From: Bart Bobbitt <bartb@hpfcmgw.fc.hp.com>
Subject: .308 for 1000 yd. Matches

Some posts and responses regarding the use of .308 Win. or 7.62mm NATO in
long range matches have been posted.  Their comments about bullets going
subsonic before going through the paper at 1000 yards echo those I've heard
on the range.

I think there are a couple of things happening that may not be obvious to
some participants in this net, as well as all those others who discuss or
experience this phenomona.

When service rifles are used and the ammo is Lake City Match M852 (Sierra
168-gr. bullet), muzzle velocity can be anywhere from 2450 to 2700.  This
depends on bore and groove dimensions that determine the actual muzzle
velocity.  I've chronographed some service rifles that shot the same lot of
ammo much slower than other service rifles.

One of the highpower competitors here in Colorado says his M1A does not keep
M852 ammo supersonic at the Nationals in Camp Perry (590 ft. altitude) but
the same lot of ammo is supersonic at the NRA Whittington Center in Raton
(6600 ft. altitude); both are at 1000 yards.  At Perry, his bullets `whoosh'
through the paper but at Raton, they `snap' as they go through.  When he
used Lake City M118 (173-gr. M2 bullet) ammo, at both places the bullets
`snapped' through the target.  This kind of indicates that the M2 match bullet
has a higher BC than the Sierra 168, or, the M118 ammo has a higher muzzle
velocity than M118 in his rifle.

In all situations where bullets `whoosh' through the paper at long range (800
to 1000 yards) they are subsonic; the ones that `snap' are supersonic and the
sonic shock wave from them causes the `snap.'  Also, when bullets are subsonic
at any range, they've always not been too accurate, which indicates a bullet
fired supersonic needs to be kept supersonic all the way to the target,
whatever the range.  But a bullet that is fired subsonic can be accurate.  In
my observations, lighter weight bullets that go subsonic in flight tend to
change directions more so than heavier ones.  That's probably because there is
more mass in them to keep them from changing directions.  For example, black
powder long range rifles used in 1000 yard matches have their bullets going
from supersonic to subsonic before going through the paper.  Yet, their
accuracy does not seem to change when this happens.

Once in a while, I've noticed that a bullet will be supersonic going through
the target at 1000 yards, yet it still keyholes.  That probably means the
bullet was either quite unbalanced or was spun too slow as it left the muzzle.

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Hot .308Win Load?
Organization: Hewlett-Packard Fort Collins Site

Jeff Boone (jeffb@ornews.intel.com) wrote:

: The first time this year (Memorial Day), the bullets
: were passing through the target sideways (Sierra 155gr. Palma @ ~2850
: fps).

One cause of bullets keyholing is they're spun too fast.  At 2850 fps
muzzle velocity from a 1:11 twist barrel, they'll do 186,545 RPM.  That
too fast for the 155.  These Palma bullets shouldn't spin faster than
about 170,000 RPM.  Especially a bullet lot whose quality is at the
high end of Sierra's spectrum; they're out of balance enough to tumble
if spun too fast.  In a 1:12 twist barrel at the same muzzle velocity,
they'll spin at 171,000 RPM; still marginally too fast.

When the bullet goes subsonic, it changes direction somewhat gradually,
not in an abrupt sence as far as I can tell.  I've shot some where half
were subsonic and half supersonic at 1000 yards.  Those that were subsonic
had probably just done so a few yards before going through the target.
But they seemed to shoot about as accurate as those which went through
supersonic.  So, I think the direction change is somewhat gentle.  The
closer the target is to the point of going subsonic, the closer the
bullet hole will be to the group of supersonic ones.  Just a theory
based on some observations.  Interestingly enough, virtually all subsonic
bullets I've heard going through the paper leave a normal hole; no
keyholing.  Even ones that were subsonic at 900 yards still made round
holes at 1000.  I convinced that a keyholed bullet hole is caused from
from a bullet that's unbalanced quite a bit and spun too fast for its
size, weight and shape.  Occasionally, the rifle's muzzle is bad, but
that's seldom seen.

BB

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Newsgroups: rec.guns
Subject: Re: [Rifle] Palma, a captain's report (GB).
Date: 1 Dec 1993 18:37:30 -0500

Lloyd D Reid (reidld@ecf.toronto.edu) wrote:

: I'm not familiar with the Leech and Wimbledon, are they iron sight
: 1000 yard prone matches?

The Leech Cup Match is an any rifle, iron sight match; prone at 1000 yds.
The Wimbledon is the same except any sight can be used.  Each also has
a division for service rifles (M1, M14/1A, M16/AR15) fired at the same
time.

: The 1992 Canadian Palma team was selected on the basis of a shooter's
: performance at 800 and 900 yards, among other criteria.  Scores at 1000
: yards were specifically excluded from the selction criteria, because in
: the opinion of the Captain, scores at 1000 yards contain a certain element
: of luck, as shot under DCRA rules.

Actually, all ranges contain some amounts of luck, such as calling a shot
center, then noting the wind had changed and you know that bullet will go
quite wide into the 8 ring.  As does the relay you shoot on; some relays
do have better conditions than others.

: Bart, would you expect that a good shooter's performance at 1000 yards
: would be materially different from his performance at 900 yards, when
: operating under the American practice of shooting handloads?  

Yes.  There's four reasons:

  * Vertical shot stringing will be greater.

  * More deflection for a given wind speed change.

  * More bullet deflection at right angles to its path due to its
    unbalance.

  * Larger holding area on target by about 11% due to the increased
    range.

: Is the
: .308, when shot with a low drag bullet and good handloads just running
: out of steam around 1000 yards, or is it still going strong?  

At 5000 ft. altitude, a 190 leaving at 2550 fps will go through a 1000-yd.
target at about 1400 fps.  A 155 leaving at 2950 fps goes through the
same target at about 1350 fps.  We've chronographed these bullets going
through two sets of screens; one set at the muzzle, the other at 1000 yds.
At the NRA Whittington Center's altitude of 6600 ft., the 155s clocked
through the 1000-yd screens just over 1400 fps.  But at 600 ft. altitude,
the 155s leaving at 2950 go through just under 1200 fps.  

So, those bullets will still be going strong; but stronger at higher
altitudes.  

: Everyone says that you gotta keep supersonic, as the bullet
: goes through the paper.  What happens if you don't?  

When the bullet goes subsonic, airflow over it gets erratic.  The same
phenomena occurs with planes as was found out by Chuck Yeager flying those
test planes.  As the buffeting caused by this erratic airflow is strong
enough to physically move the flying object, that object changes direction.
The direction it moves depends on the exact airflow patterns over the
object due to its shape.  As most match bullets are hollowpoints, I would
imagine the very slight differences in meplat size and shape are enough
at transonic velocities to cause each bullet to move in different angles.

: How much do groups open up, if you drop to subsonic? 

Lighter bullets move more than heavier ones due to their lesser mass being
easier to move for about the same amount of force.  I've pulled targets for
folks whose 800-yard scores were decent and the bullet's snap going overhead
was well defined.  At 900 yards, the same ammo was putting bullets through
the paper lower enough in velocity to have virtually no snap.  And the group
was much larger proportionally to the 12% increase in range.  At 1000 yards,
no sonic snap of the bullets going overhead was present.  Group size was
much bigger than the 25% range increase from 800 yards might suggest.  The
800 yard group was about 20 inches in diameter; at 1000, the group was near
50 inches in diameter.  That's a 250% increase in group size for a 25%
increase in range.  This guy was shooting 168-gr. HPMKs from a .308 Win.
I don't know what the muzzle velocity was, but I'm guessing 'twas about
2300 fps.  

I think some tests to determine what accuracy degradation due to subsonic
bullets would be interesting; one of these days, I just may do that.  

: Could this be what's
: happening to Canadians, with a 147 grain NATO bullet at 1000 yards?

Yes.  The easiest way to determine if a bullet is subsonic going through
the air is to listen to it going close to, but not through the target; 
like it's a miss.  If there's no sharp `snap' as the bullet goes overhead,
it's subsonic.

: What about a 300 magnum that can put it's bullets though the paper
: several hundred fps above sonic speed?  Is this extra margin
: over the .308's just-barely-supersonic worth anything?

I don't think so, except they deflect less in the wind.  As long as the
bullet stays supersonic, they tend to shoot very well.  The important 
thing for accuracy is to not let the bullet transcend the barrier.  Even
.22 LR match ammo's muzzle velocity is just below the speed of sound so
it won't transcend the sound barrier going down range.  If those 40-grain
pills did go out faster, then transcend the barrier, quarter-inch, 100-yard,
20-shot groups would never happen. 

BB