GMDR has shot many hundreds of rounds to obtain Ballistic Coefficient data for the 350gr
OT .458" FPbb at different
muzzle velocities. The study was performed for muzzle velocities that allowed for good
group sizes which therefore enabled us to use a second chronograph at 50 yards from
the muzzle. The reported ballistic coefficients were obtained with the BC Calculator
developed by GMDR, for chronograph readouts at the muzzle (starting bullet velocity) and
at 50 yards from the muzzle (terminal velocity at target). The GMDR's BC Calculator
uses the standard G1 drag function and allows to dial the specific ambient
conditions at the range for a more accurate estimate.
The BC values obtained showed a
smooth dependence with muzzle velocity for values up to about 950 fps.
A stronger dependence with the muzzle velocity was observed in between 950 fps and 1150 fps,
which seem to peak at around the speed of sound (transient region). The study has not
been extended to velocities higher than 1200 fps due to the poor group sizing obtainable
with the use of microgroove lever guns at those speeds.
These empirical BC values range from about .18 to .26 in between 600 and 950 fps, while peaking
from .26 to .46 and down again in the transient region. The following graph depitcs the
Ballistic Coefficients obtained (vertical axis) versus muzzle velocity (horizontal
axis).
For velocities in the supersonic region (1600 fps and above), the Bullet
Designer tool developed by GMDR estimates a theoretical BC of about .220 for the 350gr
.458" Oregon Trail design. The computed values and the technical specs are
shown in the following screen capture.
