One of the benefits to reloading ammo for precision shooting is the ability to adjust various various components of the load to tune it specifically to your gun. One of those adjustable variables is the bullet jump. Jump is defined as when the cartridge is in the chamber, how far the bullet is from contacting the rifling. From the other article on features of long range bullets, this is the point on the ogive that is the same diameter as the bore of the barrel and is the first part of the bullet to contact the rifling. Reloading allows you to change the seating depth to adjust how far off the lands (rifling) the bullet is sitting.

Now, the reason that this is important is because this is one feature that typically has the greatest effect on group size. From what I have seen, doing all my various load development, bullet jump and powder charge are the biggest contributing factor to the size of groups that bullet is capable of. This is also part of the reason why rifles shoot some ammunition better than other. The theory of what’s behind this is part of barrel harmonics.

Barrel harmonics is a very complex issue on it’s own, so I will save that for a separate article, but you often hear reloaders referring to finding a “node”. What that is referring to is finding a load that leaves the barrel consistently, resulting in better accuracy. The jump from the bullet into the rifling has an effect on the barrel that I don’t even completely understand, but it is noticeable. here is a video showing the progression of my group sizes by changing only the amount of jump the bullet has.

As you can see, as the bullet got closer to the lands and my jump decreased, the groups tightened. For this test that was the only thing that changed, powder charge and all other variables were the same. One potential reason for this is chamber pressure. Because the bullet takes up volume in the case, adjusting the seating depth changes the internal volume. That means that the same powder charge will have slightly different pressures because the amount of room in the case changes with seating depth. Another potential reason is the case neck gripping the case. By adjusting the bullet you are changing either how much of the bullets bearing surface the case grabs, or the location it grabs. These changes can modify the way the bullet leaves the case and how uniformly it engages the rifling.

The only thing clear about bullet jump is that it is unique in what each rifle likes. From most people I know the typical starting jump is about .020″. That is usually a good starting point that seems to produce consistent results and is a good place to start. Lately I have been trying to find a sweet spot further back, up to .100″ of jump, for reasons I’ll talk about in another post. But the typical rule of thumb is that a smaller jump is more accurate. One thing to watch for in doing this is to make sure the bullet is not seated to where the bullet is touching the rifling before firing. The rifling can either deform the bullet, or press it slightly further into the case. Both of these can have the effect of causing higher than expected chamber pressure to potentially dangerous levels. Also, the rifling engraving the bullet can cause the bullet to get stuck in the barrel and be pulled out of the case if you try to unload the cartridge without firing.

Overall, when it comes to reloading it comes down to iterations and trial and error to see what works for that particular bullet. Unfortunately there is no one size fits all, and other reloading variables are involved, but watching group sizes shrink as you get a load dialed in is very satisfying.