Why is it called a "Boattail"?

[dellet]

Twist rate IS IMPORTANT! That is what the equation shows.

So what you’re saying is that your bullets know the difference between being shot from an 1/8 twist at 1750 fps, vs a 1/10 at 2500 fps and respond differently ?

You said you used the 1/8 for the fast twist, when in reality a 1/10 at 2400 fps spins faster. This would indicate the stability issue is related to velocity and pressure, not a low spin rate.

 

[js2013]

Hi, I'm Dr. John Stutz and I teach ballistics to engineers and scientists. I also have a new bullet design (AerospikeBullets.com).

The boattail bullet design was by all accounts an accident. At the turn of the 20th century, the mass production of bullets began. Spitzer bullets were the first to put a chamfer on the base of the bullet to make it easier for a machine to put the bullet into the case around 1901. During WW1, the accuracy of the boattail became apparent during the sniper battles across the trenches.

There have been multiple studies using computer simulations and live fire testing to find the optimal angle and length of a good boattail. The results depend upon the muzzle velocity and the shape of the nose or ogive of the bullet. This subject became important to me while I was developing the Aerospike base to replace the boattail. One thing my studies never dug up was the name. Why is it called a boattail?

I haven’t read all 7 pages so excuse me if this is redundant, but can anything regarding this bullet design be patented? Good luck on your endeavor!

 

[HappyHellfire]

I haven’t read all 7 pages so excuse me if this is redundant, but can anything regarding this bullet design be patented? Good luck on your endeavor!

Yes Sir. I have a patent pending. Thanks.

 

[Dave M.]

I have contacted Dr. Stutz and I have verified that he is indeed Hellfire and the person posting on this site. Despite his bot like responses, he is indeed human and confirmed.
Dave

 

[Dave Way]

Furnish a few thousand to some of the guys here and maybe you will get some feed back

I’ll be testing the 6.5’s shortly.

Dave.

 

[dellet]

Spin rate is absolutely important. For the vast majority of bullets, if the bullet is stable out of the muzzle it will be stable to transonic. You can take the equation for gyroscopic stability and convert it to barrel twist.

The equation looks like:

View attachment 1565874
The bigger the twist number (slower twist), the lower the stability. The overturning moment coefficient changes with muzzle velocity but changes much more slowly. It is estimated at 2.5 at Mach 2.5 and increases to around 3 at Mach 1.4.

Hope this helps!

Maybe the simplicity of the question is the problem.

It appears that this bullet design requires a new stability calculation as well as the BC calculation.

A drag curve plotted on a graf generally looks like this With the bottom being subsonic velocities, M1. This new design apparently places the V in the range of M2.2. Best explanation for the numbers.

This is why the discrepancy and why conventional thinking does not apply.

This design appears to be more sensitive to air pressures than spin, unless some one can define and explain it better for a simple mind.

I chose 1.400” for a bullet length, simply because it’s the best I can do without information I asked for and never received. Using normal stability calculations

Twist. Velocity spin rate stability factor
1/10 2500 180,000 1.381
1/8. 2000. 180,000 2.040
1/8 1750 157,000 1.916
1/11.5 2500 156,520 1.044

By the numbers above, it appears that velocity, therefore pressure on balance points, has more effect than spin.

I base that on the observation that stability increases more from a drop in velocity than spin. And why the drag curve V happens closer to M2 than M1 which would be considered normal.

As with any thing in load development and Ballistics only one variable should be introduced at a time. Moving from 308 to 300 BLK velocities and simultaneously changing twist rates looks like it screwed the results.

Can any one clear that up or confirm my thinking?

 

[Dave Way]

Wait, you don't seat the pointy part in the case?

In a sub sonic situation, that would actually make sense.

Dave.

Think about a drop of water falling. It’s not rigid so gravity forces it to take on the most aerodynamic shape. A teardrop. Once a sonic shockwave is involved, things change.

 

[WSnyder]

BUT, even though he is human and confirmed that does not mean he is as educated as he would want us to believe when it comes to the different bullet shapes and how they perform due to that formation.

I have a feeling he's got a way better idea about how it all works than any of us. It may be theory but that's a bit why he came here. If people quit giving him so much pushback and worked with the guy something good just might come of it. Maybe it's because I've worked around and for scientists my whole adult life I get him and what he might be trying to do. Science is full of failure but now and then they (scientists) come up with some pretty amazing things.

 

[Dave M.]

I have a feeling he's got a way better idea about how it all works than any of us. It may be theory but that's a bit why he came here. If people quit giving him so much pushback and worked with the guy something good just might come of it. Maybe it's because eraI've worked around and for scientists my whole adult life I get him and what he might be trying to do. Science is full of failure but now and then they (scientists) come up with some pretty amazing things.

I’m literally trying hard to get some bullets to do a comparative shoot but my requests keep falling on deaf ears.
Dave

 

[Dave M.]

To be clear, if you read through my posts, I’ve never personally attacked Dr. Stutz and I even reached out to him after researching his credentials. I think he would see that I’m not only easy to work with, but also fair and impartial when it comes to data. The target rules everything in my book. The target is what told me cleaning brass and primer pockets was useless, the target is why my concentricity gauge gathers dust now, and the target is why I continue this conversation in search of the holy grail of accuracy.
Dave M.

 

[Thauglor]

I have a feeling he's got a way better idea about how it all works than any of us. It may be theory but that's a bit why he came here. If people quit giving him so much pushback and worked with the guy something good just might come of it. Maybe it's because I've worked around and for scientists my whole adult life I get him and what he might be trying to do. Science is full of failure but now and then they (scientists) come up with some pretty amazing things.

Then he is fooling you with some math and terminology in his writing. You don't think sierra, lapua, Brian litz or any of the custom bullet makers ever investigated a longer boat tail? That is all he has basically done, and he just posts drag results but not BC because BC is what actually matters. Hell on the other thread he even posted data that shows he throws out results that doesn't fit his message, and that is from his own data he posted.

He came here to sell his wares, if you believe anything different I can't help you.

 

[Ledd Slinger]

A velocity increase doesn’t make a bullet more stable like a faster twist does More velocity makes it harder to stabilize.

I have to disagree there. A good bullet with perfect concentricity from the axis will be more stable the faster you spin it. Extra velocity in any given twist rate increases RPM, which in turn increases the stability factor (SG: Specific Gravity). So the same bullet fired in the same twist rate should be more stable at 3000 fps than it is at 2500 fps due to the higher RPM, and it will maintain optimum stability farther down range.

If you use Berger’s twist rate stability calculator and simply increase the velocity in the calculation and nothing else, the SG increases.

The only way I would see an increase of velocity causing stability issues, is if you are exceeding the velocity/RPM rating of the bullet design and/or jacket.

 

[dellet]

I have to disagree there. A good bullet with perfect concentricity from the axis will be more stable the faster you spin it. Extra velocity in any given twist rate increases RPM, which in turn increases the stability factor (SG: Specific Gravity). So the same bullet fired in the same twist rate should be more stable at 3000 fps than it is at 2500 fps due to the higher RPM, and it will maintain optimum stability farther down range.

If you use Berger’s twist rate stability calculator, simply increasing velocity in the calculation and nothing else, the SG increases.

The only way I would see an increase of velocity causing stability issues, is if you are exceeding the velocity/RPM rating of the bullet design and/or jacket.

It’s a trick question, and there was a very simple answer, and it’s why words and phrasing matter. When the OP states he used the 300 Blackout to get the fast twist, it came with a reduction in velocity. That reduction in velocity allowed the bullet to be stable at 157,000 rpms. Comparing that to 180,000 rpms at 2500 fps, isn’t quite fair. Since spin does increase with velocity, the 1/8 would be spinning 225,000 at 2500 fps.

Basically the answer to my question, was in the question. Even tho the the 1/10 at 2500 was spinning faster, it’s still not fast enough to over come the drag/pressure created by the added velocity.

The potential problems with this bullet is that it is extremely light for length, copper has a density about 80% of lead so if this was a jacketed bullet it would probably weigh in around 170 plus grains. It’s not the length of the bullet that determines velocity shedding and drift, it’s mass. At some point in the bullets flight, mass overtakes speed. This is common in a cartridge like the Blackout that is velocity challenged. If you plot the drop off a 110 grain bullet at 2400 fps and 150 grain bullet at 2200, within the first few hundred yards the the drop is reasonably close, energy is within 75 pounds at the muzzle. Edge goes to the 110 grain. By 500 yards the drop has equalized, energy to the 150. By 1000 yards the 150 has 100” less drop. That’s why in another post, I mentioned true testing will start at 500 yards.

The op talked about the potential of switching to jacketed bullets. That will be another hurdle. As the spin rates required to stabilize the current design make it tough for a jacketed bullet to make it to 1000 yards, in a cartridge like the 30 PRC. 150 grains, 3200 fps, 288,000 rpm. The extra weight will slow that down, but it’s another hurdle.

Hope that helps more than an equation with undefined inputs.

 

[joshb]

I’m still waiting for an answer from the experts. Why not make a bullet with a pointed base? Maybe my first attempt sounded too much like a joke?
Serious thinking: I see in the pics that boat tail bullets still have drag inducing vortices. Wouldn’t a pointed base solve that problem?

 

[jelenko]

It’s a trick question, and there was a very simple answer, and it’s why words and phrasing matter. When the OP states he used the 300 Blackout to get the fast twist, it came with a reduction in velocity. That reduction in velocity allowed the bullet to be stable at 157,000 rpms. Comparing that to 180,000 rpms at 2500 fps, isn’t quite fair. Since spin does increase with velocity, the 1/8 would be spinning 225,000 at 2500 fps.

Basically the answer to my question, was in the question. Even tho the the 1/10 at 2500 was spinning faster, it’s still not fast enough to over come the drag/pressure created by the added velocity.

The potential problems with this bullet is that it is extremely light for length, copper has a density about 80% of lead so if this was a jacketed bullet it would probably weigh in around 170 plus grains. It’s not the length of the bullet that determines velocity shedding and drift, it’s mass. At some point in the bullets flight, mass overtakes speed. This is common in a cartridge like the Blackout that is velocity challenged. If you plot the drop off a 110 grain bullet at 2400 fps and 150 grain bullet at 2200, within the first few hundred yards the the drop is reasonably close, energy is within 75 pounds at the muzzle. Edge goes to the 110 grain. By 500 yards the drop has equalized, energy to the 150. By 1000 yards the 150 has 100” less drop. That’s why in another post, I mentioned true testing will start at 500 yards.

The op talked about the potential of switching to jacketed bullets. That will be another hurdle. As the spin rates required to stabilize the current design make it tough for a jacketed bullet to make it to 1000 yards, in a cartridge like the 30 PRC. 150 grains, 3200 fps, 288,000 rpm. The extra weight will slow that down, but it’s another hurdle.

Hope that helps more than an equation with undefined inputs.

Very cool. Thanks!

 

[dellet]

I’m still waiting for an answer from the experts. Why not make a bullet with a pointed base? Maybe my first attempt sounded too much like a joke?
Serious thinking: I see in the pics that boat tail bullets still have drag inducing vortices. Wouldn’t a pointed base solve that problem?

I think it’s been tried.

AeroSpike Bullets

Looking for a little bit of advice and guidance here. I'm a ballistician and aerospace engineer with what I think is a cool new bullet design. I call it an AeroSpike bullet after the aerospike rocket engine that I got the idea from. Basically a lot of math to shape the base so that it lowers...

forum.accurateshooter.com

 

[Knotwild]

OK, I've kind of followed this thread, so I'll bite, which end is the base and goes in the case?

 

[WSnyder]

I think it’s been tried.

AeroSpike Bullets

Looking for a little bit of advice and guidance here. I'm a ballistician and aerospace engineer with what I think is a cool new bullet design. I call it an AeroSpike bullet after the aerospike rocket engine that I got the idea from. Basically a lot of math to shape the base so that it lowers...

forum.accurateshooter.com

Maybe @HappyHellfire would explain the differences between the original design and the current design and why he changed the design? Advantages? Disadvantages?

 

[Dave M.]

Maybe @HappyHellfire would explain the differences between the original design and the current design and why he changed the design? Advantages? Disadvantages?

Or perhaps he won’t because I understand that ha “has reached his limit of suitable testers for his bullets”. Boy, I can’t wait to see these results. Someone please upload the popcorn emoji for my computer illiterate ass. lol.
Dave

 

[HappyHellfire]

Maybe @HappyHellfire would explain the differences between the original design and the current design and why he changed the design? Advantages? Disadvantages?

The original work has serious issues with the quality of the prototypes and reproduction of the data. Last summer I worked with a couple of students and we found how bad the prototypes were. We then hired a local machine shop to make a batch of prototypes and they worked great.

This lead us to buying a HAAS CNC chucker lathe and using it for prototype testing. Since we did that, the results are stable and reproduceable.

It has been a very long road.