A sniper's precision By STEPHEN HUNTER

[m25operator]

Twist rate is a decent point of beginning, but more important is velocity and rpm, that is the difference between short and long barrels, in a given twist rate, I'm gonna paste a few things here for you guys to chew on.

1st, The formula for RPM:

     (Muzzle velocity in fps) X 720
     ------------------------------  =  RPM of bullet
           (Twist in inches)

2nd,: An interesting question is why does the bullet disintegrate down
: range and not right away at the muzzle?
: The answer is that the most internal forces are generated when the
: over-stabilized bullet must follow an arcing trajectory, so it
: disintegrates when its trajectory starts requiring it to go where
: its not pointing.

I think the jacket will be under stress from centrifugal force for only
long enough for it to go past its yield point, then it will fracture.
Seems to me the greatest stress on the jacket comes from spinning the
bullet; not its downrange path or trajectory.

Some bullets have jackets thin enough that when some deep-grooved barrels
shoot 'em, the jackets thickness at the bottom of the engraved groove
won't hold 'em together at even the minimum RPM rate to stablize them.
Boots Obermeyer (match-grade barrel maker) designed his 5R rifling style
to compensate for this very thing.  His barrels typically have deeper
grooves than others.  This is fine as they last a bit longer.  But the
005-in. deep grooves were engraving the Sierra 7mm 168-gr. HPMK bullet
too much in the 1970s; a lot of 'em were flying apart in the first 100
yards of flight.  So, Boots designed a rifling groove with the sides
angled instead of straight up and down.  The lands, with their beveled
edges, engraved the bullets with less stress at the edge points and the
168-gr. bullets no longer came apart.  As luck would be, this same 5R
rifling style also shot the longer, heavier 30 caliber match bullets
more accurately than the traditional square-edged rifling.

: Taking another look at the assumptions,
: it was assumed that the coeff. of drag (Cd) was constant and this is
: not true. The faster the bullet, the lower the Cd.  This means the
: drag does not quite increase as v^2 so faster bullets will be more
: stable than predicted by the simple Greenhill formula which depends
: only on the twist.

Hooray for you!  Few people realize that the (almost 100-year old)
Greenhill formula was created when muzzle velocities were in the 1300 to
1900 fps range for rifles.  And most bullets were flat-nozed, flat-based
lead ones.

And one should note that Sierra is about the only bullet company that
says their bullets have two or three BCs; each in a different velocity
range.  Even aeronautical engineers know that a plane's aerodynamic
characterists change somewhat with speed.  Why should bullets be any
different?

: The point is that the stability should still be more sensitive to
: twist than velocity, so although an RPM range would be useful for
: practical guns and velocities used, there may be a more accurate
: way to give the lower stability limit in general.  I'll try and
: come up with more quantitative results.

I've talked to some physics gurus who have some interest in the spinning
bullet syndrome.  If you can get some definitive answers, that's great.
Meanwhile, I'm gonna talk with Sierra's folks and find out what neat
and wonderful things they might consider putting in their next revision
of their loading manual.

Speaking of RPMs, velocities and twists, compare the commonly used twist
rates vs. velocity vs. bullet caliber/weight/shape across the various
handgun and shouldergun scenerios.  Short, fat, blunt nozed bullets just
don't spin very fast to be accurate.  Long, skinny, pointy-nosed, tapered
butt, high-speed paper and game punchers really spin seemingly close to
the speed of light and fall just about on top of each other in a group.

BB