From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Rate of twist in .308 barrels
Organization: Hewlett-Packard Fort Collins Site

Each weight and shape for each bullet of a particular caliber needs to
be spun fast enough to stablize it, but not so fast that its unbalance
will cause it to wobble and deflect at right angles to its path.

For M1 rifles using bullets no heavier than 168 grains, a 1:12 twist is
about perfect through all ranges to 1000 yards.  If heavier bullets, such
as 180 or 190 grain ones, are used, a 1:11 twist is better.  If the bore
and chamber dimensions are tight enough for the load to produce a muzzle
velocity with 190-grain bullets of at least 2600 fps, a 1:12 twist will
stablize them very nicely.

For M14/1A rifles with their barrels being two inches shorter, a 1:11
twist is ideal for the 168-grain bullets.  That twist will also work
for 180-grain bullets.  Heavier bullets should not be used in M1A/14
rifles.

If Sierra's 155-grain match bullet is used in either rifle, a 1:12 twist
is fine, but this bullet is not suitable for ranges over 600 yards.  It
can't be fired fast enough to keep it supersonic through 1000 yards which
all bullets need to be in order to shoot accurately.

BB

From: bartb@hpfcbart.fc.hp.com (Bart Bobbit)
Subject: Re: Rate of twist in .308 barrels
Organization: Hewlett-Packard Fort Collins Site

P.VASILION (v111qheg@ubvmsb.cc.buffalo.edu) wrote:

: 6 groove 1/10 is best across the course and especially 1000 yards. Best
: 	ammo is 168 grain

I disagree with the above twist rate and bullet weight.  That's because
nobody uses it to consistantly produce top scores in competition at 1000 yards
with service rifles in 30 caliber; either M1s or M14s.  That twist rate spins
168-grain bullets too fast and they just don't shoot very accurate.

If a 168-grain bullet is selected, 1:11 and 1:12 twist barrels consistantly
produce better accuracy.

And given a choice, a heavier bullet is preferred.  M1 rifles can easily
handle 180 and 190 grain bullets.  With a 1:11 twist for 190s, or 1:12 for
180s, they will produce better scores.  M14s can use up to 180-grain bullets.
A 1:11 twist with them will produce better scores than a 1:12 or 1:10 will.
At least that's what they've been doing for the last 20 years.

Interestingly enough, 4- or 5-groove barrels have produced the best scores,
and accuracy, in service rifles for the last 40 years.  Lighter weight
30 caliber bullets just don't seem to shoot as accurately in 6-groove barrels
as in 4-, 5-, or even 3-groove barrels.  Competitors around the world shooting
the 7.62mm NATO bullet (144 to 147 grains) have found 4-groove barrels the
best for accuracy.  They've tried 6-groove barrels, extensively, but they
just don't produce the best accuracy.  If they did, they'd be used.

: 6 groove 1/12 is best for short ranges ( <600yd? )

The same comments apply here as did above.  Distance to the target has
nothing to do with twist.  As a bullet needs to be spun some amount in
RPMs to stablize it, that spin rate slows down very little in the 1.6 seconds
it takes to go from the muzzle to a target 1000 yards away.  Velocity will
drop to about 40% of what it was at the muzzle; spin rate will only drop
to about 90% of what it started with.  Tests done in the early 1900s proved
this.

: 6 groove 1/11 is compromise.

I don't know of any 6-groove barrel in a 30 caliber service rifle that
shoots well.  By well, I mean 20-shot, 4 to 5 inch groups at 600 yards.
All of the best shooting M1s and M14s have 4- or 5-groove barrels.

: 4 groove 1/10 is best for 173 or heavier service grade bullets (read not
: 	top of line target ammo)

This is what folks thought was the best in the early 1950s.  In the 1960s,
when the military rifle teams finally began to build really accurate rifles,
and civilian highpower match rifle builders finally did the same thing,
it was found not to be true.  Match M1 (and service) rifles had a 1:10 twist
barrel.  The M14 had a 1:12 twist barrel.  When the M14 started to be worked
over into a competition rifle, it was found that the slightly slower muzzle
velocity of the M2, 173-grain match bullet in the M14's 1:12 twist barrel
shot better than in an M1's longer barrel at higher velocity.  This meant
that the 173-grain bullet was being spun too fast for best accuracy.  But it
wasn't really well understood until the Navy started rebarreling their 30
caliber M1 rifles with 7.62mm NATO barrels with 1:12 twists.  As the match
conditioning success with M1 rifles was miles ahead of the M14 at the time,
folks took notice of the greatly improved accuracy of the M2 173-grain
military match bullet in the 7.62mm NATO M1 rifles.  Then the M14s began to
be match conditioned to shoot as well as the M1s did.  Their 1:12 twist
barrels were soon found to not quite spin the bullets as fast as they needed
to be in their 22-inch barrels.  They went to 1:11 twists and the rest is
history.

: No other devious changes is performance WRT different ROT. I reccomend 1/10,
: of course. Heaviest barrel is best, IMHO.

There's no evidence that heavy barrels shoot more accurately than lighter
ones.  Although a heavier barrel may be easier to hold steady in both benched
and position shooting, machine rest tests have shown barrel weight does
virtually nothing regarding accuracy.  As long as the barrel behaves the
same from shot to shot, all the bullets fired will go into one very tiny
group irrespective of barrel weight.  The largest group of folks to go from
light-weight to heavy-weight barrels were the military highpower rifle
teams.  When they started using heavier barrels, they felt the accuracy
level didn't change.  Instead, the heavier rifles were easier to hold steady
and recoil was less; these two things raised their scores.  Both heavy and
light barreled rifles shot to call equally well.  It was that the shooter's
calls were better on the target (closer to the center) with heavier barrels.

BB

From: bartb@hpfcbart.fc.hp.com (Bart Bobbit)
Subject: Re: Rate of twist in .308 barrels
Organization: Hewlett-Packard Fort Collins Site

Brian Woodroffe (bww@hpqtdxa.sqf.hp.com) wrote:

: However say you are required to shoot NATO ammunition (ie 150gr approx
: bullets) and that you are required to shoot to 1000yds (as you are at
: Bisley), then what would your answer be?
: FYI: The UK experience is to go with a 1-in-12 30inch barrel.

With good, well balanced bullets, a 1:12 twist will work just fine.
If the bullets are not well made, that is somewhat unbalanced, a 1:13, or
even a 1:14 twist will shoot 'em more accurately.  That's because the
increased spin rate of the faster twists will cause the unbalanced bullets
to move at right angles to their path due to increased centrifugal force
from being unbalanced.  [Even car tires, or tyres if you choose, spin very
nicely until their unbalance at faster speeds makes 'em go thump, thump.....]

BB

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: .308 Palma Bullet Loads
Organization: Hewlett-Packard Fort Collins Site

sfaber@intgp1.att.com wrote:

: According to the above the 173 grain match bullets were used with
: 1:12 twist barrels.   My best current calculation predicts a
: maximum 1:11.6 in. twist needed for that bullet.  This would easily
: change to about 1:13 in. in the Rockys where the air is less dense,
: but the above suggests the 1:12 twist worked in general.
: The guy who sold me the 173 gr bullets also implied they would not
: work well in a 1:12 in gun (in Iowa.)
: Maybe I need to go back to the drawing board on this bullet or
: use a more accurate estimate of the B.C. if it turns out the
: 1:12 was good at standard air density.

: If anyone knows the B.C. for the 173 gr mil. match bullet I'd appreciate
: it.

Steve's calculations are very much correct.  The M2, 173-gr. machine gun
bullet (later the match bullet used in M72 and XM118 and M118 ammo) has a
ballistic coefficient about half way between Sierra's 180-gr. and 190-gr.
HPMK bullets; about .530.  I've used BC numbers in that range with Sierra's
software to duplicate actual trajectories through 1000 yards at altitudes
from sea level to 8,200 feet.  Errors between calculated and actual bullet
drop were less than 2 inches at 1000 yards; near zero at 200 yards.

BB

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: .308 Palma Bullet Loads
Organization: Hewlett-Packard Fort Collins Site

sfaber@intgp1.att.com wrote:

: The guy who sold me the 173 gr bullets also implied they would not
: work well in a 1:12 in gun (in Iowa.)

He was probably not aware of the great success folks had with the 173s
in 7.62mm NATO M1 rifles built by the Navy's Marksmanship unit.  Those
barrels had 1:12 twists and would shoot M118 ammo at about 2625 fps for
the 173-gr. bullet loaded.  Many matches were won and records set with
than ammo in 1:12 twist barrels.

And, Winchester's M70 target rifles in .308 Win. had 1:12 twist barrels.
They shot M118 ammo very well; typically better than service rifles did.
Velocity with M118 was about 2650.

But wait; there's more........

Many folks had Winchester target barrels with 1:12 twists put in their
accurized M1s.  Don McCoy (best M1 maker) put dozens and dozens of M70
target barrels in his .308 M1 rifles; they drove tacks, er...I mean they
shot sub-MOA groups at 600 yards with a good lot of M118 ammo; even better
with handloaded 173s.

BB

From: bartb@hpfcla.fc.hp.com (Bart Bobbit)
Newsgroups: rec.guns
Subject: Re: Overstabilised Projectiles.
Date: 30 Oct 1994 13:34:15 -0500

Ian Cameron (icameron@enterprise.powerup.com.au) wrote:

: There seems to be some controversy about the deleterious effects on 
: accuracy of "over spun" projectiles. 

What controversy?  Spinning a bullet too fast as it leaves the muzzle
will reduce its accuracy.

It's the same physics thing as a wheel on a car being a bit out of
balance.  When it spins at lower speeds, no thumping is felt.  As its
spin rate increases with increased vehicle speed, there's a point where
its out of balance can be felt by the thumps it makes on the road.  
Bullets behave the same way; the more unbalanced they are, the more
forces there are as their spin rate increases to move them at right
angles to their flight path as they are no longer restrained by the
barrel upon exiting its muzzle.

: Will a fast twist open up the light bullet
: groups just a few thou', or will it turn a tack-driver into a scatter-gun?

It all depends on how well balanced the bullet is when it leaves the
muzzle and how fast it's spun.  Groups can open up anywhere from 1/8th
1/2 MOA at one hundered yards; sometimes more at longer ranges.  Whether
or not someone can observe this with their rifle and ammo is something
else.

: Also, what twist would I need to stabilise say an 80 gr VLD at swift 
: velocities? 

80-grain VLD 22 caliber bullets probably need to be spun at least 
270,000 RPM at the muzzle velocity a .220 Swift will shoot them.  Either
a 1:9 or 1:10 twist may work well.  As 50-grain 22 caliber bullets need
to be spun in a spin bandwidth at Swift muzzle velocities of about
165,000 to 185,000 RPM for best accuracy, they'll spin over 280,000 RPM 
at 3900 fps from a 1:10 twist barrel; too fast for best accuracy.

If one wants to shoot a wide range of bullet weights from a given barrel
with maximum loads, like 50, 52, 55, 60, 63, 68 and 80 grains from a
.220 Swift, best accuracy will be attained with those whose weight varies
no more than about 15 percent; i.e., the 50 through 55 grain bullets will
shoot most accurate from a 1:16 twist.  Or the 68 and 80 grain ones will
probably shoot most accurate from a 1:9 or 1:10 twist.  

The same thing happens in other calibers when bullet weight varies from
light to heavy by 160%.  For example, 30 caliber bullets from a .308 Win.
ranging from 155 to 250 grains.  The 155-gr. bullet leaving at 3000 fps
requires a 1:13.5 twist for best accuracy.  A 250-grain bullet at a
muzzle velocity of 2200 fps needs a 1:8 twist for best accuracy.  When
the 155-gr. bullet came out a few years ago, folks tried maximum loads
with it in typical 1:11 or 1:12 twist barrels; it wasn't as accurate as
when used in 1:13 to 1:14 twist barrels.

You can use the formula:

	  (velocity in fps) X 720
	  -----------------------  = spin in RPM
	    twist in inches

to find out how fast a bullet will spin with a given muzzle velocity and
twist.

You'll probably get the best accuracy with a wide range of bullet weights
by using a twist that's correct for the heavy ones, then use the best
quality lighter-weight bullets when those are desired.  You could use
the lighter bullets at reduced velocities and their accuracy may well
improve.  But some compromises are needed when bullet weight range is
great for a given rifling twist.  You get to decide which bullet you
want the best accuracy with, then pick a twist to produce it. 

BB

From: dmc@io.org (Daniel Chisholm)
Newsgroups: rec.guns
Subject: Re: How to pick rifle twist rate?
Date: 27 Jul 1995 22:27:47 -0400

In article <3v6usj$925@xring.cs.umd.edu>,
davi mcbride <st88d@Bayou.UH.EDU> wrote:
#Daryl Garrett (garrett@aurxc5.aur.alcatel.com) wrote:
#: The post-ban Colt HBAR rifles (223 caliber) are offered in several
#: twist rates, as shown below.
#
#:   MT6700 Match Target Competition HBAR  -- 1 in 9"  twist
#:   MT6601 Match Target HBAR              -- 1 in 7"  twist
#:   MT6430 Match Target Light Weight      -- 1 in 10" twist
#:   MT6830 Match Target Light Weight      -- 1 in 12" twist
#
#: So, what difference does the twist rate make, and why would someone
#: pick one twist over another?  My interest is target shooting,
#: typically 100-200 yards, but maybe 300 yards on occassion.
#
#
#Generally speaking, the tighter the twist rate, the heavier the bullet that
#can be stabilized.  If long rang match shooting is your primary agenda, I
#would think that the heavier low drag bullets would be on the menu, there-
#fore the 1 in 9 would be the ticket.  Someone in the BR crowd should ex-

1 in 9" is fine for 68/69 grain match bullets, but just a tad shy
for the Sierra 80 grain or JLK 75 & 80 grain match bullets.  There's
no real reason to use these bullets if you're not going to shoot
beyond 300 yards.

If you want to be able to shoot the heavier match bullets (for improved
wind bucking), you should get a 1 in 7" (or 1 in 8" if available) twist for
your .223.

#pound on this but I believe there is a correlation to twist, bullet weight
#and accuracy.  As I understand it, you want no tighter twist than is neces-
#sary to stabilize the chosen weight.  If, for instance, varmints are your

Correct.  Not enough twist is disastrous for accuracy, too much twist
causes very minor degradation.  For serious target shooting, it makes
sense to get every advantage you can, so an attempt is made to find
the minimum required twist rate.

#primary targets and 50-55 grain projectiles are the preferred weight, the
#1 in 12 twist would be applicable.  1 in 9 would suffice, but likely not be
#as accurate as the 1 in 12 (with the 50-55 grain weight). As I understand
#it.  Is that perfectly clear?

1 in 9" may not be "ideal" for 50-55 grain varmint bullets, but keep in
mind that the accuracy degradation will be minor.

I shoot 80 grain Sierras, 75 grain JLKs, and 55 grain Nosler ballistic
tips out of a 1 in 9" twist 26" barrel bolt action target rifle.  They
all shoot excellent, I am unable to see any differeence in group size
between 55 grain hunting bullet (the Nosler) vs. the long and heavy
match bullets (typ 5 shot group at 100 yards is 0.5" for all three
bullets).

Anything other than the 1 in 12" twist should be fine for shooting
all miltary ball ammo, both 55 grain and 62 grain.  The 1 in 12"
will only work for the 55 grainers.


- Daniel

From: bartbob@aol.com (Bartbob)
Newsgroups: rec.guns
Subject: Re: AR-15 Twist Rate
Date: 15 Dec 1995 12:15:34 -0500

There's been a lot of myths and misconceptions regarding the M16
rifling twists.  I hope this will help folks out.

In the beginning, a 1:14 twist was used.  It was well known that 50-55
grain .224 spitzer bullets leaving at 3000-3100 fps needed a 1:14 twist
to properly stabilize them.  So the Army used that in the first M16s.
But it was soon learned that the muzzle velocity, on average, wasn't
quite as fast as they'ld hoped for.  Especially in cool/cold weather.

That's when a 1:12 twist was implemented.  At the lower muzzle
velocities, that spun the bullets fast enough.  Accuracy was excellent
and no, the bullets didn't tumble; they flew point on through 1000 or
more yards.

When heavier bullets began to be used, they needed to be spun faster.
The 1:12 twist wasn't fast enough for the velocities they left the
barrel at.  So 1:10 down through 1:7 were tried.  The rest is history.

You can determine a bullet's spin rate by multiplying the muzzle
velocity times 720, then dividing that by 12.  A bullet leaving at 3000
fps from a 1:12 twist barrel spins at 180,000 RPM.  70-gr. bullets from
an M16 need to be spun about 250,000 RPM.

BB

From: towle@vssteg.enet.dec.com (Clark Towle Gunsmith)
Subject: Re: M16A2 barrel wearing out prematurely?
Organization: Digital Equipment Corporation

In article <9209111726.AA00896@nuc.Utah.univel.COM>, Thi_Huynh@univel.COM (Thi Huynh) writes...
#Hi folks,
#
#I've heard that US Army M16A2 barrels are wearing out prematurely with
#NATO SS109 (US M855) ammunition due to its 1x7 barrel twist.  I've also
#heard that the 1x7 twist was originally designed to handle the M856
#tracer round (which has longer bullet than the M855 round).  Can anyone
#confirm this?

 The 1/7 twist seems to cause premature throat erosion. The cause is because
the twist is so fast that the bullet actually skids a few thousands of an
inch before the rifling takes hold and can stabilize it. This condition gets
exagerated with each shot. I've seen barrels that swallow the throat erosion
gage in as little as 2000 rounds. The 1/7 twist was originally deemed
necessary to stabilize the SS109 round. There is no truth to the rumor it was
needed to stabilize the tracer round.

#Can the 1x9 twist effectively handle the SS109 round?  What about heavier
#bullets such as the 69-grainers?  Can the 1x9 stabilize these too?

 Yes. As a matter of fact it is generally thought that the 1/9 twist is the
best compromise twist for all 223 bullet weights. 1/9 barrels generally yield
impressive accuracy results. 1/10 barrels come in next.

#What is the exact weight of the SS109?  I have seen 62-grain and read of
#63-grain!

 62 grains.