How does the HP of a BTHP work?

[flakbait]

I understand the the hollow point of of a BTHP (Boat tail Hollow point) bullet is designed for accuracy and not rapid expansion as in a traditional hunting bullet. How does the hollow point contribute to accuracy?

I know the boat tail part helps streamline the bullet as it cuts through the air...I don't quite understand how having a hollow point tip helps. Intuitively, a single pointed til would seem to be more aerodynamic??

BTW, I shot a 100 lb hog (poorly) through the liver broadside with a 174 grain Hornady BTHP bullet from a .303 Enfield this weekend and it dropped dead on the spot. The exit wound was bigger than 1 inch and impressive. I would have used a better bullet for hogs, but there is not much hunting bullet selection for the .303.

 

[PotatoJudge]

In part it's a remnant of the manufacturing process. As far as I've heard, it has very little mass and little effect on aerodynamics..

 

[Ridgerunner665]

PotatoJudge is right...

And about the exit whole on your hog...many of these type bullets have very thin jackets (J4) and will expand violently and rapidly, one might even call it explosively...if the velocity is high enough.

 

[jpwilly]

How does the hollow point contribute to accuracy?

In "Match King" bullets it's there to make an air pocket in front of the bullet reducing drag as it flies through the air.

 

[Shadow 7D]

^^ jpwilly nailed it
it's not remnant, potatojudge is VERY wrong

It is there for the aerodynamics
The hollow point breaks up air smoothly and allows for a consistent flow over the bullet, look at a rocket, at a fighter or any airplane, sharp, straight, and pointy doesn't work (conical) The hollow point traps a pocket of air and that is what smooths the flight rather than the point of the bullet.

Unfortunately the small HP is more prone to go straight through and not tumble when hitting like a spitzer?? (5.56 ball) which was designed to up end when impacting flesh.

 

[Sheldon]

"In part it's a remnant of the manufacturing process. "

I believe this is true. The base of the bullet is the last thing effected by the rifle and as such all match bullets have a closed base bullet. If the base is closed then the tip will be open and where the lead is inserted and when the tip is closed and shaped the small open tip is where the bullet is pushed out of the shaping dies.

 

[USSR]

The base of the bullet is the last thing effected by the rifle and as such all match bullets have a closed base bullet.

Correctamungo. It's not so much the hollowpoint that contributes to accuracy, it's the more perfect base of a HPBT bullet, as opposed to an open base FMJ bullet.

Don

 

[Ol` Joe]

Flat base can be just as accurate as boat-tails. The flat base design is actually easier to manufacture and get consistent. Look at the bullet used in most 100-200 yd benchrest competitions and you will find lots of top scores with them.
The hollow point adds to the bullets accuracy by keeping the weight towards the rear of the bullet. This helps stability, and the longer profile improve BC which helps fight wind drift.

 

[Tirod]

The Matchking boattails were found to be 20 to 32% more accurate than flat based military rounds.

http://www.blackfive.net/main/2006/01/army_jag_bans_e.html

The JAG decision in 1985 goes into detail about this, and how the manufacturing process for the open tip compares to other bullets. It's apparent the AMU delivered a lot of background on the subject. Open tip bullets were approved for military competitive use in 1980.

S l o w l y b u t s u r e l y their use is expanding , but watch out for mislabeling an open tip as hollow point. I've done it. They are two different bullets, just as much as a flat base isn't a boattail, nor is a hollow point necessarily a dum dum.

 

[LRS_Ranger]

The hollow point part of the bullet is there for aerodynamic reasons, NOT just a leftover from the manufacturing process. The hollow point creates a high pressure shockwave which breaks the air, as opposed to the metal of the bullet itself. This creates less drag, and allows a higher BC, which in turn allows hits at longer ranges.

There is a lot of misunderstandings that people have about ballistics. I HIGHLY reccomend "Understanding Firearm Ballistics" by Robert Rinker. It is 400-something pages of more than you ever thought existed about firearm ballistics. It goes very in depth in parts, but it is written in a way that most of it you don't need a degree in Physics to comprehend. If you have a serious desire to learn more about how your bang stick works, I HIGHLY urge you to get this book or one like it instead of asking the peanut gallery on the internet. FWIW... here is a link to amazon, though you can buy it in a Barnes and Noble for 20 bucks or so.

http://www.amazon.com/Understanding-Firearm-Ballistics-Robert-Rinker/dp/0964559846

 

[jhansman]

I don't pretend to know much of anything about ballistics, but I can tell you the 60 gr. SMKs I shoot out of my bolt .223 are extremely accurate. They are HP flat-based bullets. No non-HP bullet I've yet tried has proven as accurate in this gun.

 

[Boberama]

It shouldn't expand.

At high velocity it creates a bubble of air, making the perfect point.

 

[Coronach]

Also, as far as it being a remnant of the manufacturing process, that has a kernel of truth in it. It is relatively difficult/costly to perfectly machine a tip to be 100% centered and of equal profile the whole way around. Engineering it to be open-tipped is easier and as other benefits (as mentioned).

Mike

 

[stubbicatt]

Two advantages:

A bullet is like a boat in that it steers from the rear. The hollow point bullet shifts its mass rearwards, increasing stability. The smooth transition on the rear of the bullet created by a uniform jacket as opposed to an open base eases the bullet through the shock wave more uniformly. In essence there is no irregularity on the bullet base to tend to "steer" the bullet by interfering with airflow, which in combination with rotational velocity, contributes unwelcomely to precession.

Secondly, the hollowpoint creates a pocket of air that allows the bullet to slip through the air more easily increasing its ballistic coefficient a skosh.

 

[Mags]

Hollow points help accuracy due to the jacketing ending more uniformly at the hollowpoint versus the base of the bullet, other than bullet seating I have no idea to the benefits of the boat tail design.

 

[taliv]

The hollow point part of the bullet is there for aerodynamic reasons, NOT just a leftover from the manufacturing process. The hollow point creates a high pressure shockwave which breaks the air, as opposed to the metal of the bullet itself. This creates less drag, and allows a higher BC, which in turn allows hits at longer ranges.

There is a lot of misunderstandings that people have about ballistics.

I take it you're not convinced meplat trimmers and pointing dies improve BC and consistency?


I don't think that pocket of air does anything to improve the BC. essentially all it does is prevent the uneven meplats from hurting accuracy.

 

[eddism]

increasing the profile of the bullet tip increases the bullet's meplat and adjusts the bullet's ballistic coefficient. and lowers its drag coefficient.

 

[MTMilitiaman]

Two ways actually.

First, the base of the bullet determines gas seal as the bullet exits the muzzle. So while bullets can still be accurate with relatively large deformations in the tip, even small nicks or inconsistencies in the base of the bullet can lead to erratic performance and poor accuracy, much in the same way a flaw in the crown of a rifle does the same. The manufacturing process of the BTHP match puts the hollow point in the nose during the manufacturing process because it makes it easier for manufacturers to keep the bullet's base as consistent as possible.

Plus, the hollow point moves the center of gravity to the rear. When properly stabilized, this also has the effect of making the bullet more accurate.

 

[Shadow 7D]

Taliv hate to tell you, it does increase the ballistic coefficient, by trapping the bubble it smooths the flow and the bubble can react to changes in pressure, where the hard point does not.

What it does is use the disturbance that is naturally going to exist no matter what to help rather than hinder.

 

[desidog]

Does anyone have any super-slow-motion video or computer animation of these conditions?

I had thought (apparently misguidedly) that the hollow point shifted the center of mass back, increasing the BC, BUT that the open point hurt aerodynamics....although the increase in BC outweighs the decrease in aerodynamics, so it is still a good trade-off.

I'm skeptical of an air pocket acting as the point in flight as a positive attribute; I imagine that the void at the point would be less aerodynamic, and performing more like a flat-nose in flight.

Would a hp bullet traveling through water create- or take an air bubble with it? That would also be good slo-mo video.

 

[Tirod]

Be careful of interchanging the term hollow points with open tip. Hollow points are big enough to insert the tip of a FMJ bullet. Open tips will barely take a paper clip, the hole is barely .035.

Boundary layer air theory is the missing ingredient, the same "bubble" of trapped air surrounding an object in flight. Jet pilots are familiar with it effecting low level flight under 50 feet. The boundary layer is actually what moves the air aside. It converges at the tip enough that a .035 hole doesn't upset it. Air moves over the boundary layer, not the actual surface.

Boundary layer is exploited as much as possible in intake manifolds, and is one reason we get more power per cubic inch today. How you control boundary layer is how air is moved.

 

[SaMx]

Taliv hate to tell you, it does increase the ballistic coefficient, by trapping the bubble it smooths the flow and the bubble can react to changes in pressure, where the hard point does not.

This doesn't sound right to me. The bullet is going well over the speed of sound, pressure changes won't be able to propagate except via a shockwave, which exists anyway.

I'm going with the manufacturing consistency theory, as well as moving the center of gravity back. It would also make the bullet longer for it's weight, so the angle of the nose should be lower, and the shock weaker. It might also have a larger moment of inertia about it's axis, because more of the lead is in a cylinder shape and less is in cone shape.

 

[SaMx]

besides, If there was a good aerodynamic reason to have an open tip, you would probably see them on more supersonic cone shaped objects, like airplane fuselages and missiles.

 

[HarleyFixer]

Well I am not a trained ballistics specialist but I am a trained aerodynamisist. The reason for the boat tail is to reduce parasitic drag on the rear of the bullet and the purpose of the hollow point is to produce a uniform pressure wave at the front of the bullet to equalize the aerdynamic forces in flight.

 

[Tirod]

He said, she said. A lot of this is not using specific terms and showing hi speed photos.

I'm with Harleyfixer, it's consistent with most of the stuff I've read over the last thirty years. If anyone has high speed photos you've taken in your lab proving otherwise, we'd like to see them. There's new stuff all the time. Open tip bullets have been winning matches since Palma in the late fifties, that was certainly counterintuitive.

If there's a precision long range record using something flat base round nose, please post that, too.

Ok, back to he said she said.