375-408 CheyTac (Kirby Alert!!!!!!) question

[Nicholas]

Blackdiamond,
I have shot Dean's 375/408 with lost river ballistic bullets and it shot under 1/2 moa at 100 yards easy. I haven't shot the 408 at 100 yards but have shot it out farther and it did quite well past 1500 yards.

Dean is a good guy and I respect him.

 

[Brown Dog]

…..I'm still keen to learn how the faster bullet is ignoring Newton's First Law of Motion by somehow 'remembering' the force that caused its initial deceleration for the rest of its flight? /ubbthreads/images/graemlins/confused.gif



…. I believe, as others have pointed out, the only explanation for the higher mv bullet being 'beaten' at long range must be (since both bullets will have identical zero yaw drag when at the same velocities) that the faster bullet (in the case of the '50yds less' example) must have had greater yaw drag for its entire flight (ie it hasn't been properly stabilised).

...presumably the doppler data quoted only treated the projectile as a point mass ....and gave no direct yaw data?

/ubbthreads/images/graemlins/smile.gif

 

[waha]

Brown Dog you are right-on. The tipoff forces create this nutational motion from fast High "C" bullets and will cause this and we never have a clue because the bullet has stabilyzed at 100 yards. This only occurs occassionaly but it can seriously effect the B.C. in the first 100 yards. Check out Sierra's 4th (50th anniversary manual), and 5th manuals. I don't think this velocity differential is that significant but enough that Sierra and others recognize it.

 

[Mountainsheep]

Brown Dog
As a point of consideration I would like to offer another explanation to illustrate how a bullet with an initial greater velocity could decelerate at a greater ratio than a bullet launched at a slower speed. This example will adhere to Newton's First Law of Motion and no credence is given for any memory of force. But, I do believe that Newton's Second Law of Motion and the First Law of Molecular Adherence are also applicable to this situation and will indicate a significant Accumulation of Resistance.

As the bullet travel forward it contacts atmospheric particles that apply a negative or resistant force to the bullet. These particles do not accelerate away from the bullet but remain in contact with its surface until other particles collide with them and push them along the sloped surface of the bullet until they reach a point at the apex of the sloped surface. Due to the extreme rate of Frequency of Collision, the frontal surface of the bullet should quickly become coated with a layer of atmospheric particles, thus increasing the overall mass and surface area of the bullet. A visual illustration of this would be driving a car into rain without using the wipers. As the car moves forward the droplets hit the windshield and are pushed upward along its surface by air until reaching the top and blowing away. If the rain is steady and you are driving at a significant speed, the windshield will soon become covered with a solid sheet of relatively slow moving water.
Now if we apply the First Law of Molecular Adherence to the now coated surface of the bullet, a substantial percentage of subsequent atmospheric particles that make contact should bond, compress and add additional mass and area to the surface, thus creating conditions for greater resistance; the greater the speed of propulsion, the greater the volume of accumulation.
In an effort to support my application of the First Law of Molecular Adherence, I'll ask this question: Is the "trace" or "trail" that sometimes appears to follow a bullet in flight, nothing more than atmospheric particles compressed to a density that is capable of reflecting a visual amount of light? (I don't know the answer, just guessing)

Does this theory seem valid or I'm I way off base on this one? I'm not trying to debate but just offering additional ideas and testing a theory.
Dave

 

[lerch]

That is a pretty **** interesting statment. From what I have seen of "bullet trace" what you have described is exactly what I have thought it to be. I have always noticed a more pronounced trace in certain lighting conditons, morning sun with the light coming from behind you, and in very calm conditions where the compressed air would be more stationary.

As for the compression of molecules on the forward area of the bullet would their not be a kind of maximum point of this happening. Where it would happen on both bullets fired at the same speed. though it would happen to the faster bullet quicker it would also happen to the slower bullet though it would take a little longer. IF both bullets were identical I would think that the amount of substance catching on the forward end would be the same and though it would happen to the faster bullet sooner due to its speed the bonding would reach its maximum amount faster also leading to both bullets reaching the max amount of bonding and then becoming equal again. I would think that the initial velocity advantage of the faster bullet would still prevail since both bullets would be comprimised in the same amount.

Like I said if anyone here looked at my math scores in college you would know i have no place in this conversation but I figured what the hell, gotta learn some how.

take it easy
steve

 

[Brown Dog]

Dave,

Thought provoking post; I think my dog must have wondered what I was preoccupied with on our walk just now!

I'm not familiar with laws of molecular adherence, at the levels I've worked we've been more interested in the behaviour of the boundary layer rather than its mechanism of construction…

….but in terms of the 'added mass' idea; my mental 'common sense' check would say no; not as a measurable effect: Lets SWAG the adherent layer as being X air molecules thick. The volume of adherent molecules will be tiny….the surface area of the bullet x the depth….and so the mass involved will be tiny tiny….

…..lets double, triple, quadruple the thickness………the weight of that layer will be still be tiny tiny …..and insignificant when compared to the weight to weight variations of, even, bullets sorted by weight.


..What I believe would be more significant (and measurable in terms of effect) would be the behaviour of the boundary layer (ie the layer in which the effect of air viscosity is significant).

…no doubt I'm teaching you to suck eggs here (does that expression translate across the atlantic?! /ubbthreads/images/graemlins/smile.gif) but when the boundary layer forms, initial flow is laminar…..as the thickness of the flow increases (viscosity/adherence) it becomes unstable and 'falls over' in a transition region to become turbulent. Obviously, the position of that transition region on the projectile will vary………if I'm not expressing this well, I'm thinking whilst I'm typing here………..I think we'd agree that with the higher MV proj, this transition region would move forward….and the resulting turbulence would be greater.

…..now, here's the weird bit, despite the fact that this increased turbulent layer will produce increased 'skin friction' drag…………..its overall effect is to reduce total drag.


Why? The turbulent flow will significantly decrease excrescence (base) drag (a far more significant effect)(by allowing greater mixing of the flow it reduces the pressure gradient at the base.)


To summarise / paraphrase all that waffle:

Greater adherence would, in terms of mass increase, be insignificant.

But, greater adherence would increase skin friction drag through increased boundary layer turbulence…

But, the effect of that boundary layer turbulence is an overall reduction in drag due to the effect it has in reducing base drag.




…clear as mud? /ubbthreads/images/graemlins/smile.gif

Interested to hear your take on that /ubbthreads/images/graemlins/smile.gif


...so I would contend that your adherence theory, rather than slowing the projectile, actually contributes to an effect that decreases total drag and thus improves performance /ubbthreads/images/graemlins/shocked.gif /ubbthreads/images/graemlins/smile.gif /ubbthreads/images/graemlins/smile.gif

I remain convinced that increased yaw drag must be the only answer! /ubbthreads/images/graemlins/smile.gif

All the best

Matt

 

[Mountainsheep]

Matt,
Thanks for your attention and consideration of my theory. I am most enlightened and I am in total agreement with your explanation. Your deduction proves my observation of surface expansion and drag was a bit shallow, (pardon the pun) and that the reduction of base drag is defiantly a more significant factor in determining the resistance force. Every learning experience is a valuable contribution that provides opportunity to further expand our boundaries. I'm by no means an expert on ballistics or mechanical physics, but I do feel that by openly sharing our thoughts and reasoning in an open forum will help the general public realize that there is quite a bit of in-depth research and intellectual thought involved in our sport. Exposing the science and technology involved in our equipment development will hopefully debunk any perceptions of machismo that might be associated with the use of larger and more powerful ammunition.

Have a great weekend,
Dave

 

[Brown Dog]

Dave,

I enjoy the chance to be forced to think about things that I haven't considered in detail for a long while; thanks for the chat. (I'm no ballistics expert either; but -as Kipling had it- in the land of the blind, the one-eyed man is king!)

You have a great weekend too.

Matt

 

[Guest]

This is a good discussion and the theories are great but the guy that started all of this (triggerfifty) has still not come back to explain his theory. I would still like to hear your explaination triggerfifty.

 

[overbore]

I hate to enter these "highly turbulent" waters but if I read the bottom line of a great theory correctly, does it not translate, even across the Pond, into this: a rebated base ULD will have less base induced drag than a nominal BT or FB bullet? Now to go way, way out, how about integral turbulence generating patterns engraved upon the circumfrance of the bearing portion???This works in a fluid medium and at supersonic air velocities---- Overbore /ubbthreads/images/graemlins/confused.gif

 

[critter]

Ouch! my head hurts! /ubbthreads/images/graemlins/confused.gif

Triggerfly Sir, Please explain your info in simple simple layman's terms. thanks! --critter

 

[Ballistic64]

I got it! The bullets werent on the same race track.One was fired east and the other fired west. /ubbthreads/images/graemlins/grin.gif

 

[overbore]

Contacted LR about the recent, November, Precision Shooting article "announcing" the 375/408 cartridge. They have no actual data nor any release date. How does that square with those of you who have an insight? Are any of you going ahead on your own or are you being updstaged by LR? Overbore /ubbthreads/images/graemlins/confused.gif

 

[Brown Dog]

[ QUOTE ]
I hate to enter these "highly turbulent" waters but if I read the bottom line of a great theory correctly, does it not translate, even across the Pond, into this: a rebated base ULD will have less base induced drag than a nominal BT or FB bullet? Now to go way, way out, how about integral turbulence generating patterns engraved upon the circumfrance of the bearing portion???This works in a fluid medium and at supersonic air velocities---- Overbore /ubbthreads/images/graemlins/confused.gif

[/ QUOTE ]

Overbore,
Apologies, only just noticed the above. I'm not sure what you're asking....I think we're agreeing /ubbthreads/images/graemlins/confused.gif /ubbthreads/images/graemlins/smile.gif ...can't comment on the effect of engraving on the bearing surface of a bullet but, from memory, the turbulence created by the engraved notches in a shell's driving band (for example) does contribute to a reduction in the shell's base drag..... /ubbthreads/images/graemlins/crazy.gif /ubbthreads/images/graemlins/smile.gif