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.
Thank-you for the straight talk. This kind of honest communication for the good of the sport will reward you.
As stated in another thread I have recieved my rifle from Lawtons. It is a 30 in 8 twist on an 8000 action in a Manners stock bedded by Lawton. Noel Carlson presented me with samples of prototypes of his banded solid 375 bullets. He sent me samples of two different configurations. The first is a copper 431 gr. and the other is a brass 417 gr of identical demensions. The copper by far is the most interesting of the two. I started out with 130 gr of US869. The holes were round and there was no excessive pressure. I continued to step it up and attained 2965 fps with 150 gr of US 869. At this point the bolt became a little sticky and there were slight ejector marks on the case. The cases from Jamison are first class, I only used 3 cases for the tests and have 32 loadings on each case. There have been no split necks and the pockets are still good. I encountered stability issues at the higher velocities and Noel is tweaking the design to correct this. At lower velocities accuracy is very good, 100 yd 3 shot groups were .3 to .4 moa. The most outstanding display of accuracy was a 1/4 min. three shot group at 600 yds. At 1020 yds a half min. was attained. I was having more ES than desired on velocity at the lower velocity end of the spectrum which caused vertical stringing at distance. This was caused by the 125 gr of BMG which left a huge amount of the case unfilled. The load that produced 2900 fps was 145.5 gr of US 869. This load produced ES in the single digits and showed no signs of excessive pressure. This bullet could probably be driven very close to 3100 fps from the 408 improved versions. Think of this, you have a 431 gr projectile traveling at 3100 fps with a BC ------ OK here goes----- around 1.2 +. Fowling is a non- issue. Cleaning this rifle takes less effort than any of my custom 308 barrels. I fired into a couple of different medians and the nose bends past 90 degrees. I shot one into a clay bank, when it was recovered it was a round circle with the center touching. Anyway we are on the verge of seeing something awesome, if Noel can make the tweaked version fly as well at 2900-3100 as the first ones did at slower velocities, long range shooting just got a boost.
Augustus,,, On your post above, you estimated that this bullet might see 3100 fps on the improved version of the 375 cheytac,,,,,Were you using the improved version on your tests above????
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, for what it's worth, I really hope you continue to refine your 431 gr banded solid for a 1:8 constant twist barrel. I'm still waiting for the results from the NV shoot, but even in this early stage the 1:8 twist looks promising for the 431 gr.
I think you will always have two oppisites sides of the spectrum with regards to shooters (long range hunters and long range shooters). But there is a growing group of guys like me that are hunters and long range shooters.
In the cheytac 375 cal, it looks like we can shoot a handful of bullets including the 350 smk's with the Lawton type of 1:8 twist. And although this looks to be the bottom floor (in the twist dept), of the SMK's, it looks to be the upper end of the 431 banded bullet. And even though the 1:8 looks to be the raged edge of both bullets, it gives guys like me direction when choosing barrels.
I've been waiting to order my 375 allen mag barrel pending the results of the NV shoot, but I'm not sure I can wait. I may just go with the 1:8 and hope that my assumptions are correct about both the smk and the 431 banded bullets. If the 431 can stabilize in a 1:8 constant twist, I think this bullet will be a great product!!!!!