| Introduction:
Setting precise bullet-jump to achieve
accuracy gains is a common practice in rifle circles, but is much less
common (non-existent?) in pistol shooting. Essentially, one reduces the
distance the projectile must travel before engaging the lands in an
attempt to avoid longitudinal misalignment of the projectile with the
main bore.
A projectile whose long axis is perfectly
parallel with the main bore axis will exit the barrel with an
"ideal" spin about its long axis, less/no "wobble",
and will generally travel to the target more accurately than one with a
less than perfect bore alignment. A side benefit of eliminating the
"wobble" is that the frontal area presented by the bullet is
reduced, which will decrease in-flight drag and increase velocity at a
given powder charge. The concept is similar to throwing a perfect spiral
with a football. This is a highly simplified
explanation of the subject, but sufficient for purposes of this article.
Bullet-jump reduction comes in basically
three flavors: 1) adjusting bullet seating depth to place the projectile
closer to the lands, 2) reducing the free-bore (lead) in the barrel
design, and 3) a combination of 1 and 2 above.
My analysis of the stock Glock stock
barrel and a KKM Model G22D1 "Match Grade Barrel" showed that
it is impossible to reduce bullet-jump sufficiently by adjusting seating
depth alone.
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In the picture above, the round in the
middle depicts a "typical" 40 S&W round with a cartridge
OAL of 1.126". The round on the right illustrates how far the
bullet seating depth must be reduced to achieve a zero bullet-jump in
either the stock Glock or KKM G22D1 barrels. Obviously, a 1.301"
long round isn't going to fit in the magazine. This is due to an overly
generous free-bore specification in both barrel designs. It is my
contention that a bullet-jump of 0.175" is excessive, increasing
the probability of projectile misalignment to an unacceptable level.
The round on the left? Read on and I'll
tell you about it...
And
then? |
