Light, high BC bullet

[Warren Jensen]

txhunter,

It will be difficult using drawn-jacket, lead-core, swage technology to get a BC to .800, let alone above .900 or 1.00. There are limits to where these techniques can take you. We have .338 BCs in the high .800s, now, 30 cals. in the .900s and some new 50s way above 1.00. The tangential ogives used in those older methods are inferior ballistically at supersonic speed. Secant ogives are superior to Mach 5, or hypersonic velocities where cone ogives become superior. We can computer model a projectile to optimize it and when I have what I want I can input it to the big CNC and 5 minutes later I have that bullet in my hand to go test. If it needs adjustments I can do that very easily. I can do several design, manufacture and fire tests in an hour. Do you have any idea what the cycle time for drawn jacket technology is to do that same thing? It is weeks to months.

The difficulty is in the firearms, specifically, the barrels. Barrels have been refined for many, many years to shoot drawn jacket lead core bullets very accurately. Some of the things that make a barrel shoot lead core bullets very well, make shooting solid bullets difficult. For example, it is well know among barrel makers that to get top level benchrest accuracy you make the barrels to squeeze down on the bullet more than normal. You make a minimum dimension or subminimum dimension bore and groove. In .308, for example, you make a bore that is .300" or less and a groove that is .3076" or less. These tolerances cause me no end of headaches. I can get sub-quarter-minute groups at 400 and 500 yds with standard run-of-the-mill barrels, but when you use one these top of the line benchrest barrels the accuracy is terrible. Convincing the top barrels makers to start doing things differently than what they have learned, is also difficult. Custom fitting bullets for each barrel is time consuming. Eventually we will get there. I have proven to myself with several projects, the .408 being one, that this is the way of the future. The projectiles are simply superior.

The second thing that will have to be changed is the limitations of magazines and SAAMI specifications. Most of the limits on what can be done with a bullet and cartridge are set by the receiver that will be used. This too will changed with time.

Expect in the next 10 to 20 years that long range shooting will be done commonly with bullets that have BCs above 1.00.

[ 07-26-2001: Message edited by: Warren Jensen ]

 

[steve smith]

Shot down again. Another idea gone down in flames. Oh well, the next one will be a ringer. I just know it.

Thanks for your time Warren! I know you probably have much more important things to do than to answer our questions and I don't know if you realize how much we apreciate your presence here. Having some one to bounce all of our hairbrained ideas off of is really nice if not convenient. Again Thanks.

PS. Are you sure that the laws of physics can't bent a little. Just this once. Please

[ 07-26-2001: Message edited by: txhunter ]

 

[Warren Jensen]

Txhunter,

I think that stuff from burnt cookies that David mentioned may pierce the limits of known phenomena. I am pretty sure that some of that secret tank armor was made from such materials.

 

[steve smith]

I just assembled the first round of 27/300 Ultra Mag. This thing looks like an intercontinential ballistic missle. That burned cooky heat shield material may come in handy after all.

 

[cronhelm]

So what is more important, BC or muzzle velocity?

Obviously having both factors fairly high will always result in a superior long-range projectile but if one has to compromise, which should take precedence?

I did a bit of informal research into BC and muzzle velocity. Both generally peak in the middle of the calibre range with conventional bullet designs. Off the top of my head I believe it was the .264 Mag that had the highest velocity with the highest BC bullet.

Peter Cronhelm

 

[Dave King]

I'm of the belief that if you are going to keep your shooting inside 500 yards or so, you can get away with going for velocity and shoot lower BC bullets (within reason). If you are going long range, greater than 500 yards or so you need to seriously consider high BC bullets and sacrifice a little velocity.

Run several charts comparing one cartridges' bullet selections. Run light lower BC high velocity in comparison to heavy high BC lower velocity. (Actually weight is not the consideration, BC is what we need to compare but is hard to break the weight/BC bonding ritual.)

 

[cronhelm]

Isn't that the thing though? Every factor is related to every other factor and you can't gain in one without losing in another.

While BC is related to weight, weight has a stronger relation to muzzle velocity. More weight (usually) means a higher BC but it also means a lower MV.

In my case, I lucked out and found a 6mm bullet that had a high BC and weighed 12 grains less than the big 107gr MK.

The Berger 95 grain VLD has a BC equal to the bigger MK. So I can get ~100 fps more velocity without sacrificing BC. This is what allows me to take a .243 Win out to such outrageous distances.

The bullet reminds me of a tiny javelin.

FWIW I did an estimation of the energy of the 95 gr bullet at maximum distance and it is about equal to that of a .380 ACP handgun. Not exactly big game material but it could definatley do some damage with proper placement.

Peter

 

[ss8541]

I believe when velocities are within 150 fps that you should go for High BC and accuracy. Lately I have been working with both Warrens' 140 grain J36 (.644), and the 130 grain HV from GS customs (.550), in my 7 STW. I am very impressed with the J36 which I get 3 shot groups in the High .2's and 3's at 3500 to 3550fps from my 27.5", 1-10 twist barrel. I just started shooting the HV's and If I only want to reload a case 2 times I can shoot them at over 4,000 fps(4,070). It looks like with good case life I will shoot them between 3850 and 3900 fps with accuracy in the .2's and .3's. My calcs show this bullet having a clear advantage under 1000 yds. I am working on loads for Mule deer and antelope out to 800 or so. I still haven't shot either of these bullets at anything other then 100 yds yet, but next week I will hopefully shoot both at 600 to get an idea of how they are holding up in comparison to each other. It just seems to me that even with a difference in BC of .094 in favor of the J36's that the extra 300 to 350 fps tips the scales toward the HV bullets. If I was planning to hunt with these past 800 or so I would have to reasess the situation. At that point I would really start worrying about Energy and expantion, not just trajectory and wind. Like I said earlier I have only compared these bullets out to a thousand yards at this point, on a balistics program, and if the J36's shot noticeably better from my gun. Say a difference of .2" or more then I would go for the accuracy.
Vince Foster

 

[ss8541]

I just compared the 140 grain J36 to the 130 grain HV bullet. With a 350 fps difference in velocity the HV shoots much flatter, 7.8 inch difference at 800 yds, and 12.7" less at 1000. The J36 is the best on wind though, with it's higher BC. With a 10 mph cross wind the J36 drifts 2.4 inches less at 800, and 4.5" less at 1000yds. If I was picking a bullet to use mainly out past 650 yards the J36 would get my vote. But since I am just looking for a good fairly in expensive (HV's go for 48$ per box of 50 delivered) all around loading right now (300 to 600 yards)I am leanning towards the HV, but this may change when I shoot them both at 600.
I used the standard G1 curve on the JBM ballistics software to do the comparison. The designer of the HV says that you should use a VLD drag curve, but I won't use anything other then a G1 unless this curve proves to be way off at 600 next week.
I have a question for Warren. Do you see any point in using a curve other then the G1, for bullets of VLD design? In some ballistics software I have seen the G5 listed for boat tails and the G7 curve for VLD designs. There is a huge difference between them when you run the numbers. what is more reliable from your experience.I understand that this will vairy from manufacter to manufacturer, and even bullet to bullet. I don't have enough experience shooting at long range with the VLD design to know. While in the Marines I shot thousands of rounds of 308 with 168 and 173 grain millitary special ball out to 1100 yards or so. And I have several thousand rounds through the Barret 50 cal. out to past 2,000. But I never had other bullets with a good design to compare. We had our drop charts and you just shot and made some adjustments to them for the lot, and the rifle under those conditions. I never got to pick and choose, or do my own testing. By the way Warren I can't waite to try the 200 grain J40's in my 300 Ultra on rockchucks out past 1000 yds.
Thanks,
Vince Foster

 

[Warren Jensen]

Vince,

The explanation you want would be easier to give with the aid of graphics. I may get a little long winded, but bear with me.

Using the correct Siacci Function (G1,G5,G6,G7) for the bullet will give a better approximation of the bullet's trajectory than strickly using the G1 curve for all bullets. (Take note of the word approximation, as I will expand on that later.) I am not aware of a popular ballistics program that uses anything other than the G1 function. The G1 is for flat-base bullets with tangential ogives of about 2R. The G5 function is for boattail bullets with tangential ogives of about 6R. The G6 function is for flat base bullets with tangential ogives of about 7R. The G7 function is for boattail bullets with secant ogives of about 10R. All of these functions are mathematical models. All will provide good numbers in two planes of motion, but completely ignore the third plane. For ranges out to 1000 yds for 308 class cartridges and 1300 for the bigger stuff they are good enough. Beyond that the errors start to grow, unless the bullets are balanced, in which case the G functions will be real close. The plane that the Siacci Functions does not account for is the Z axis, drift due to precession, and the axial components of this. When computing the drag coefficient only the side cutaway of the bullet is considered.

To get a more accurate trajectory model you have to consider the bullet in three axis and include it's rotational characteristics. When most folks picture a bullet in flight the picture they see is that of the bullet as it was taken out of the box before it was loaded in the case. A bullet in flight does not look like this at all. It has been extruded down a tube, engraved by the rifling, and is slightly longer than it was coming out of the box. It's flight WILL be effected by these changes and more particularly the specific changes that the engraving made to the physical characteristics of the bullet. This is where the word APPROXIMATE that I mentioned above comes in. No ballistics program, none, zero, zip, nada accounts for this. The rate of spin, the rate of decay of that spin, the width and depth of the engraving marks, the density of the projectile, and the surface area of the projectile all have to be accounted for if the trajectory is to be predicted accurately.

Let me explain. It is a rule of thumb that rotational (axial) drag is always less than linear (horizontal) drag. (At least it used to be.) This means that with time the bullet will be spinning faster and faster relative to it's forward velocity. It is also a rule of thumb that a bullet is most accurate when it is just stable. Spin is necessary to keep the bullet pointed nose forward, but every bit of spin above what is just necessary starts to cause unwanted things to happen. The more spin above what is necessary the more the unwanted things increase. These include precession, yaw, Magnus moment, and slow and fast mode oscillations. Precession and yaw are the ones that have the most direct effect on the trajectory. As the spin increases the bullets longintudinal axis begins to pivot such the the nose of the bullet is flying a little high and to the right of the oncoming air. It is drawing little cirles in the air. As the precession increases this offset increases and the size of the circles increase. The bullet will begin to yaw and buffet. The effective form factor is decreasing as is the effective BC. This is not really noticeable on round nose or G1 projectiles because there is no real different between the aerodynamic center of the nose and being slightly offset. It really becomes apparent with long, sharp nose bullets, and is the reason that bullets with very fine meplates have been considered inaccurate by many. They simply require a much finer level of tuning than do less sharply pointed bullets. Anyway, this precession, yaw, and increase in drag is completely unaccounted for in the Siacci functions.

The rate that the precession occurs is a function of the relationship between the forward drag and the axial drag. The axial drag is effected by the twist rate, the number, width and depth of the lands, the surface area of the bullet, and the effective roughness caused by the engraving. Different twist rates, different land configurations, etc. will impart to the bullet different axial drag numbers which in turn will effect the actual precession that the bullet will have. Therefore, to know exactly what trajectory your bullet will follow, especially past 1000 yds, you have to know these characteristics.

OR, you can shoot your bullet from your rifle and plot where it impacts at the various long ranges. This will give you the same effective information, but without all the math. Your trajectory will be slightly to greatly different from the same load and bullet fired from a different rifle with a different twist, and/or, land and groove configuration. It will not match exactly any of the Siacci Functions.

 

[ss8541]

Warren,
Thanks for your informative reply. That was exactly what I was looking for.
Thanks,
Vince Foster