What Is Bullet Jump?

One of the benefits to reloading ammo for precision shooting is the ability to adjust 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’s 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.

Why Jump Affects Accuracy

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

Barrel Harmonics and Finding a Node

Barrel harmonics is a very complex issue on its own, so I’ll save that for a separate article, but you often hear reloaders referring to finding a “node.” What that’s 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’s noticeable. Here’s a video showing the progression of my group sizes by changing only the amount of jump the bullet has.

Why Jump Changes Group Size

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, which means the same powder charge will have slightly different pressures since the amount of room in the case changes with seating depth. Another potential reason is the case neck’s grip on the bullet — by adjusting the seating depth, you’re changing either how much of the bullet’s 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.

Finding the Right Jump for Your Rifle

The only thing clear about bullet jump is that what each rifle likes is unique. From most people I know, the typical starting jump is about .020″ — that’s usually a good starting point that seems to produce consistent results. Lately I’ve 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 when doing this is to make sure the bullet isn’t seated to where it’s touching the rifling before firing. The rifling can either deform the bullet or press it slightly further into the case, both of which can cause higher-than-expected chamber pressure, potentially to dangerous levels. Also, the rifling engraving the bullet can cause it 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 iteration and trial and error to see what works for that particular bullet. Unfortunately there’s 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.