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

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11 thoughts on “RELOADERS CORNER: Why Not Flat-Base?”

  1. For short range accuracy (100 meters) & much less throat erosion I shoot the flat base 35 & 40gr Nosler Varmagedons; they are spooky accurate in Rem 222 barrels & don’t need a heavy load of CFE223 to push them either; trifecta!

  2. Good Read ! I hope young shooters pay attention to your words. I have found, out to 300 yards, that I (my guns) prefer Flat Base. I get better accuracy…Period. I was amazed at how much my groups tightened up with Flat Base Bullets. 3/4″ groups became one ragged hole ! Try em…you will like em !

    1. Accuracy or tight groups? These are two different characteristics. Tight groups is precision. Hitting point of aim is accuracy. Shooting for score (close to center) requires accuracy. Good accuracy shot to shot gives precision.
      Group shooters really don’t care so much about accuracy.

      1. If one is getting a good group accuracy can be increased by adjusting the sights. Can’t blame the bullet for the shooter’s failure to center the group on the X..

      2. I agree 100%. I shoot nothing but groups and a target is necessary only to have precise aiming point such as a round bullseye or the diamond shaped ones which I prefer. I could care less if a group is off center as long as I get good groups. Shooting for a good group is a pain in the arse if you’re always adjusting the sights to hit dead center. I’m always shooting a different COAL, powder type, powder weight, bullet weight or type of bullet, etc. Group size is the only thing that determines accuracy. But I’m not a hunter, nor do I shoot competition. I can’t walk more than 5-10 minutes before my legs & feet give out on me. Between my age 77y/o & diabetes, my walking days are over.

    2. If the military uses it (boat tails)it bears consideration to go in the opposite direction for accuracy,as accuracy does not seem to be high on their list of requirements. They do all that testing and then go with cheapest supplier they can find which kind of defeats the purpose of accuracy. I have always leaned toward flat based bullets with Sierra bullets at the top of my list for accuracy at a reasonable price. I will shoot boat tails but have found them to be less consistent when developing a load.

  3. I shoot the Hornady 165 grain bullet in all my 308 rifles. I get great accuracy out to 300 yrs. They fall off a little after that distance but still great for hunting.

  4. In my varmint or hunting rifles, it’s up to the barrel. I have rifles that only shoot well with boat-tail bullets (my Ruger Ultralight 257 Roberts only shoots boat tail 117 or 120 grain bullets well), most prefer flat based (one of my 22-250 rifles insists). Flat base cast bullets always slug up to fit the barrel much better than commercially cast bullets with a tapered base. I understand commercial casters use bevel base molds because the bullets fall out easier. I also understand reloaders often prefer boat tails because they seat into the case easier. But unless you are a long range competition shooter, there is little reason to choose boat tails.
    Your barrel is a temple, feed it what it likes.

  5. I haven’t run the numbers, but I think your aerodynamic argument is the driver. BT bullets probably have less drag and smoother flow. Sharp angle corners on a high-speed projectile are discontinuities disrupting the flow of air around the projectile.

    For example, almost all large aircraft now have winglets at the wing tip. A plane flies because the pressure on top of the wing is less than the pressure underneath the wing. Without winglets, the pressure flows directly from the bottom of the wing across the wingtip to the top of the wing. This flow results in violent vortices which trail behind the aircraft sucking away propulsion energy and causing problems for other aircraft. Winglets mitigate that flow of air.

    A flat-based projectile would have a similar issue as the flow of supersonic air around the ogive nose hits the sharp corner of the flat base. The disrupted flow would burble behind the projectile. Energy to create the burble effect comes from the finite amount of muzzle energy as the projectile leaves the barrel. This parasite drag would slow the bullet and possibly destabilize its flight.

    Computational Fluid Dynamics (CFD) could answer the question with hard numbers. But I bet the best aerodynamic configuration would stretch the bullet’s boat tail out to a spitzer point on the trailing edge of the projectile.

  6. Great article. I am old enough to remember a time when boat tails were rare exotic bullets. I stopped shooting for a few decades. When I came back, I saw BTs dominate. Intuitively it seems that BTs would be better aerodynamically. I just assumed that they are better shooters in all situations. I was surprised to read this article and learned that is not the case.

    Thanks, Midsouth for having this column. It has been interesting and educational. I will try to throw business your way whenever practical.

  7. I was once told by a genuine, bonafide, external ballistic expert, boat tail bullets are more accurate at distance, but they are also much harder to MAKE accurately because of the extra working of the bullet jacket. Plus, anytime you add a step to a critical process you also add another chance for something to go wrong.

    Also, I think most Benchrest shooters would agree that uniform wall thickness the the bullet jacket is the most critical component for consistent accuracy. After all, the jacket forms the largest diameter of the bullet that’s spinning at 165,000rpm (typical benchrest bullet), so any jacket defects will negatively affect bullet stabilization. Personally, I’ve experienced some large variations in accuracy due primarily to different jackets from different lots/

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