Accuracy & Precision vs. Bullet Jump

@JDinFbg, one of the reasons why Barnes recommends at least .050" or more is because of the high engraving force for solid copper bullets. A longer jump allows case pressure to drop as the bullet moves forward to make contact with the lands.

It's simply not true that the best loads are always found with the bullet .020" or less from the lands, or even into the lands.

Berger has interesting articles on their website about bullet jump.

Berger Bullets | VLD - Making it Shoot

Setting Up Your Loads for Optimum Accuracy | Berger Bullets

I have found excellent accuracy with jumps of anywhere from -.010" to .250". The factory Hornady 147gr ELD Match 6.5 PRC load has a jump of around .080" in my custom Kimber and shoots like this for 5-shot groups.

I had fully given up on Barnes and other monometal bullets in rifles after trying and failing on multiple occasions to achieve even remotely acceptable groups. Then several years ago, a colleague pointed out they need at LEAST 50thou jump. I revisited, jumping the bullets 50-120thou, and sure enough, they shot extremely well.

Alternatively, with lead-core bullets such as the 105 Hybrid, I have fired groups as small as I can with anything, ever, while jumping anywhere from 5 thou to 150 thou. I don’t spend time chasing lands in my PRS rifles, and 6 creed eats throats like crazy, so I start close and let it work it’s way out on its own.

I'm not the most knowledgeable person on stuff like this. But based on my reading certain bullets are more sensitive than others to bullet jump. I've gotten mixed results with both Berger and Barnes bullets. By all accounts both of those can certainly be extremely accurate, if you get them seated to the right spot. But for me it was just easier to switch to another brand of bullet that was less sensitive. I've had better luck with Lapua Scenar and Hornady bullets.

jmr40 said:

I'm not the most knowledgeable person on stuff like this. But based on my reading certain bullets are more sensitive than others to bullet jump. I've gotten mixed results with both Berger and Barnes bullets. By all accounts both of those can certainly be extremely accurate, if you get them seated to the right spot. But for me it was just easier to switch to another brand of bullet that was less sensitive. I've had better luck with Lapua Scenar and Hornady bullets.

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The only problem I've ever had with Barnes bullets is related to less than ideal barrel twist rate and air temperature due to solid copper bullets being long for caliber/weight. Berger, Lapua Scenar and Hornady have all been excellent as well.

Tangent ogive (e.g. SMK) are supposed to be more forgiving than secant (e.g. Lapua Scenar) when it comes to jump. If anyone's wondering about the difference between tangent and secant bullets it refers to the arc of the ogive and how it blends into the bearing surface of the bullet. Tangent means no discontinuity between the ogive and bearing surface i.e. the angle between the two surfaces at the interface is zero degrees. With secant bullets you can see a discontinuity between the two surfaces in the form of an annular ring because the angle between the two is not zero. I hope this makes sense.

JDinFbg said:

I've read that Weatherby rifles are designed with very long throats, which results in a long bullet jump, and that many folks achieve very good accuracy with them.

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Yes that is correct, in their proprietary cartridges. Pretty much for the same reason Barnes recommends a long jump with their projectiles, to control pressures.
Weatherby magnums digest massive amounts of smokeless, some nearly 100gr.
Gotta give that bullet some un hindered movement down the tube with that kind of fireball behind it.
I worked on a 300 Weatherby for a guy yrs ago, made a dummy cartridge to check the freebore, and found out quick they wasn't enough bullet to hang out the end of the cartridge. A little research, and I learned that little tidbit about the Weatherby magnums. And yes, it was a fabulous shooter, but only 2 cracks at it for me.
Makes my shoulder ache just thinking about it

I've always had great results out of most of my rifles with a .010" - 0.020" bullet jump, but shooting 140gr Berger VLD-H bullets out of my 6.5-06, I have to seat them to have a 0.070" jump in order for the rounds to fit in the blind magazine. That rifle will still shoot well under MOA with those bullet's jumping 0.070".

I have a .308 that tolerates jump as long as I’m using the right bullet geometry. The limitations here are loading to fit the magazine, which generally nets around .120” jump. Still able to consistently keep hunting loads in the 1/2 MOA 5 shot groups range across multiple loads

@JDinFbg, that's excellent information there. Thanks.

I look at jump differently as I have no clue in either if my .308’s what the jump measurement might be. I utilized the seating depth to determine what the rifles liked - same science but different method. I really do not care about jump as my seating depth tells me everything at the target - again the jump, I haven’t got a clue.

Jumping right around 70k getting .5 .6 5 shot groups with nosler rdf 130 gr. For this rifle I loaded all test rounds to max mag length and just tested a variety of charge weights. Basically just finding the best combustion and let the rifle do what it’s supposed to do.

I don't doubt that bullet jump affects accuracy but I can't wrap my head around why it affects accuracy.
I can't understand how thousandths difference changes anything when the bullet is traveling 20 inches or more down a barrel.

The mechanics of it is a mystery to me.

DukeConnor said:

I don't doubt that bullet jump affects accuracy but I can't wrap my head around why it affects accuracy.
I can't understand how thousandths difference changes anything when the bullet is traveling 20 inches or more down a barrel.

The mechanics of it is a mystery to me.

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There are a lot of things like that in load development. You have inputs that you can control and outputs that you can measure. Everything else goes into a black box

Many people say load development is essentially three areas:
Combustion
Harmonics
Ballistics
Im not positive where seating falls in but it definitely falls in, ill go with Harmonics.

Im curious as to what the experts say.

DukeConnor said:

I don't doubt that bullet jump affects accuracy but I can't wrap my head around why it affects accuracy.
I can't understand how thousandths difference changes anything when the bullet is traveling 20 inches or more down a barrel.

The mechanics of it is a mystery to me.

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You're not alone here. It's a mystery to most everyone, even the big hitters such as Bryan Litz. Many put out plausible explanations but until someone pursues a PhD in this stuff we'll probably never know why it makes a difference. I find it interesting that Berger recommends dialing in bullet jump before powder charge. QuickLoad has no subtle provision for engraving forces or allowance for the bullet being on or into the lands, but you can see that a 0.1gr change in charge weight has a larger effect on case pressure and barrel time than a .005" change in jump.

I'll pipe in as an aspiring student of the NRA high power mid range prone game. If you've ever had to remove a chambered round and had it come apart and spill powder in your action (don't ask me how I know) you might be less enamored of "jamming". I still hear competitors talk about jamming but sometimes the line has to go cold, the electronic targets go down or staples will fail and a target starts a flappin' so a ceasefire gets called and the potential for swearing as meticulously assembled uniformly neck tensioned rounds getting disassembled ensues. Jamming aside here's a very interesting explanation of jump, jam and the quest for the accuracy node that I came across the other day.

Enjoy!

I know that seating depth does change things in a load. Just like testing combustion with a set length you can test a set length for combustion by trying different charge weights. I suspect it has to do with the over all balance of the load just like the other variables. Some say it’s about finding the sweet spot where the bullet engages in the lands with the least rickets possible. Each rifle is a little different

@bpshooter13, thanks for the video, it was interesting but it leaves me wondering about those of us that start out with a load at max COL for the magazine. We can't push the bullet out in .003" increments to chase the lands as the throat erodes. This indicates that for rifles limited by magazine length, you want to start out with a load that's "significantly" shorter than magazine length so that the bullet can move out.

I was also interested to hear that jump nodes lie within a .006" window so .003" difference between loads is the way to go when doing jump testing.

How it effects dispersion, is in the name. When an unfired cartridge is in the chamber the bullet is held in reasonable axial alignment with the bore by the case neck. Normal case design has all or almost all of the cylindrical body of the bullet inside the case neck. In order to keep the bullet axially aligned in the bore, the cylindrical section of the bullet cannot get cocked in the free-bore (or leade) before engaging the rifling. When the pressure starts to push the bullet out of the case the friction is not equal all around the case neck, and the bullet will tilt toward the draggier side of the neck. The free-bore is not the same size as the bullet but has a small clearance, so it can tilt. The bullet must "jump" from the case neck to the start of the rifling and maintain axial alignment. The longer the distance from the ogive to the start of the rifling, the more opportunity for tilt. So, the tilted bullet goes down the bore with the long axis sweeping out a cone, due to the spin. When the bullet leave the constraints of the barrel, aerodynamic and gyroscopic forces will realign the bullet axis to some equilibrium yaw angle, and thus generate side forces on the bullet and cause the bullet to drift in that direction. Since the initial tilt direction is random, the resulting drift direction will also be random.