From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: [RIFLE] [RELOADING] Load development
Organization: Hewlett-Packard Fort Collins Site

Todd Enders A262 857-3018 (enders@warp6.cs.misu.NoDak.edu) wrote:

:      This is a very worthwile comment for those who have wind flags and
: know how to dope the wind.  Unfortunately, most hunters/plinkers/non-comp.
: shooters/etc. don't know how to do this.  For them, shooting only on dead
: calm days would probably yield better results.

Nor do they typically not care enough to learn.

: #Shooting at 100 yards is a waste of time if the load will be used at
: #greater distances for accuracy shooting.
:
: Indeed good advice.  One of the raps against 6.5mm cartridges, especially
: with 140 gr. bullets is that "they don't group" at 100 yds.  However, if
: tested at 200 or 300 yds, the groups, on an M.O.A. basis are *tighter* than
: at 100 yds. The bullets just needed a little time to "settle down" in their
: trajectory.

I've heard for many years of these `raps' with just about all bore sizes.
So some time ago, I did some tests to find out what causes this `wait until
they settle down [go to sleep is a common expression]' philosophy.  When
bullets were fired at muzzle velocities fast enough to spin them too fast,
they always grouped smaller in MOA at longer ranges than shorter ranges.
When fired just fast enough to spin at the lower end of their required RPM
range, groups were equal in MOA through 300 yards or thereabouts.  Groups
did enlarge due to velocity spread and time of flight which is what the
laws of physics predict.

Benchresters typically get group sizes very near the same MOA for ranges
through 300 yards.  But they shoot their bullets at a velocity and with a
rifling twist rate just fast enough to stabilize the bullet.  They'll
even go from a 1:14 to a 1:13 twist barrel in colder weather as the bullet
needs to be spun a bit faster with the increased air density.

: You may have to determine the capacity of the case to the base
: of the bullet (in grains of H2O) as seated and use that weight as a *guide*
: to the loading data to indicate powders that will achieve this range of
: loading densities.

Or just fill a case with powder until it reaches the point where the bullet's
base will be when seated.  This is a much more accurate method, besides
being simpler, of determining case capacity.

: For benchrest, I can see the wisdom in loading at the range, but have to
: wonder in the case of highpower match rifles or hunting rifles where one is
: by necessity carrying pre-loaded ammo.

Some of the top long-range highpower shooters bring primed cases to a match,
then the night before shooting, they'll charge the cases and seat bullets.
Powder is either pre-weighed and put in vials or is thrown and weighed.
Long-range highpower ammo tends to shoot about an eighth MOA better this
way.

: The headwind/tailwind component should be
: at least somewhat obvious from the wind flags, though.

Oops.......this is another one of those old wives tales.  Consider how many
miles per hour your bullet leaves the muzzle at.  Then consider how many
miles per hour those bullets will vary within their velocity spread.  And
finally consider what the difference is between a head or tail wind in
miles per hour and the difference in muzzle velocity.  A 20 fps velocity
spread equals a 14 mile per hour velocity spread.

:      The assessment that "For many folks, maximum accuracy doesn't matter
: enough to go to the trouble it takes to get everything exactly right for
: load testing." is, no doubt, true.  However, few folks actually *need*
: maximum accuracy in the first place.

I've found out over the years that most folks do indeed want as good of
accuracy they can get.  When asked the question: "What is the greatest
distance you want your bullet to strike away from where the sights are
aligned on the 300-yard target when fired?" they more often than not say:
"No more than half an inch!" or near equivalent numbers.  When I respond
to their objectives by saying they need a one-third MOA grouping rifle and
ammo to do that, they finally realize what accuracy is all about; placing
the bullet very close to where the shot is aimed, even if it's at the
animal's left hind foot or the paper target's 7-ring at 4-o'clock.  They
really want the bullet to go where aimed.

BB

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Disappearing bullets
Organization: Hewlett-Packard Fort Collins Site

Bullet spin rate is found by:

   Muzzle velocity times 720
   -------------------------   =   RPM
    Twist rate in inches

Your 1:7 twist will spin a bullet with 3000 fps muzzle velocity at
308,571 RPM.  That's really fast for the 55-gr. 22 caliber bullets
you used.  Those bullets are best spun at about 155,000 RPM when they
leave the muzzle.  Considering the bullet's are spinning at about
twice what they shoot best at, no wonder the jackets came apart.
There's a lot of centrifugal force on bullet jackets.  A lot of
folks have experienced bullet desintagration when they're fired
from faster than correct twists for the velocity used.

If you use a powder type/weight that will give you about 2000 fps,
your 1:7 twist barrel will still spin 'em about 200,000 RPM.  That's
probably OK and the bullets won't fly apart.  But your accuracy may
not be as good as a 70 or 80 grain bullet fired at max velocity from
that twist.

I've often asked Sierra Bullets to put some information in their
reloading manuals about bullet spin rates.  The section that discusses
each bullet caliber, weight and style is a good place.  Useful ranges
are mentioned in this manual, but the important thing for accuracy is
each rifle and pistol bullet needs to be spun in a narrow RPM range
to be accurate.  Once this RPM range is identified and the above formula
modified to give rifling twist rate for the muzzle velocity the bullet
will be fired at, getting best accuracy is easy.  I'm gonna do some more
talking with the Sierra rep at the highpower nationals this year.  Maybe,
just maybe, he'll be receptive to my suggestion.  I will offer to write
this section up for each bullet; all they'll have to do is supply the
velocity and twist data each of their bullet's need to be most accurate.
I already know what their 30 caliber match bullets need to be spun at to
get best accuracy.  Other bullets would be easy to determine the best
RPM range they need.

BB

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Disappearing bullets
Organization: Hewlett-Packard Fort Collins Site

Bill Preuss (cccbbs!bill.preuss@uceng.uc.EDU) wrote:

: How fast do Sierra's .30 cal bullets need to spin to produce the
: best accuracy - and give that formula again.

Here's the minimum and maximum spin rates in RPM I've found works best
with Sierra's 30 caliber match bullets:

     .308 dia.      Min.       Max.
      Bullet        RPM        RPM
     ---------    -------    -------
     150 HPMK     140,000    160,000

     155 Palma    145,000    170,000

     168 HPMK     150,000    175,000

     180 HPMK     155,000    185,000

     190 HPMK     160,000    190,000

     200 HPMK     170,000    210,000

     220 HPMK     195,000    230,000

     240 HPMK     210,000    250,000

A bullet lot whose bullets aren't quite as well balanced should be kept
at the lower range.  An extremely good lot of bullets (like those in the
brown boxes of 1000 we used to get 10 years ago) can be spun at, or a tad
above the maximum RPM and will shoot very, very accurately.

The formula for RPM:

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

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Disappearing bullets
Organization: Hewlett-Packard Fort Collins Site

sfaber@ihlpb.att.com wrote some good and interesting comments about
bullet spinnin', including these:

: 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 yeild 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