On Wed, 17 Dec 2003 10:31:18 -0500, Tom Quackenbush wrote:
Bob Swinney wrote:

Yep! Stands to reason the rotational velocity would sustain long after
forward velocity.

snip

Yes, it's still rotating, but is there enough spin remaining to
stabilize a bullet falling base first? The rifling twist is usually
chosen to impart the necessary spin to a limited range of bullet
weights within a range of speeds (relatively fast speeds, compared
with falling) for a bullet fired point first. It's counter-productive
to spin the bullet much faster than is required for normal firing
conditions.

Right, but normal firing conditions are worst case. That's because
the axis of rotation diverges from the line of flight (normal bullet
path is a parabola). This divergence allows aerodynamic forces
to disturb the gyroscopic action, causing the bullet to nutate
around the line of flight. This saps rotational energy, and causes
the bullet spin to slow faster than if the forces were aligned.

A bullet fired vertically *does* have all forces aligned. The line of
flight does line up with the spin axis. So the bullet loses rotational
energy much slower than when fired horizontally. Add to that the
bullet doesn't have to spin as rapidly to maintain stability when
the bullet is moving slowly (because the disturbing aerodynamic
forces are less). So a vertically fired bullet has the best chance
of maintaining gyroscopic stability.

Let me go out on a limb and say that sometimes the bullet remains
stable and sometimes it doesn't.

That's probably true, but a tumbling bullet isn't as aerodynamically
clean as one falling base or point first. So if stability isn't maintained,
terminal velocity will be even lower.

Gary