small cases where setting the bullet back, significantly reduces the powder space, or at least it has been reported to be the case on this web site, with heavy bullet 6BR loads. I have no experience with this. My 68 gr 6PPC loads all seem to behave in more or less standard fashion, producing less pressure, with a given amount of powder, with bullets seated off the lands. If you are going to jump bullets, pay particular attention to loaded round concentricity, and uniformity of neck tension within a particular set of ammo.

 

[DrJeckyl]

If it were purely a case of making sure the bullet were fully straight in the bore, I'd make the case to start the barrel as a smooth bore, and only have the rifling start after the entire bullet's bearing surface were in the smooth bore.

Because the bullet would be traveling extremely fast in such a short distance, starting the rifling too late I would think that the bullet couldn't grab the rifling right away and basically try to plow down the barrel against the rifling, possibly damaging the jacket, start it twisting from the start and you will have a more uniform pattern of rifling on the bullet.

Trying to justify that a bullet pushed into the rifling by a case, or pushed into the rifling by an explosion will result in one being straighter than the other does not make intuitive sense.

Would not make much difference if you could GUARANTEE that a jumped bullet is is facing DIRECTLY down the bore and needs no further alignment as it's sitting in the chamber waiting to be fired.

I'm not a gunsmith, but the actions that have been properly trued and the chambers that have been cut into the barrel that are basically perfect, then having both the action and the barrel matched to each other, can more or less guarantee a bullet will leave a accurately prepped case and enter the rifling perfectly straight, off the shelf guns generally cannot guarantee this will happen, some do shoot the lights out though.

Making the math easy by assuming a 30" barrel and a 3,000 fps projectile, you gather 100 fps speed for every 1" of barrel length. After only a 0.020" jump, you will only have 2 fps of 'running start'. Even if you make the case that acceleration is not linear, it's still a small number.

The bullet will reach it's peak velocity within the first 4-6" of barrel length, the gasses will keep the bullet moving depending on powder burn rates, it doesn't accelerate at or near a linear fashion to the entire length of the barrel.

The thing on the jumping is this will greatly reduce pressure spikes. Giving the bullet a "running start" even if it's only .020 or .030 of an inch, it's already moving when it contacts the ogive. This will in turn keep moving as the pressure is building allowing a larger volume of space behind the bullet for the pressure to build, jam it in the lands and the pressure will build very high before it can get the bullet moving. How much force does it take you to push your car from a stop as opposed to keeping it moving on a flat surface at a slow speed? Pressure inside the case rises extremely fast, if you are in the lands, be sure you have reduced the charge a bit and work back up, dont take a max load and start in the lands.

Take a dart and try to push it into the dartboard, then throw it using a normal throw, dart is not moving very fast but buries itself deep in the board as opposed to being pushed, the bullet having a running start has ALOT of momentum but may not appear to.

Another kinda cool bit on pressures, but a different application and principal altogether...take a piece of newspaper, 1 full sheet, and lay it on the table right to the tables edge. Place a standard school-issue wooden ruler under the paper leaving about 3-4" overhanging the table, keeping the newspaper on the table entirely, press down on the ruler slightly and allow the air to get under the newspaper and you can push the ruler all the way down, raising the paper.

Do this again but whack the ruler with a piece of 2x4 and the ruler will likely break before the paper lifts, all from about 14 PSI of pressure on the paper pushing down.

Now imagine 50,000 PSI.