How does the HP of a BTHP work?

[Jon_Snow]

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.

SaMx, the air in front of the shockwave created by the bullet is supersonic, but unless you've got a really fast bullet, it's likely subsonic downstream, aka exactly where the bullet is. I think it's likely that the HP is a manufacturing artifact that doesn't impact the aerodynamics because of the boundary effects Tirod mentioned.

 

[Float Pilot]

The open tip does two things. It does create a small pressure bubble and mostly it shifts the bullets center of gravity rearward.

It also allows the bullet be longer for its weight than would otherwise be possible. This increases the BC and increases the possible range.

The boat-tail lets the passing air close in behind the bullet with a reduced amount of drag. I flat base tends to create a vortex behind the bullet which induces drag like the tail-gate of a pickup truck.

Other bullets like the Nosler Accu-bond and Ballistic tip are also made with sealed tails as the nose is the last part being finished. They simply have a plastic nose cone placed into the nose cavity area.

Sierra holds most of the target accuracy records due to their superior quality control as well as the bullet design. Years (decades) ago they figured out the best ratio to use in their jacketing material, so the bullets have a good gas seal while having limited drag within the bore.
They also pioneered the checking process to make all of the match bullets super close in weight and concentrically,.

The Ballistic Coefficients of a bullet change at different velocity ranges.
Here is how a 142 grain, 6.5mm (264) Sierra Match King changes.

595 @ 2850 fps and above
.580 between 2850 and 2400 fps
.575 between 2400 and 2050 fps
.550 @ 2050 and below

Now here is the 140 grain SMK. It is only slightly shorter.. BUT, look how much the B.C. change has become.

.535 @ 2800 fps and above
.526 between 2800 and 2000 fps
.521 @ 2000 fps and below

Here is a photo showing how much the bullet lengths can vary. This really makes a big difference when different rifling twist rates are applied.

 

[millertyme]

Like the dimples on a golf ball

 

[flakbait]

These are all great answers! I didn't know I needed a PhD in Aerodynamics to figure this out.

It's seems to be a manufacturing process (open tip to inject lead into the core of the bullet) that has some sort of aerodynamic benefit...otherwise I'm sure the bullet makers would have tried to close the open tip if there was a reason to do so.

The million dollar questions is do these bullets typically expand reliably as in a traditional hunting bullet? I assume that the small open tip gets filled up with soft tissue and pushes on the jacket to start expansion.

 

[Float Pilot]

do these bullets typically expand reliably as in a traditional hunting bullet

No.
The jacketing thickness, core to jacket bond, ojive shape and core material were not made with reliable expansion in mind. They sometimes act like a FMJ and sometimes they fragment when striking bone.

 

[SaMx]

SaMx, the air in front of the shockwave created by the bullet is supersonic, but unless you've got a really fast bullet, it's likely subsonic downstream, aka exactly where the bullet is. I think it's likely that the HP is a manufacturing artifact that doesn't impact the aerodynamics because of the boundary effects Tirod mentioned.

it's an oblique shock, not a normal shock, so the mach number downstream should still be supersonic. Based on the waves coming off the crimping groove and forming around the wake I'd say it is.

 

[336A]

The simplest way to get this information is straight from the horses mouth. I have contacted Sierra via both phone and e-mail in the past and it has always been a pleasant experience. Here is the a link at the top of the page is a 1-800 contact number or just e-mail them if you prefer. http://www.sierrabullets.com/

I'm willing to bet that the answer you will get is that the Open Tip on the SMK bullets is a byproduct of the maufacturing process. After all they have to get the lead in there some how.

 

[Ridgerunner665]

Finally...336A suggested it.

I did that several years ago (talked to Sierra)