Index Home About Blog
From: Gale McMillan <" gale">
Newsgroups: rec.guns
Subject: Re: Short Range Instability?
Date: 29 Apr 1997 19:35:55 -0400

Steve Podleski wrote:
# Bartbob wrote:
# # Every bullet fired in unbalanced.  The most perfect balanced bullets made
# # become a tiny bit unbalanced upon being slammed into the barrel's throat;
# # sometimes more so depending on how crooked they were in the case, how much
# # off bore axis the throat is, and by how uniformly smooth the leade is.
# # Once they exit the muzzle, their out-of balance condition causes them to
# # take a small jump sideways.  This is one reason why bullets, even with the
# # exact same muzzle velocity and BC don't shoot into the same hole at any
# # range.
# During a 300yd prone match with the International target, I used 47.5
# gr. of IMR4895 with the Speer 145 7mm match bullet in a a .284W case
# shoot out of 1-in-8 twist 30" Krieger barrel.  Results were terrible:
# the bullets danced around in the 9-ring. I got much better results with
# 51 gr. of VV160 and the Sierra 168.  All bullets were molied.
# I was wondering if the cause was:
# 1)too high a twist for the Speer 145 bullet (high spin rates on
# unbalanced bullets)
# 2)too low a charge density for the Speer 145 bullet.
# 3)too low velocity for the molied bullets.
# Would you have any ideas?
# --
# steve podleski
# Everett, WA

What most shooters who go to the overly heavy and overly long bullets
never stop to realize is when you rotate a projectile 25% faster the
quality of the bullet must be several times better quality to fly
equally well.  Which they are not!  A 165 grain 7 mm jacket is just as
ununiform as a 140 grain.  Quality match bullet jackets will run from
.0002 to .0005 thicker on one side than the other.  This goes for a 240
grain 30 cal bullet.  If you fire a 240 gr. 30 call bullet at 2600 fps
out of a 1 in 8 twist the rotational spin is 207,999.99rpm as compared
with a 190gr. fired out of a 1 in 12 twist at 3000fps doing 180,000
rpm.  With both bullets being of the same quality and having the same
variation of bullet jacket thickness it doesn't take a Ph.D. to know
which one will fly the truest.  Calculate the centrifugal force on a
jacket rotating 207,999 rpm.  When you rotate a thin jacket that fast
out of eve 20 rounds you will sling a jacket loose and the core will
rotate at a different speed than the jacket.  Ask any high power shooter
who shoots a 7 mm mag how many times he has been drilling the 10 ring
and have a round never get to the target.  I spent 30 days working with
the Navy Seals evaluating the 240 and 250 gr. 30 cal bullets out of a
300 W ma.  I built two rifles identical except one was an 8 twist and
one was a 12 twist.  One to shoot 240 and 250 gr. bullets and one to
shoot 190 gr.We laid two shooters on the line with instructions to take
a dead hold and fire on command of the coach.  The coach tried to call
the most opposite conditions that he could.  When we scored each target
and figured the most drift the 190s out shot the heavy bullets by a
considerable amount.  Since that test the Navy has adapted the 180 gr.
match king.  This was not just a few targets but 8 hr. a day 5 days a
week for a month.  When it was completed I rebarreled their 1 in 8
barrel to a 1 in 12.  Things like the tests we ran is what keeps me from
being a heavy bullet fan.You can imagine what would happen to a 190 shot
out of the 1 in 8 at 300 fps faster and that increase in rotational
speed over a 250 gr.

Gale McMillan

Index Home About Blog