Great data, Cedric.... Here is how the bullets compare.... The Form Factor is a measure of how "slippery" the bullet is, and the lower the better.... If it is over 1.00 it means the bullet has more drag than the G1 Standard Projectile....98.5 gr. BBT:Your BC = 0.203 @ average velocity of 958 fpsSD = (98.5/7000/0.284/0.284) = 0.175FF = 0.175/0.203 = 0.859130 gr. LBT:Your BC = 0.249 @ average velocity of 860 fpsSD = (130/7000/0.284/0.284) = 0.230FF = 0.230/0.249 = 0.923As you realize, the BC you calculated is only valid at that velocity, how the BCs compare at some different velocity can't really be determined without multiple data points to discover the shape of the Drag Curve for each bullet.... However, we CAN calculate the Drag Coefficient for each data point by multiplying the Cd of the G1 Projectile by the FF determined, and then we can plot those to see how they compare to the standard G1 and G7 Drag Models.... Here is that chart....The distance the data point is below the G1 curve is an indication of how good the bullet is, taking Sectional Density (bullet weight) out of the equation.... The further it is below the G1 line the better.... Since the LBT bullet is pretty close in shape to the G1 Model, we might expect it to have a drag curve that is quite similar to the model.... while the BBT, having a boattail, might have a drag curve somewhere in between the G1 and G7 models, because it does have a boattail, but not an extreme one.... What that means is, that if you could push the LBT 100 fps faster, it should have a lower BC than the BBT.... and the same thing if you slowed down the BBT 100 fps.... However, that isn't going to happen with the gun you have, you would normally take advantage of the velocity it can produce with either bullet.... and the retained velocity and energy of the two bullets are really all that matters....All in all, I am quite pleased that the boattail is doing what it is supposed to do.... reducing the drag, and therefore the wind drift, at any given velocity.... and it looks like you are getting great accuracy as well....Bob
Cedric, all you guys shooting at long range are playing a balancing act between trajectory and wind drift.... If you look at that graph, and think of wind drift on the vertical axis and trajectory on the horizontal axis, you will have a pretty good grasp of the problem.... Low Cd means low wind drift, but high velocity means flatter trajectory.... Since the velocity is predominately a factor of bullet weight, and the faster you go, the closer you are to Mach 1 where the drag takes a huge increase, you are trading off one for the other.... A more pronounced boattail, in a slightly longer and heavier bullet might the best compromise.... Look at the 5 designs in this thread for a .257 cal....http://www.gatewaytoairguns.org/GTA/index.php?topic=94920.0In 7mm, those designs would be 88, 104, 120, 135, & 150 gr.... Looking at the 104 gr. and 120 gr., they would have an SD of 0.184 and 0.213.... You are shooting the 98 gr., with an SD of 0.175 at 980 fps (210 FPE) and the 130 gr., with an SD of 0.230 at 870 fps (220 FPE), so we might reasonably expect the 104 gr. to shoot about 212 FPE (~960 fps) and the 120 gr. about 217 FPE (~900 fps).... The theoretical twist required is about 12" for the 104 gr. and 11" for the 120 gr.... I have a gut feeling that the 104 gr. would be OK in your 14" twist, but the 120 gr. would be marginal or unstable, so let's look at the 104 gr. more closely.... In .284 cal, at 2.5 calibers long, it would have a LOA of 0.71".... The nose radius, if scaled up, would be 0.88", but Veral doesn't have one of those, but he does have a 0.8" and a 1.0" Ogive pattern.... Either one of those will provide a much sleeker, lower drag nose than the 98 gr. BBT you have now.... The big change, however, is the longer, shallower 8 degree boattail.... The combination of those should reduce the drag to a significant degree.... Using the same FF as the 98 gr. of 0.859, the BC should increase to 0.214, but at the slightly lower muzzle velocity that should increase a bit.... With the changes to the nose and tail, if should go even higher.... If you're interested, I think maybe we should start with the 104 gr. 2.5 caliber long boattail, and tweak it a bit to meet your needs.... Veral now has the new 8 degree boattail pattern done, it is used on the Whiteout.... He has the 0.8 and 1.0" Ogive patterns, either of which could be made to work.... If you don't like the bore-riding nose, there are two alternatives, one with the back of the Ogive at groove diameter and the area between it and the rear driving band 0.002" smaller than the lands.... The other would have conventional drive bands and grooves.... Veral can make any of them without making new patterns.... If you let me know what you want, including the weight, I'll draw up my suggestion.... I would suggest staying between 100-115 gr. at the outside....EDIT: I had some time tonight, so came up with a suggestion for you to look at....The dotted line is where the Ogive would start to engage the rifling, so the total length engaged by the rifling would be well over 1 caliber.... The center section is about 0.276", just clear of the lands.... The stability is 1.3 at 1000 fps in a 13" twist, but I am quite confident it would work fine in your 14" twist.... I'm not sure how much longer you could go, however, this might be pushing it.... Here is the existing bullet for comparison....As you can see, the new design is quite a bit sleeker, and should have a significantly better BC, even though it is not a lot heavier....Bob
Boattails make the bullet act longer, both in terms of reduced drag and increased twist required, so at least in theory they need faster twists than the equivalent length flat base bullet.... The twist calculators have a correction for boattails, but it seems from the evidence we are slowly accumulating that the rebated boattails I use don't need the full correction factor that is built into the twist calculator.... A flat base bullet with the same nose and overall length, would in theory only need an 18" twist for the same stability as the boattail in a 13" twist.... My guess is that 15" is probably OK for the boattail, maybe even 16"....The theoretical drag curve for the 105 gr. boattail above is virtually flat, out to Mach 0.85 (949 fps), and only increases about 20% at Mach 0.90 (1005 fps).... so it should be running right in the sweet spot in your gun.... The calculated Cd is 5% better than the original 94 gr. BBT at Mach 0.85 and below, and 28% better at Mach 0.90.... The blunter nose and tail of the original caused the rapid increase in drag above 950 fps (~50% increase by the time it reached 1000 fps), and you were already starting to get into that speed range.... With the 12% higher SD, that should, in theory, mean an increase in BC of about 18% up to 950 fps.... I hate making predictions, but that should mean a BC of about 0.24 at 950 fps....Bob
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