On the topic of bullet stability it appears there may be some misunderstanding about the topic and I offer the following for whatever it may be worth.
The need for inducing gyroscopic stability stems from the imbalance of center of gravity (CG) and center of aerodynamic pressure (CP) in conical projectiles. In round balls they are one and the same location, but as length increases the CG remains constant while CP moves forward. In short, longer bullets require more spin to supply sufficient angular momentum for stability. Velocity is only pertinent for a given length in context of the degree of moment generated between the two points. Velocity has significant influence on that issue as it presents different levels of drag. In typical circumstances as drag increases so to does the moment. The graph below represents a comparison between drag and velocity, with the former represented as Cd, or coefficient of drag. Cd is a fundamental component of BC calculus.
The point I wish to make is that if a bullet is stable at Mach 1.5 it is also stable at Mach .80. As the speed of sound is variable subject to temperature I'll let you twiddle the fine numbers if you wish, don't get lost in a mathematical fog. You will notice from the graph that as represented that the drag increases sharply as one approaches the speed of sound, peaks and then drops with some rapidity. Not shown on the graph is the CD numbers found up around Mach 4. The slope continues its downward journey and in this realm drag is actually quite low, as is the necessity for fast twist.
Couple of points on this and I'm on my way. Most cast bullets dwell in the high drag regime or what is known as the transonic range of velocity. Roughly Mach .80 to 1.3 in general terms. I'm NOT talking about bullet velocity so much as I am the velocity of air in the flow field. At subsonic velocity air is not compressible. The form of the bullet weighs heavily upon flow field velocity as air seeks to null compression factors by accelerating about the bullet. In short, the bullet might be cruising at 1050 fps but the air velocity around the ogive/meplate may be considerably higher. That means increased drag and a higher requirement for angular momentum. Bullets with more pointed or elliptical nose forms suffer far less from this. Now you know why marginal stability in long range guns leads to bullet upset/tumbling at long range when the bullet falls to lower velocity, and likewise, why this can be fudged to small degree with higher muzzle velocity. Higher velocity equates to higher angular momentum to a very small degree. Twist rate is the more influential metric in the equation.
Some time back a fella at a respected barrel shop told me a few things that were wrong. 1) cast bullets with a suppressor was not practical. 2) 1:12 twist would not stabilize a 180 gr bullet at subsonic velocity. He was quite wrong about that. Do your homework before diving into the deep end of the pool. There are many programs available on the 'net which will allow proper calculation on these fine points. http://www.jbmballistics.com/
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