These Von Karman projectiles are seven calibers in length, and are designed to function optimally from an exponential gain-twist barrel with an exit rate of ~20 calibers. In the case of Terry's .375, this would translate to a 1: 7.5" twist, which is slightly tighter than the 1: 8" constant-twist Lawton employed with a degree of success recently.
The .338 would require a still tighter rate of 1: 6.75".
A recent fall in copper prices has helped my pricing structure. The projectiles will be more than competitive with comparable solids, but I want to make the necessary refinements prior to formal marketing.
2,800fps-plus barrel exit perturbations/blast instability needs to be resolved in the, otherwise, dynamically stable flight properties of this design. There is also need to ensure that the engraving bands are stout enough to be interchangable with a constant-twist rifling geometry.
It should be noted that this is only the first component of a complete ELR barrel/cartridge system.
Noel, Im interested in shooting the 431gr in a 375....You mentioned Lawton had some good luck with the 1:8 twist....Im considering the 1:8 for my new barrle or the 16:8.....How does the 16:8 perform with this 431 gr??
The original 431 grain ZA375/7.0 went through significant tail revision in order to achieve dynamic stability beyond 2,600 fps in a 1: 8" twist, although it was highly accurate when fired at that velocity. It just goes to illustrate the limitations of an SG calc.
The 2nd (420 grain, solid copper) version gained another 150 fps in stable flight from a 1: 8". The 6.5" twist added even more potential, but the case capacity permits velocities of up to 2,950 fps, and the ZA/7.0 copper solid is still not fully stable at that velocity. I expect the PDT core to confer a significant gain in dynamic stability, without a net change in projectile mass. The ultimate twist requirement is unestablished, for the ZA375/7.0-W, at this point.
Terry gave you good advice. Hold on for a few weeks.
Expansion testing was done only in 338, and even that caliber pushed the gelatin beyond practicality... in terms of the quantity required.
What I know is this;
The bonded tin/bismuth core expanded reliably to 2+ calibers, at ~1,400 fps, in soft targets. The PDT core is more prone to vaporize the nose, but I do not have actual impact data as yet. I anticipate equivalent low velocity frangibility.
Kelly, the barrel I fired the RKY MTN bullets from was a constant 8 twist. It turned them to dust; however, I am sure they were the thinner jacketed version that 408 referred to. The new ones may work.
I do not like what I am seeing out of the gain twist barrel I have. It is a 13-6.5 and there is significant side swadging of the material on the bearing surfaces of the projectiles I recovered. I am going to order a 8-5.5 to test the 7 cal solid copper. As stated before I believe one should not get too radical in the deviation of the entrance to exit twist. Here again, we are treading in new waters and I may be totally wrong, more testing is in order.
Thanks for all the info guys....Wow, you guys are really pusing the envelope with these bullets.....I will wait fo the results of the tests in NV....It sounds like these bullet makers are more sensitive to the needs of long range shooters than the main stream makers. Hat's off to these guys!!!!!!!!!!
Two issues which have been raised that need some clarification, and elaboration.
First; There is nothing "theoretical" about the effect which an increase in the specific gravity of a projectile will have upon gyroscopic stability. Tungsten is more than twice as heavy, per unit volume, as copper. The correlated gain in SG is direct. The only unsettled question is how much of this gain can be extracted from a PDT core, that is not full caliber diameter, consequent to design constraints. That the twist requirement will be reduced is not up for serious debate, it is simply a matter of how much.
Second; The gain-twist rifling geometry has questionable value for any jacketed projectile. It has been tried many times, over the years, in a conservative gain profile. If recent competition results are a good indicator there is one thing that can be concluded... it need not impair accuracy.
Engraving bands have an entirely different mechanical dynamic with the rifling than jacketed bullets. ZA projectiles actually "float" within the gun bore. The only thing in contact with the bore surface are the bands, and only one of these bands acts as a gas seal (the rear one). As the bullet proceedes down the barrel, the forward bands deform in a controlled manner which act to progressively re-align the projectile's axis with the bore axis. This is one of the primary reasons that the ZA is capable of such extreme accuracy, but the benefits are not limited to accuracy. Axial torque is spread out along the length of the barrel more gradually, which makes the larger caliber loadings much more comfortable to shoot than an equivalent, constant twist geometry, is capable of. The gain-twist profile may also have a positive effect on barrel vibration.
In the 6.0+ caliber length projectiles, there is a metallurgical issue that needs to be resolved for future development flexibility. Tellurium copper, as used in hunt bullets, machines very easily, and expands/fragments at low velocities when designed to do so, upon impact. The ZA/6.5&7.0 do not have these fragmentation requirements, and evidence some shearing, as opposed to swaging, in the process of bore transit due to their longer bearing footprint. This leaves a small amount of shear residue in the barrel which, while not observed to detract from shot-to-shot consistency, does present an undesirable condition for some of the future developmental needs.
One possible short-term solution has already been suggested; increase of the breech twist, to reduce the overall net gain at the muzzle. In the long-run that is not a solution at all, as we will be firing at higher velocities, from shorter barrels. The ELR projectile will need to be as strong as the t-copper, but more ductile. I will be using SAE 660 bearing bronze in that application, which has a specific gravity that is identical to t-copper.