Good evening gents,
The issue of BPCR bullet stability has been quite the puzzlement, for quite a while, to this crank. For the past 6 years there has been a considerable focus on optimizing bullet design for a given twist and vise-versa. Some solidification of understanding is starting to take place through extensive testing by shooting bullets through heavy corrugated cardboard at distances from 50 yards to 1,000 yards and applying a measurement technique to estimate bullet yaw angle or wobble about its center of mass or gravity. The short story is, all is not as it would seem.
Now for the medium length story about to date findings, theories and hypothesis concerning stability of BPCR cast bullets.
Common wisdom, a reasonably approach to the problem, says that bullet length and barrel twist are all one should be concerned with, well and muzzle velocity. So far that has been disproved beyond a shadow of a doubt in this crank’s mind through actual live fire testing at various ranges using numerous calibers, barrel twists and bullet designs. All is not as it would seem nor is it what we typically hope for when attempting to design The Long Range Bullet.
Where to begin, where to begin, let's start with the easy stuff and work through some basics required for bullet stability and then proceed to experimental findings and current thinking.
The easy stuff has nothing to do with bullet design, barrel twist or muzzle velocity concerning appropriate bullet stability. It is simply a loading technique issue. Since our bullets are cast with relatively soft lead alloys the back of them bumps-up to fit the bore. If the nose of the bullet is not in line with the barrel-bore axis the back of the bullet will bump-up and be centered about that axis but the bullet-nose axis will be out of alignment with the bullet-shank axis. That is a less than optimal way for the bullet to start its flight down range. It will have an alignment induced yaw during its trajectory which no amount of barrel twist or MV can overcome. To make sure that does not happen one must load their ammo so that the bullet is as far into the barrel as possible when the shank bumps up due to the pressure created from powder ignition. I’ve personally seen holes through a Creedmoor target that were quite elliptical created by a 45-110 launching the Lyman Postell bullet to over 1,400 fps out of an 18-twist barrel. There was plenty of rotational stability for the bullet, but it was no longer a symmetric bullet along its longitudinal axis after bumping up and was therefore wobbling a lot. When this happens to a bullet in the transonic range, where our bullets spend most of their time from muzzle to target, the drag on the bullet will be much higher than it should be. That means the BC plummets, trajectory angle is steeper and the wind moves the bullet around more. The way to overcome this problem is to use a bullet that fits your chamber and barrel snugly and stick it as far into the barrel as possible and still be able to close the action. Simple, no? Oh, and put plenty of powder behind that puppy if shooting LR matches. At this time I’m convinced that faster is better so long as you can deal with the recoil, accuracy is topnotch and fouling is not an accuracy reducing problem in the weather conditions you will be shooting in.
Now for the next issue, what forces are making the bullet stable and what forces are trying to make it wobble or tumble? Most of us know that the barrel rifling spins the bullet which helps it spin on its longitudinal axis as it flies down range. Base drag, the drag caused by the base of the bullet also helps to keep the bullet spinning on its longitudinal axis, sort of like fletching on an arrow or fins on a rocket. The aerodynamic forces on the bullet as it flies from muzzle to target exert an overturning moment that can induce bullet wobble. This force is called the Center-of-Pressure. The Center-of-Pressure (CP) is the vector summation of aerodynamic forces acting on the bullet in flight. It is always in front of the Center-of-Gravity (CG) for a spin stabilized projectile. We really don’t need to know much about the CP except that the aerodynamic forces can be summed to act at a single point on the spin stabilized bullet in front of the CG and it will make the bullet wobble or tumble if not spun fast enough. Think of the distance between the CG and CP as a lever. The longer the lever the greater the force applied to making the bullet wobble. If the CP and CG acted at the same point the bullet would not tumble even if shot from a smooth bore. That type of bullet is commonly known as a “round ball.” Through bullet design we can work to shorten the distance between the CP and CG and improve bullet stability.
One issue that should be discussed when talking about spin stabilized bullets is that increasing MV does not always deliver enough added stability to optimally stabilize a particular bullet. The stability does do up due to the increased spin rate which is a function of the velocity squared but the over-tuning moment or CP also goes up with velocity but only in a linear fashion. Designing the bullet for the barrel twist and desired MV is really the way to go, not stuffing more powder into the case and hoping for better bullet stability.
Some findings that were a bit of a surprise might shed some light on this bullet stability issue. A very good example can be shown by relating what bullet holes looked like at 1,000 yards through the target using two different rifles and bullets. One rifle is a 10-twist 38-72 and the other is a 12-twist 38-70. The bullets in question are a 1.496” long grooveless G3 design that weighs 405 grains and the other is a 1.54” long spire point what weighs 373 grains. One would assume using current common wisdom that the 405-grain, 1.496” long bullet would be more stable out of the 12-twist barrel than the lighter, longer spire point. Not at all! The 1.496” long bullet produces quite elliptical holes through the target even at 800-yards. It shoots great out of the 10-twist barrel. That seems reasonable. But here is the kicker, the 1.54” long spire point that only weighs 373 grains will punch round holes at 1,000 yards when launched out of the 12-twist barrel. Go figure! So what is going on? My current understanding is that the CP for the spire point is much closer to the CG than that distance for the 1.496” long, 405-grain 3G bullet. This phenomenon has been seen with a number of other bullet designs, calibers and twists also.
We can use this information to optimally design bullets for a given caliber and barrel twist as well as targeted MV. When doing so for BPCR Target Rifle competition we are not concerned about target knock down, just accuracy, wind deflection and trajectory. For BPCR Silhouette we need to add ram knock down to the decision process if shooting one of the smaller calibers, more on that later.