Tag Archives: JLK

RELOADERS CORNER: Why Not Flat-Base?

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A better question, given that the vast majority of popular rifle bullets are boat-tail, is why flat-base? KEEP READING

flat base bullet

Glen Zediker

Good question! I have something that at least has elements of an answer.

A boat-tail bullet is the standard for the majority of rifle bullets, and the domineering choice of long-range shooters. Competitive Benchrest shooters favor flat-base bullets. Flat-base is also popular with varmint-hunters: the stellar Hornady V-Max line for good instance.

Hmm.

We all want best accuracy, so why the difference? Consider the overriding characteristic of a flat-base bullet: it’s shorter. Now, since not all flat-base bullets are shorter overall than a same-weight boat-tail (they’re usually not), I seriously need to clarify that!

Clarification: a flat-base can be shorter, and lighter, than it would be if the same ogive or nosecone profile used then added a boat-tail. More: if they’re both the same weight and at least similar in profiles, a flat-base often has a longer bearing area than a boat-tail bullet, again because the boat-tail is sticking down there, or not. These are both a bonus to Benchrest or any other shorter-distance circumstance where utmost precision is the goal. (When I refer to capital-b “Benchrest,” I’m not talking about a shooting rest, but a competitive sport.) Shorter bullets allow slower barrel twists (bullet length, not weight, chiefly governs needed twist). Slower twists offer a miniscule improvement in damping a bullet’s orbital pattern in flight, and considering the likewise near-caliber-size 5-shot groups these folks are after, that matters. Bullets fly in a spiral, like a well-thrown football. Again comparing those with similar profiles, flat-base bullets stabilize faster and sooner than boat-tails, it’s a smaller spiral. Bullets with longer bearing areas tend to shoot better “easier,” less finicky. And, flat-base bullets can provide more cartridge case capacity.

vld and ld compare
Here’s unique. Jimmy Knox of the original JLK Bullets once made flat-base versions of his Davis-designed VLD (very low drag) boat-tails. So this is a .224-caliber flat-base 65gr LD (low drag), which is the same as his 80gr VLD shown with it, just no boat-tail. Why? It was more of a “Why not?” Idea was to provide better downrange performance for those with slower-twist-rate barrels, and to retain the flight pattern and in-barrel characteristics he liked about flat-base (and way on more speed). This idea was popular among some better High Power shooters about 15 years ago.

All those good points make it sound like flat-base provide superior accuracy. They might. By my experience, they do, but! Distance defines the limit of that truth.

The boat-tail provides an aerodynamic advantage, and the farther it flies, the greater this advantage. There are well-founded beliefs that boat-tails are less influenced by gas pressure thrusting against the bullet base. A good and most knowledgeable friend at Sierra told me that a boat-tail has an effectively more concentric radius at the base due to the junction point created by the angle on the tail and the bearing surface. Further, a flat-base, is, in effect, harder to make so that the base will have a radius that’s as concentric with the bullet bearing surface. Manufacture care and quality (related), of course, makes that more or less true or false. If the idea is that a good boat-tail is “easier” to make, that this shape makes the end product more forgiving of manufacturing errors, then I’ll accept that since it’s pretty hard to argue against, but, again, I really don’t think that boat-tail designs simply take up slack in quality tolerances. I’m sho no rocket-surgeon but I know that the tail slips the air better.

LD_ and Hornady 68
Same LD bullet compared to a Hornady 68gr HPBT. The 65 is a tic shorter overall but, because it’s a 15-caliber (!) ogive, way less bearing area (exception to the “rule” big-time) than the boat-tail next to it. The 65 had a higher BC but was über-tricky to get to shoot well. I could get these to just over 3000 fps in a 20-inch .223 Rem. Mostly because of the tiny bearing area.

This can get pounded completely into the ground because adding a boat-tail (and I’ll show a great example of just that) to a similar nosecone also adds weight to the bullet, and that increases BC. It’s not exactly a chicken-egg question, though, because the tail helps otherwise.

barts bullet
Here’s a 52gr boat-tail from Hornady (right) next to a 52gr custom Benchrest bullet. I said the overriding difference is that a flat-base bullet is shorter, but that’s not referring to overall length. A flat-base is shorter than it would be as a boat-tail, if the other dimensions were the same, and usually has a longer bearing area.

You might have also heard said that boat-tails shorten barrel life because the angled base directs burning propellant gases more strongly at the barrel surface. They do, and many steadfastly uphold that as a reason against them. More in a bit. However! Beyond 300 yards, at the nearest, there are no disadvantages in using boat-tail bullets that come close to surpassing their advantages.

There’s another debated advantage of a flat-base and that is they tend to shoot a little better in a barrel that’s about to go “out.” I’m talking about a good barrel that’s pushed the limit of its throat. That one is true too!

And speaking of barrel life, another is that flat-base bullets produce less flame-cutting effect than boat-tails. A barrel lasts longer if fed flat-base. True! Flat-base bullets “obturate” more quickly. Obturate means to “block,” but here it means to close a hole, which is a barrel bore, which means to seal it. The angled boat-tail creates a sort of “nozzle” effect. Can’t much be done about that, though, because when we need boat-tails we need them. That is, however, a big score of help for the varmint hunter.

There is a relatively obscure “combo” out there called a “rebated” boat-tail. This has a 90-degree step in from the bullet shank (body) to the tail. It steps in before the boat-tail taper is formed (they look like a flat-base with a boat-tail from a bullet a couple of calibers smaller stuck on there). It’s common for competitive .308 NRA High Power Rifle shooters, for instance, to switch from the popular Sierra 190gr MatchKing to a Lapua 185 rebated boat-tail when accuracy starts to fall off due to throat wear. Sure enough, the Lapua brings it back for a couple hundred more rounds.

rebated boat tail
Here’s a rebated boat-tail. 115 grain 6mm from David Tubb.

If anybody with heavy equipment making bullets for sale out there is listening: I’d like to see some more rebated boat-tail designs! It is, though, a challenge to make precisely.

So. What? So what? Well, if you are big into small groups, I very encourage some experimentation with flat-base bullets. Again, distance is the only limit to their potential goodness. 100 yards, yes. 200 yards, yes. 300 yards, no!

vld chamfer
One thing is for certain: Flat-base bullets are not nearly as easily seated! Some have an edge-radius, some don’t, but, they are very easy get started crooked, or difficult to get started straight, same effect. I strongly recommend taking steps to square case mouths and use a generous chamfer.

This article is adapted from Glen’s books, Handloading For Competition and Top-Grade Ammo, available at Midsouth HERE. For more information about other books by Glen, visit ZedikerPublishing.com

RELOADERS CORNER: Bullet Jump 2

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Tips that help take bullet jump out of the accuracy equation. Find out how!

leade
This is an aluminum staub cut with a chamber reamer. It’s easy to see the transition to the lands. The more smoothly a bullet enters the lands, the better it will shoot. When seating depth can’t be idealized, choosing a “gentle” bullet is the best defense against ill effects of jump.

Glen Zediker

Last time I shared some insight about bullet “jump,” and specifically with respect to the viability of setting up a “zero-jump” chamber/ammo combination.

To hit the highlights: Jump is the gap the bullet must traverse when it leaves the case neck to engage the lands or rifling. Generally, best (and better) accuracy comes with this gap is reduced to a minimum amount, or at the least reduced. Better is better.

To go farther into this topic, it’s worthwhile to move the bullet around, seating it more or less deeply (nearer or farther from the lands at rest) to maximize accuracy. Clearly, there’s a limit on cartridge overall length if the rounds have to fit into a magazine box so they can feed right. In NRA High Power Rifle competition, the AR15 pilots are specifically not allowed to have the rounds feed from the magazine in semi-auto mode; each round must be loaded into the chamber one at a time for the “slow-fire” segments, which includes the 600-yard event. That means competitive High Power shooters using AR-platform rifles are free to move the extra-long 80+ grain .224-caliber bullets out to near or on the lands when chambered. That doesn’t really matter but it explains the popular “Wylde” chamber we tend to use. It’s got a long enough throat to free more case volume and also provide a bigger “expansion chamber” for burning propellant gases, but it’s not as long as a NATO-spec so should perform better with bullets that do have to be loaded deeply in enough to fit the magazine box. Something like a Sierra 80gr or 82gr Berger won’t usually shoot worth a flip loaded to mag-length. That bullet, and others similar, are simply too dang long for a .223 Remington case. A huge amount of the bullet swallows up the case interior.

Sierra bullets compared.
It’s not all in the ogive specs, but it’s influenced by it, because those specs influence the overall profile of the bullet. Here’s a .224 Sierra 77gr MatchKing next to an 80gr MatchKing. The first is approximately 8 calibers, the 80 is approximately 12. The marks indicate the location of the first point of coincidence of land diameter. Considering the overall profile differences, it’s pretty clear that the 77 jumps with better results when each is loaded to the same cartridge overall length. There’s just 3grs difference in these bullets but they’re worlds apart in both tolerance and performance.

The best defense against ever worrying over jump, meaning whether you’re getting good accuracy regardless of the amount of bullet jump (well, at least within reason…) is bullet choice. Specifically, a tangent-profile bullet with a conservative ogive. Recollecting from some materials I did a while back, a “secant” profile is a sharper taper-in from bullet body to bullet tip; a tangent is a smoother transition. Secants, more or less, have a “shoulder” indicating a more abrupt taper rather than a smooth arc. For examples: true VLD (very low drag) and the Hornady A-Max are secant.

Bullets with relatively shorter nosecones and relatively longer bearing areas (length of the bullet that’s in contact with the rifling) are likewise more tolerant of jump.

Sierra 69, JLK 70 VLD
Here’s an example of different .224 bullet profiles at essentially the same weight. A Sierra 69gr MatchKing on left and a JLK 70gr VLD. The tangent 8-caliber-ogive (approximate) Sierra shoots great when it’s jumping; the secant 15-caliber-ogive VLD tends not to shoot well at all unless it starts touching the lands.

There’s been a trend for many years now toward creating bullets with higher ballistic coefficients. Worthwhile pursuit! Only issue is that when a bullet design features better aerodynamics, the features of that are, yep, longer nosecones with shallower angles. The ogive (what I’ve been more descriptively calling the nosecone because it’s easier to picture) usually is expressed in calibers. Technically it’s “calibers of ogive,” and that’s the ogive radius divided by the caliber. To me it’s easier to picture looking at the “other side” of the equation: the arc that scribes the profile in multiples of the bullet’s caliber. So, a 7- to 8-caliber ogive is a tighter circle (more rounded profile) than a 12- or 15-caliber ogive. Most of the “high-BC” profiles use a 15, some more. In other words, they’re stilettos.

calibers-of-ogive
Here’s an illustration of calibers-of-ogive from Sierra. That transition area from bullet diameter to first point of contact with the lands (which will be land diameter, and at least 0.005 smaller) has influence on how well a bullet endures jump. A lower-number is favorable in this regard. In this illustration the ogive radius, 2.240 inches, divided by the caliber, 0.308, gives 7.27. That should tolerate jump well.

I’m kind of breaking this down farther and faster than exercising good technical care in covering this topic should warrant, but: comparing both same-weight and same-caliber bullets, the longer it is the more sensitive it’s going to be to jump.

I have shot way too many high-X-count 300-yard cleans with bullets jumping 0.030+ inches to say that it’s not possible to have good accuracy unless jump is minimal. I admit that’s only a 1 moa group. I’m also using what some makers call a “length-tolerant” bullet, and specifically that’s a 77gr Sierra Matchking, and the same goes for a Nosler 77 or Hornady 75 HPBT (not A-Max). It’s the bullet form, not just its weight, that has the strongest influence on all this.

So, do you have to abandon better ballistics to attain better accuracy? Maybe. At least to a point. With the smaller calibers, which don’t have other advantages larger calibers have simply by virtue of weight and sectional density, there tends to be an effectively greater discrepancy between the lighter and heavier (again, it’s really shorter and longer) bullet ballistic performances.

A rifle with a generous-length magazine box provides greater jump-reduction via loaded round architecture. If there’s enough room, a bullet can be scooted out to the limit of the space within the box.

As always, well at least usually, there are tools! Get them and use them. A gage “set” from Hornady is well advised. There are others similar. I’ve been using their LNL Overall Length Gage and Bullet Comparator for many years and receive needed results. The first tool indicates the seating depth that touches the lands, and the second provides more reliable and accurate means to measure and record it.

Hornady LNL gage
This Hornady LNL gage pair gives you the tools needed to determine the jump you’re getting with the bullet you’re using. Check it out HERE

The leade, which, again, is the transition to the lands and determined by the chambering reamer (or throating reamer if custom-done) does influence tolerance for jump. The shallower the angle the better, but, that’s a two-edged issue. Take a commonly-used 3-degree leade and make it a more preferable 1.5-degree leade and that takes way on more than double the distance (length of cut) to attain. Again, when there’s a magazine getting in the way of bullet seating depth flexibility, a shallower leade eases transition into the barrel bore for a jumping bullet, but also increases jump. There are some cartridges, like David Tubb’s 6XC, that were designed specifically to “perfect” all these relationships: magazine-mandated cartridge overall length, bullet choice, and leade in, and it’s one reason it owns the records it does. Otherwise, it’s often a compromise… But don’t compromise accuracy for anything. A smaller group is, in the long run, the best defense against both wind and distance when it comes to hitting a target. Reliable feedback equals correct adjustments.

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