April 2026 (2) - Flipbook - Page 34
the e昀昀ects of the PIP.
It is noteworthy to mention here
that we don’t always have unexpected accuracy issues at 900
yards. Yet it does happen enough
in the past 29 years to have
caught our attention.
So just what exactly happens to
our spinning projectile when it
drops down to .8 Mach?
The Aerodynamic “Wall” at 0.8
Mach the PIP...While the term
“ballistic wall” often colloquially
refers to a physical barrier, in
昀氀ight mechanics, it describes the
sudden increase in aerodynamic
drag and pressure changes as an
object nears the speed of sound.
• Transonic Drag Rise: At 0.8
Mach, local air昀氀ow over parts
of a projectile (like the nose or
boattail) can reach supersonic
speeds even while the object
itself is subsonic. This creates
localized shockwaves that signi昀椀cantly increase drag, a phenomenon often described as hitting a
“wall”.
entered or passed through the
transonic range (roughly Mach
0.8 to Mach 1.2), where aerodynamic forces shift signi昀椀cantly.
At these speeds, the following
factors compromise 昀氀ight stability:
• Center of Pressure (CP) Shift:
As a bullet slows into the transonic region, the center of pressure—where aerodynamic forces
act—shifts forward. If the CP
moves too far ahead of the Center
of Gravity (CG), the “overturning
moment” increases, making the
bullet prone to pitching, yawing,
or tumbling.
• Base Drag and Air昀氀ow Detachment: For boat-tail bullets, the
air昀氀ow often detaches from the
tapered base as it drops below
Mach 0.9. This creates a turbulent wake that can “bu昀昀et” the
tail, leading to erratic movement.
Flat-base bullets are typically less
a昀昀ected by this speci昀椀c issue.
• Magnus Moment: Changes in
the air昀氀ow around the bullet
base can trigger the Magnus
moment (not to be confused
with the Magnus e昀昀ect), which
destabilizes the projectile’s spin
and can lead to dynamic instability even if gyroscopic stability
remains high. This is The Phase
inception point. Where our bullets are very susceptible to odd
wind or condition abnormalities,
often due to range topographical anomalies, or strange wind
sheers.
• Dynamic vs. Gyroscopic Stability: While a bullet’s spin (RPM)
decays much slower than its
forward velocity—meaning its
gyroscopic stability factor (SG)
often increases as it slows—its
dynamic stability decreases due
to the shifting aerodynamic loads
mentioned above.
• Magnus Force E昀昀ects: For
spinning projectiles at 0.8 Mach,
the largest local Magnus forces—
which cause lateral drift—typically occur near the cylindrical
and boattail sections. This can
destabilize the 昀氀ight path compared to lower subsonic speeds.
• Predictability Issues: Ballistic
software often struggles at this
exact speed; below 0.825 Mach,
drag is frequently underestimated by 2–5%, leading to potential
errors in long-range trajectory
calculations.
Bullets traveling at Mach 0.8 or
slower encounter stability issues
primarily because they have
34
Kenny’s world record 5 shot group from the
Pedersoli 5 @ 200, July 2008, Raton, NM.
April, 2026 - Issue #2
