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DIMINISHING RETURNS OF POWDER VS.
VELOCITY

By Wayne McLerran
Posted 1/15/20

The following should be especially interesting to Browning .40-65 BPCR
owners.  While working on an article on the Buffalo Arms (BACO)
JIM410410M1 bullet (408.7gr using 16:1 alloy) in the Browning with Swiss
1.5Fg.  I needed some velocity data vs. powder capacity.  I thought I’d share
the data.  The loads were not shot for accuracy, only over a chronograph to
determine average velocities for each load.  5 shots were fired with each load
with wiping between shots.  See the data chart below.  BTW, primers were
Federal Large Pistol Match GM150M and 0.060” fiber wads were used over
the powder.
Since the chronograph provided standard deviation data, it was included
although it should not be taken as an absolute indication of accuracy due to
other factors also affecting accuracy.  But with that said, the two lower SD
values in the chart do coincide with the best two accuracy loads for my rifle
with the bullet.

Another observation: if you do the math you’ll note that the changes in
compression is approximately 0.026” per grain, give or take 0.001” or so.  
This is due to the Browning chamber having a constant diameter 0.5” long
neck resulting in each additional grain of powder being compressed the same
amount.  In chambers such as the Shiloh Sharps .40-65, the neck has a constant
taper resulting in more compression required for each additional grain of
powder assuming the bullet seating depth does not change.

Wishing you great shooting,
Wayne