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Author Topic: Flat Base or Boattail how and why:  (Read 793 times)

Offline Dutch-Hunter

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Flat Base or Boattail how and why:
« on: May 18, 2014, 02:40:18 PM »
Well Bo you got me to thinking and it hurts. Looked back in my notes and archived files and put together this little ditty.

The bullet does the actual killing. The rifle is simply a means to deliver the bullet to the target. So, a basic knowledge of bullets seemed to be a prerequisite for studying rifles.

As hunters, we put a lot of thought into the tips of our bullets.  Roundnoses, spire points, plastic tips and such are all worried over to ensure that the bullet weíre using is right for the game weíre hunting.  But have you ever given as much consideration to the other end of your bulletĖthe base?

In the scheme of things, the base of your bullet probably isnít going to make any more of a difference between a hit or a miss, a clean kill or a wound, than the bullet tip, but it is something to consider if youíre on a once in a lifetime hunt and want to eliminate as many variables as possible that could leave you standing there wondering what went wrong if a trophy animal bounds away seemingly unscathed.
There are generally two types of bases on hunting bullets, flat base and boattail. The practical differences between them may not be what you think. Both have their benefits, and their drawbacks. For example, when it comes to pure accuracy, flat base bullets are inherently more accurate than boattails, and thatís why you see the ďshort rangeĒ Benchrest shooters using them. The reason is because itís easier to make their bases perfectly square with the sides of the bullets than it is to make boattails perfectly concentric and on straight.
Another benefit of flat base bullets is that for a given caliber and bullet weight, theyíre shorter overall, so are more easily stabilized in a slower rifling twist. That shorter length, however, puts their center of gravity more toward the rear of the bullet, and that tends to offset stability gained from the shorter length.

Finally, flat base bullets generally experience less jacket to core separation on impact. If you think of the bullet jacket like an ice cube tray and the lead core as the ice, it should be obvious that the right angle inside of a flat base jacket holds onto the core better than the tapered inside of a boattail jacket. On big game, you want a bullet that penetrates, and that means using a bullet that stays together and retains its weight instead of coming completely apart during expansion.

Iím not suggesting standard cup and core boattails is the wrong bullet to use for hunting. They have their place and their advantages, such as at very long range on light-bodied game. Boattails increase the ballistic coefficient of bullets, which helps them overcome air resistance and wind deflection. The difference in the amount of bullet drop between flat base and boattail bullets wonít amount to much until well past the range of 400+ yards, but the boattailís better ballistic coefficient makes errors in wind deflection correction and range estimation more forgiving. Whatever loss of accuracy there is from the boattailís inherent manufacturing flaws are more than offset by their ability to overcome adverse or unknown shooting conditions. And if youíre a handloader, boattail bullets are simply easier to start into a case mouth and seat with less chance of crumpling or bulging the case neck.
Thankfully, bullet technology is at a point where we really donít have to choose between a flat base and a boattail for good terminal performance. There are plenty of bullets that give you the benefits of both without some of their drawbacks.
From a design standpoint, Nosler has its Solid Base boattails that retain their cores during penetration better than standard cup-and-core boattails. With the Solid Base design, the entire boattail base is solid copper, so instead of being tapered inside the base, the lead core bottoms out against a flat surface.  Itís like Nosler made a flat base inside a boattail so thereís less a chance of the core popping out as it impacts and passes through game. That design also shifts the bulletís center of gravity forward helping with stability.

There are also bullets that mechanically retain the core such as Hornadyís InterLock boattail bullets. An InterLock is essentially a raised edge inside the bullet jacket that grips the core solidly when itís swaged into place during bullet forming. Iíve shot a lot of big game with Hornady InterLock bullets and I canít recall an instance of jacket/core separation.

Another technique to help boattail bullets perform better is by bonding the jacket and the core.  Often, this bond is so strong that itís not uncommon to recover expanded bullets with significant amounts of lead still bonded to the petals instead of being ripped or wiped off. Swift Bullets uses bonding in its Scirocco II line, and if you section one youíll also see that the lead core sits in a flat-bottom cavity much like Noslerís Solid Base bullets.
Of course, we canít forget all-copper bullets such as Barnes. They offer the sleek design and high ballistic coefficient of boattail bullets, and expand reliably without any lead core at all.
The of Benchrest and military specialists demanded bullet makers to make boattail bullets that behave like flat bases at all ranges, and flat base bullets that behave like boattails at long range. Those efforts include rebated boattail designs that have an abruptly reduced bullet diameter before the boattail is formed, and FBVLD, or flat based very low drag bullets that have an extremely long nose, or ogive, for minimum nose drag to compensate for the greater base drag. Berger Bullets has their VLDís too. They use a method of forming boattails so that they are more concentric in conjunction with hybrid ogiveís used.
 
Well I guess since I have gotten this ball started I may as well keep it rolling.

Bullet Shapes & Characteristics

In addition to the variations of expanding bullets that we have already covered, there are five different general shapes of hunting bullets: flat point, round nose, semi-spitzer (semi-pointed), spitzer (pointed), and boattail spitzer (pointed with a tapered heel).
Flat point and round nose bullets have a blunt nose, the difference being merely whether it is flat or rounded. Typically, this allows a lot of lead to be exposed at the nose of the bullet, which makes for reliable expansion, especially at lower velocities. These designs also tend to penetrate in a reasonably straight line, and are reputed to be less apt to be deflected off course by intervening leaves, twigs and brush that they might encounter on their way to the target. As a consequence of their blunt nose, RN and FP bullets exhibit greater air drag, lose velocity more rapidly, and therefore exhibit a more curved trajectory.

Spitzer and semi-spitzer bullets have a pointed nose and a flat base. Spitzer bullets are sharply pointed, minimize air drag, and therefore shoot "flatter" than bullets with a blunt nose. But, spitzers have a tendency to tumble on impact if they don't expand, and don't initiate expansion as reliably as flat point or round nose bullets.

The most streamlined hunting bullets are boat-tail spitzer bullets. These have a pointed front-end and a tapered heel (amazingly like the stern of a sailboat). Boat-tail spitzers exhibit the least air drag and shoot flatter than the other shapes. Compared to spitzer bullets, the advantage of the boat tail is most pronounced after the bullet has fallen below the speed of sound. So the boat tail spitzer is a good design for very long range shooting, say in excess of 300 yards. Their terminal performance is similar to other spitzer bullets, but they have a reputation for more readily losing their cores.
Frangible Bullets
Frangible bullets break up into very small pieces upon impact with the target or the background. The penetration of this type of bullet is limited and the inflicted damage is typically near the surface of the target. They are the safest type of bullet to use in semi-populated areas, as the risk of ricochet is minimized.

Frangible bullets are normally used to take animals weighing less than approximately 30 pounds (These small animals are typically called "varmints.") by creating significant damage within the first few inches of the impact point. The frangible bullets that are typically used to hunt varmints are called "varmint bullets." Well DAH!

Because controlled expansion bullets and non-expanding bullets are more strongly constructed, they may simply pass through a small animal causing less than immediately lethal damage; the animal will painfully suffer as it heals or slowly dies. A humane hunter wishes neither, so frangible bullets are the ticket for hunting small animals.

Non-Expanding Bullets:
Non-expanding (FMJ) bullets typically retain their general shape as the bullet penetrates and passes through target. The penetration of this type of bullet is usually much greater than frangible or expanding bullets because the frontal area of a non-expanding bullet does not increase as it penetrates.

Since the wound channel is typically much narrower than that of an expanding bullet, the damage caused by a non-expanding bullet is usually much less, and quick kills on deer size game are rare. For this reason non-expanding FMJ bullets are illegal for big game hunting almost everywhere in North America. In round nose form they are favored by some African hunters for use on the largest and toughest game, principally on elephant and rhino, where very deep penetration against heavy hide and bone is required.

A hunter's objective is normally to cause significant soft tissue damage, especially to the heart and lungs, so that the animal dies quickly, minimizing the chance of losing the kill. Non-expanding bullets do not provide that kind of performance and should not be chosen for hunting North American big game.

Expanding Bullets:
Expanding or "controlled expansion" bullets are designed to deform or "mushroom" as the bullet penetrates and passes through the target. Expanding bullets are the most complex and difficult type of bullet to design, and also the most useful and numerous type of bullet. Almost all big game hunting bullets are of the expanding type. The penetration of an expanding bullet may be measured in inches or feet, depending (among other things) on the bullet's design, the bullet's sectional density, the expansion medium the bullet hits, and how fast it is traveling when it hits the expansion medium.

Expanding bullets are normally used to humanely kill animals greater than approximately 30 pounds in weight by creating significant tissue damage as the bullet passes through the animal. Expanding bullets are generally constructed to mushroom to approximately twice their initial diameter as the bullet passes through soft to firm tissue, such as skin, fat, muscle, small bones, and internal organs. The objective is to cause catastrophic damage to vital organs, especially the heart and lungs, so that the animal dies as quickly as possible.

Coaxing the optimal expansion from a bullet can be a tricky feat for the bullet designer and the hunter. For optimum expansion, the bullet must pass through tissue of approximately optimal density within the optimal speed range. There are minimum and maximum impact velocities and target densities for any expanding bullet design.
Most expanding hunting bullets have a lead alloy core protected by a jacket of some harder metal. Manufacturers vary jacket material, jacket thickness and jacket design, as well as the hardness of the lead alloy core, to control the expansion characteristics of soft point hunting bullets.

By the way, the term "soft point" refers to the lead exposed at the tip of the bullet, which helps to initiate bullet expansion upon impact with the target. There are hunting bullets that are not soft point types, but for now let's stick with those that are, as they are the kind most commonly seen. The popular Federal Soft Point, Federal/Speer Hot-Cor, Nosler Solid Base, Remington Core-Lokt, Sierra GameKing and Pro-Hunter, and Winchester Power Point are all common examples of soft point bullets found in factory loaded ammunition.

If an expanding bullet is traveling at relatively slow velocity at impact, and especially if it passes only through soft tissue, its expansion may be limited if at all, behaving much like a non-expanding bullet and causing less than immediately lethal damage to the animal. If an expanding bullet is traveling too fast at impact, and especially if the bullet hits a large, hard bone, it may expand so violently that its penetration ceases before it reaches the animal's vital organs. This creates a large and very painful surface wound, but less than immediately lethal damage. Once again, the animal is likely to endure great suffering and travel a considerable distance before it succumbs to its wound or is pulled down by a predator.

It seems that most expanding bullets are designed to optimally mushroom as they pass through the chest cavity (skin, fat, muscles, rib cage, lungs, and heart) of the animal for which they are intended at the average range of speeds expected at the typical hunting ranges of the cartridges for which the bullet is designed.

As the bullet is subjected to conditions more severe than optimum (i.e., higher speeds, impact with larger bones, larger than average deer, etc.), the bullet may over-expand, thereby reducing its penetration and effectiveness. Conversely, if the bullet is subjected to conditions less severe than optimum (i.e., lower impact velocity, smaller than average deer, etc.), the bullet may under-expand or not expand at all, also reducing its effectiveness.

Expanding bullets are intended for a wide variety of animals that weigh approximately 30 pounds or more. Logic suggests that bullets normally used to kill 70 pound antelope are probably not the proper bullets to kill 1000 pound moose. Logic further suggests that, while maintaining the desired expansion characteristics, larger and tougher bullets must be used to take on larger and tougher animals.
The bullet manufacturers are fully aware of the bullet weights that hunters typically use to hunt specific animals and design the performance characteristics accordingly. Some manufacturers (Federal and Winchester in particular) indicate the intended class of game for which their bullets are intended, while others do not.

OK! Iím wrapping this up now. So in conclusion:

Boattail bullets for hunting are design limited in some instances. I have used them for decades. In the shorter lighter spitting the core has happened. This is not that critical on small to midsized game, up to deer size.

Flat bases take up less room in the case, which allows a bit room for fuel. As discussed above flat based bullets limitations are range. The biggest advantage is their ability to control expansion.

When it comes to nose shape it is also dependent on the prescribed use and range.

With all that has been said most differences are relatively small. If you have found something that works well for you stick with it.

Once again I've attached a PDF copy if you'd like to download or print it.
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Offline MichiganLouie

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Re: Flat Base or Boattail how and why:
« Reply #1 on: May 18, 2014, 09:23:48 PM »
Nice job Dutch.  If I may add my 2 cents worth, I think the Geneva Convention mandates that military bullets have to be FMJ (full metal jacket).  BTW, the 7.62x51 NATO round uses a boattail FMJ. The U.S. government has performance requirements that the bullets have to meet, like penetration at some distance, like 400 meters, etc. 
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Offline Split toe

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Re: Flat Base or Boattail how and why:
« Reply #2 on: June 10, 2014, 10:46:04 AM »
Another great read Dutch and as always thanks for sharing your knowledge!!!
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Offline dave 1211

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Re: Flat Base or Boattail how and why:
« Reply #3 on: June 11, 2014, 10:56:27 AM »
 --9-908   as always great info          ##$%#115 
turkey season was a bust hope deer season is better




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Offline Zos41

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Re: Flat Base or Boattail how and why:
« Reply #4 on: August 18, 2014, 08:31:37 AM »
excellent write up, thank you
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Offline Flintlock1776

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Re: Flat Base or Boattail how and why:
« Reply #5 on: August 21, 2014, 02:48:19 AM »
Nice stuff!

I bought the RCBS system , brass centrifuge etc when Bill Clinton got elected. Back then people were concerned that he would restrict our ammo opportunities. Obama is doing it by having the Pist Office and other government divisors buy up the ammo.

I got busy raining my kids and never got to even set up the RCBS . Still here unused. I should try and sell it all. Very heavy to ship I reckon. I won't be greeting into reloading at this stage. I wonder if there is a good market to sell all of it in one big package?
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Offline wvwhitetail

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Re: Flat Base or Boattail how and why:
« Reply #6 on: August 25, 2014, 09:44:17 AM »
Very interesting read. Thanks for writing it.

 

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