RELOADERS CORNER: Bullet Jump: does it really matter? (Part One)

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The distance a bullet travels to enter the lands is a topic of much concern to the precision shooter. This series takes a look at why it matters, and also when it doesn’t…

Glen Zediker

bullet jump
Here’s jump: it’s the distance from the end of the case neck portion of the chamber to the first point the bullet will engage the lands or rifling.

Bullet jump: the open space a bullet must span until its first point of sufficient diameter engages the barrel lands.

Last week I had a long phone conversation with a fellow who had been bitten by two bugs, two somewhat conflicting bugs (at least seemingly so on the onset). The one was a regrouping equipment project for USPSA-style practical rifle competition, and the other was for a desire to maximize accuracy, which is to minimize group size. This fellow had been involved in competition long enough to decide to stay with it, and was re-upping his AR15 upper with a new custom barrel. He wanted to have the best accuracy he could buy, and that’s a worthwhile pursuit as long as there’s a budget that supports it.

The subject of bullet jump became the dominant topic.

Yep, he had read my books and a few others and developed the impression that minimizing bullet jump was one crucial component to maximizing accuracy. That’s fair enough. I’ve gone on about it, as have others. Adjusting bullet seating depth can make a big, big difference in shot impact proximities. However! The reason bullet jump matters — usually — is largely, almost exclusively, because of some bullet profiles being more finicky than others. Namely the longer and spikier “very-low-drag” type bullet profiles.

The first point of “major diameter” on a bullet is what coincides with the land diameter in the barrel. If that’s a .22 caliber with 0.219 diameter lands, then the first point along the nosecone of a bullet that’s 0.219 is the distance. Gages that measure this distance (Hornady LNL for instance) aren’t necessarily going to provide perfect coincidence with land diameter, but still provide an accurate bullet seating depth that touches the lands.

If you find the cartridge overall length, which really means bullet seating depth, that touches the lands (coincides with land diameter) then subtract that from what you then measure when the bullet is seated deeply enough to fit into a magazine box, that right there is the amount of jump.

Hornady LNL gage
There are other ways to find it, but the Hornady LNL Overall Length Gage makes it easy. I’d be lost without this tool. Use it to determine the current distance to engage the lands for any bullet you’ll use (it works also as a way to monitor throat erosion). Get one HERE

Dealing with an AR15, or any other magazine-fed rifle, assuming we are wanting the rounds to feed from the magazine, is that there’s a finite cartridge overall length that will fit into the magazine. So. We’re almost always going to be dealing with some amount of jump, unless one or two things can be manipulated to reduce or eliminate it.

AR15 loaded magazine
A box magazine sets the effective limit on overall cartridge length. Getting a bullet to sit close to the lands when the round is chambered requires either some trickery in chambering specs or, mostly, bullet selection. However, with selection there is also limitation. For safety’s sake, no factory loaded round is going to approach lands-on seating structure. When a bullet touches the lands at rest, pressures will, not can, spike. All good, if it’s accounted for. This sort of “fine-tuning” is strictly a (careful and knowledgeable) handloader’s realm.

The one is that the influence of rifle chamber specs with respect to either more or less jump is pretty much exclusively in the leade or throat. That’s the space that defines the transition from end of the chamber case neck area to entry into the lands. The closer the lands are to the chamber neck area the shorter the jump will be with any bullet. That is the leading difference between a SAAMI-spec .223 Remington chamber and a 5.56 NATO chamber. The NATO has a much longer throat. I’ve written on that one a few times…

A shorter throat has goods and bads. The main good is that, indeed, any and all bullets are going to be closer to the lands in a round loaded to magazine-length.

But the “two” in the things that influence jump is bullet selection. It is possible to find a combination that will easily have the bullet sitting right on or very near the lands at the get-go. That’s going to be a short, and light, tangent ogive bullet within a SAAMI-spec .223 Remington chamber, or (and this is what I have done) a barrel chamber finished using a throating reamer to get even closer. In general: the nearer the first point of major bullet diameter (remember, that’s the land diameter) is to the bullet tip, the shorter the jump will be, and that’s because this point is “higher.”

Hornady 52gr HPBT
Looking through a good factory loading manual down amongst the “lighter bullet” selections, take a notice of the overall round length used in the test ammo. Magazine box maximum for an AR15 is 2.25 inches (it’s actually 2.26 at a maximum, but don’t cut it too close). If you see a length less than that, then there’s a bullet that can be seated on or near the lands at magazine length. Simple.
Here’s a great example that I can tell you absolutely will engage the lands in an AR15 loaded to magazine length. As a matter of fact it will be over 0.020 into the lands at magazine length, so certainly must be loaded to an overall length well less than that… It also shoots little groups. Check it out HERE

Throat erosion is going to lengthen the throat. Can’t stop that. The cartridge structure that was jumping, say, 0.005 on a new barrel is jumping more than that after literally every round fired through it. After some hundreds of rounds it’s jumping a few multiples of 0.005. (How much or how many is not possible to forecast because way too many factors influence the amount and rate of throat erosion. Just have to keep checking with the gage I suggest you purchase.) This is the reason I specify a custom dimension to get reduced jump: with the right hands using a throating reamer it’s easily possible to maintain land contact at magazine length seating even after a lot of rounds have gone through. Bullets will begin being seated more deeply and then get nudged out as the throat erodes.

So, where the conversation ended was this: If (and only if) someone is willing to take the time and make the effort to carefully establish and then control a reduced or eliminated amount of magazine-loaded jump then, yes, it’s a fine idea! It’s also an idea that likely will result in the best accuracy. I’ve done it in one of my AR15 match rifles, and it’s the best shooting I’ve ever owned. The hitch is that the rifle becomes what I call a “one-trick pony.” It’s not always going to accept bullets and loaded round architectures that stray from the carefully calculated dimensions originally set down. It’s also not likely going to perform safely with every factory-loaded round out there, and you can forget (totally forget) ever firing a NATO-spec round.

There’s a whopping lot more to this whole topic, and we’ll look at the other end of the spectrum next time.

The reason that reduced amounts of bullet jump increase accuracy, in a perhaps overly simple but entirely correct way to understand it, is because there’s simply less potential for disruptive entry into and lands and then through the bore. There’s less misalignment opportunity, less jacket integrity disruption opportunity. There is a lot more that can be discussed in finer points, of course…

The preceding was adapted from Glen’s newest book, Top-Grade Ammo, available here at Midsouth. For more information on this book, and others, plus articles and information for download, visit ZedikerPublishing.com

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10 thoughts on “RELOADERS CORNER: Bullet Jump: does it really matter? (Part One)”

  1. Thanks, as usual, for the info. I hand load my 7mm. Remington Mag. I take a cartridge and slightly open the throat so that the bullet fits loose. Then I use a black permanent marker and coat three quarters of the bullet from the bottom up towards the tip. Then I put pressure on the edge of cartridge mouth until the bullet “holds”.
    I place this in my rifle carefully, and lock the bolt, then remove it. It will engage the lands and grooves when locking the bolt and it will give you that distance for that particular bullet. Any other bullet must be repeated. I have mine 0.003 to 0.005 of a gap. There is a big difference from factory ammo. I get less than 1/2″ at 100 years from a bench. And I’m still a novice! Thanks again for any insight and perspective.

    1. I load all of my Hunting Ammo, That they just start to touch the Lands, On all of my Rifles, this has worked for me for as long as I can remember etc, I get perfect groups & when hunting the Ammo id Dead on Target etc.

  2. Sometimes this can fool you. The late Dan Hackett, writing in the 1995 Precision Shooting Reloading Guide, mentioned having a Remington 40X in 220 Swift that average 1/2″ groups of five shots with nothing better than 3/8″ nor worse than 3/4″ at 100 yards. That sounds good to most of us, but he was a benchrest shooter, so that was unsatisfactory to him. Then one day, he switched from one bullet to another 0.015″ shorter during a loading session, accidentally turning the micrometer adjustment on his seating die the wrong way. This resulted in bullet ogives seated 0.050″ from the lands instead of 0.020″, as he’d intended. He had 20 rounds loaded before he noticed the error. He considered pulling them to “correct” the seating depth, but instead decided just to shoot them in practice. To his astonishment he got two 1/4 inch groups and two true bugholes in the “ones” (one to two tenth inch range).

    In another example, Berger has on their site under their Tech Talk heading, an article called, VLD—Making It Shoot. In it they describe how some guns won’t shoot long secant ogive bullets until they are 0.150″ off the lands, even though most like them closer or jammed -0.010″ into the throat. They look for the seating depth sweet spot in gigantic 0.030″ steps, like the one Dan Hackett accidentally took.

    The main takeaway I got from the information from Hackett and Berger is that the guys starting with bullets 0.015″ or 0.020″ off the lands and creeping forward in 0.002″ steps, squinting hard at their groups to see anything that might be a statistically significant improvement, are at risk of not seeing the forest for the trees. Try some bigger steps and try them further out than conventional wisdom suggests should help or even work at all.

  3. Since this has been brought up again, I’m going to bring this back to read. Previously written on……

    July 11, 2016 at 2:32 am
    Since I started reloading in 2009 I have come across several articles as well as to my own way of thinking as to regarding this ” Land Loading”, for reducing jump. I have certainly found out that it improves the accuracy by a (guess) 300% . More or less like much like Robert, the way I had read and interrupted how to go about doing it was to set a case, THAT HAS BEEN FIRED IN THAT RIFLE ( most accurate head space) , cleaned and other wise left unchanged, perhaps with the slight exception for perhaps a light “crimp” if needed to hold the bullet in place, that is almost ready for primer, powder, and bullet…less the primer, powder, just insert the bullet.
    I’m probably going to say all the wrong words to describe this operation, hopefully you will understand and follow what I mean. I loosely insert the bullet (colored with a magic marker) in the cartridge case and making sure that it will not slide much and hold it’s position in the case, insert it in the chamber and bolt it into position as if ready to fire. I then eject as carefully as possible trying not to upset the relationship of the brass and the bullet that was seated to the lands of my rifle. I do repeat this operation several times on like length cases and bullets to ensure all measurements are VERY close together. An average is the best form of accuracy over several like items. At that time I transfer those measurements to paper, don’t trust the mind so much anymore, subtract another 0.002or 0.003 more to give me magazine clearance from OAL gathered at this point and that becomes my OAL for the “Land measurement” to reduce jump from 0.22 more or less to 0.001 jump to lands. Out of 6 rounds fired 5 were in the same hole the other was on the edge of that hole.

  4. I have to say that after looking at the sketch and reading the caption I almost closed the browser and moved on. My thought was that “This guy doesn’t have a clue about bullet jump. Bullet jump has nothing to do with the case neck or the area of the chamber that allows for the case neck.” I did continue to read the article and he did accurately describe “bullet jump” in the text. After reading the article I can say that Mr. Zediker does have a great deal of wisdom about reloading. I enjoyed the article and as I read it I anticipated when we would get to the point where he would have some new insight into the subject that I hadn’t read elsewhere. Sadly he never did, but I am sure that the reloader new to the subject found this article very interesting and informative. I thank Mr. Zediker for sharing it with us.

  5. I load all of my Hunting Ammo, That they just start to touch the Lands, On all of my Rifles, this has worked for me for as long as I can remember etc, I get perfect groups & when hunting the Ammo id Dead on Target etc.

  6. And, once again, it seems to be assumed that the leade is only there to help bullets be more inaccurate. Get rid of it, seems to be the opinion. Well, the closer you put a bullet to the rifling, the more it must be pressed into the lands rather than using acceleration to run fast and hard into them. That is like pressing a stake into the ground rather than using a hammer. Pressure goes much higher. That is my one concern, telling people to minimize the free-start distance of the bullet without mentioning its effects on pressure. This is one of the main reasons that there is a safety consideration about using 5.56 NATO rounds in a rifle designed for only .223. On .223 only rifles, the leade is normally cut shorter, or such has been the rule. The increased pressure of the load of a NATO round is also a concern, but the short leade is the main opportunity for problems. How short a leade one can get by with, well, you can chat about that over tea all day, at least until you virtually eliminate it, and then you may have other things to worry about.

  7. Need more of this kind of info. For me it’s right on the border of understanding the content. When I read the article more than once, It seems to help to clarify the material.

  8. with bullet in contact or close contact with lands would the initial presssure not be higher than in the case of more bullet jump. to me seems likely but I am not a ballistic specialist. thanks joe

  9. I have tested from zero out to .060 on my hunting rifle and it prefers. 040 for best accuracy. I would not hesitate to test out to .100 off if the results are getting better. Best group 1.250 inches at 400 yards. And this is a stock sporting rifle!

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