Concentricity - How much more accurate?


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Lovesbeer99
October 1, 2011, 10:00 AM
I've been investigating the benefits of concentricity and the investment in the tools to measure it along with some neck turning tools but I want to know if I'm wasting my time.

I shoot service rifle and with my current load recipe I shoot under 1 moa. I'll guess about 3/4 actually. I'm using LC brass that has been fired multiple times with flash holes deburred, primer pockets uniformed, and trimmed to legnth. (I have another AR with a scope on it also that shoots similarly)

OK .... so if nothing else changes and I start using the neck turning tools how much more accuracy can I achieve? Are we talking 1/2 moa or 1/4 moa?

If if helps I'm regularly shooting 2" groups at 300 yards with my scoped AR from a prone/bipod position but I use my service rifle most of the time.

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243winxb
October 1, 2011, 10:31 AM
Just buy a neck turning tool. Lyman & others make them that fit on a case trimmer. Skip the measuring tools. Let your target tell you if it helps accuracy or not. Sorting brass by weight may help get rid of flyers.

Clark
October 1, 2011, 11:20 AM
from "The American Rifleman" ~ 50 years ago, and the NRA says it is ok to quote.

"Gauging Bullet Tilt"

THE MOST PRECISE AMMUNITION
FROM A LOT CAN BE SELECTED
WITH A BULLET ALIGNMENT GAUGE.

By A. A. ABBATIELLO

Other factors being normal, bullet
tilt with respect to the case center-
line affects group size. If the barrel
length and twist are known, it has been
found possible to predict the direction
from the group center in which the tilted
bullet will strike. If the amount of tilt
is known, the distance from the group
center can be predicted.
Significant score improvement has
been noted by those who have tried
such gauged ammunition.
In cal. .30 long-range shooting, the
best match-grade ammunition will group
in one to 2 minutes of angle under test
conditions. Part of this spread is due
to the bullet tilt with respect to the
case centerline, imposed by the bullet-
seating tool. This tilt displaces the bul-
let’s center of gravity slightly to one
side; in bullets such as the cal. .30 Ml,
the amount is about 1/8 the displace-
ment of the bullet point. It enlarges
groups by amounts up to one minute.
These deviations become proportion-
ately less as the tilt is reduced. Tilts
over .O04" do not seem to increase the
dispersion of the group beyond the ex-
pected one minute. Perhaps this is
because a well-fitting chamber has a
tendency to straighten any rounds
which are excessively tilted. Other ex-
planations are possible.
The gauge consists of a V-block
which permits rotating the round about
the bullet point and 2 tangent spots
near the case head. A dial indicator
which reads in tenths of thousandths of
an inch (.0OO1") bears on the bullet
near the case neck. Half the total indica-
tor reading is used as the displacement
for determining the classes into which
the rounds are separated. The high point
is also marked at this time for orienta-
tion of the round in the rifle chamber.
Rounds with .0O2" tilt or less can
be considered good enough for long-
range use, while those with .O03" and
.OO4" tilt are best used only at short
ranges. In general, it was concluded
from target results that each .0Ol" of
tilt will increase the group spread about
1/4 minute of angle, up to a maximum
of .OO4" as mentioned above.
Under test conditions, it was found
that when the rounds were chambered
with the high point always in the same
orientation, the groups were smaller
than when it was randomly oriented.
Gauging and orienting the rounds can
produce the smallest groups of which
that ammunition is capable.
These ammunition refinements are
becoming important, particularly in
long-range matches.
The essentials of the tilted bullet were
discussed in detail no less than 50 years
ago by Dr. F. W. Mann in his book
"The Bullets In Flight". He pointed out that
the balance of the bullet and the spiral
path of the center of gravity are of
high importance in accuracy.

Following a discussion between
George L. Jacobsen of Frankford Arse-
al and the writer at the 1959 National
matches, a trial of the effect of neck
concentricity was carried out by Jacob-
sen. He described his results in ".30-’O6
Cartridge Cases And Accuracy", which
appeared in THE AMERICAN RIFLEMAN,
January 1960, page 20.

SEATING TOOL A FACTOR

The effects which Jacobsen found,
though small, are essentially in agree-
ment with the work reported here.
However, he did not separate the effects
of neck eccentricity and the bullet cen-
ter-of-gravity location with respect to
the bore. The angular direction of the
bullet seating tool is a controlling factor
in the initial position given to the bullet,
rather than merely case neck eccen-
tricity. Case necks can be centered or
eccentric, and the bullet can be inclined
in completely random directions. The
tilted bullet is believed to be the main
cause for center—of-gravity side shift.
The cal. .30 boattail bullet of 173 grs.
weight was selected for these tests be-
cause it is in common use and is of
sufficiently high quality for use in the
National Matches.
Using the gauge shown, 42 ammuni-
tion lots were sampled and the high
point was marked on each round gauged.
These rounds were grouped in steps of
.OO1" bullet tilt, and the data tabu-
lated. The results gave a bell—shaped
curve for 829 rounds of match ammu-
nition, peaking at about .0O2" (see
illustration). Measurements on Service
ball ammunition produced a curve of
similar shape, but peaking at about
.0025" tilt.
This graphically illustrates that even
match-grade ammunition has appreci-
able variations. There is a large spread
among particular lots and boxes. In
general, 10% to 20% of each lot, de-
pending on ammunition quality, falls
into .0O3", .0O4" or even up to .O10"
tilt. Run-of-the-mill ammunition can
thereby enlarge groups to about twice
the size which the same ammunition
can show when it is gauged before firing.
Since the tilt angle of the bullet is
so small (about 1/4 °) it is difficult to
perceive visually. The gauge, however,
makes the sorting a fast, routine step.
A mathematical solution of this prob-
lem was also tried (see box) and is in
good agreement with the results ob-
tained. It is gratifying to find the math-
ematical solution and the experimental
results in agreement.



MATHEMATICAL SOLUTION

A laterally displaced center of
gravity moves through the rifle bore
in a helical (screw) path. The pitch
of this helix is the pitch of rifling,
and its radius is the lateral displace-
ment of the center of gravity. On
leaving the muzzle, the center of
gravity continues in the direction it
had at that point. For example, if it
leaves at top of the bore and rifling
is to the right, the departure will be
to the right. The bullet travels ap-
proximately 2l.5" in a 24" barrel,
making 2.15 turns in the 10" twist
of rifling. The number of turns
shows the orientation on emergence
compared with that in the chamber
before firing. The angle of emer-
gence is that angle whose tangent is
2 pi times the lateral displacement
divided by the rifling pitch. For
.004" point displacement and I0"
rifling pitch, the tangent is 1/8(2·pi)
(.004)/l0 and the corresponding
angle is 1.1 minutes.
The displacement on target from
this cause is proportional to the
range and can be obtained without
noting the angle. For example, ,004"
point displacement gives in l0"
rifling pitch, so far as this mecha-
nism goes, a target displacement at
100 yds. (3600") indicated by the
proportion .00l· pi /10=X/3600, from
which x =1.1".

243winxb
October 1, 2011, 11:58 AM
More on the subject here > http://riflemansjournal.blogspot.com/2009/07/reloading-audette-neco-case-checker.html Benchrest Central > http://benchrest.com/showthread.php?70706-Case-run-out

ranger335v
October 1, 2011, 03:41 PM
"...how much more accuracy can I achieve? Are we talking 1/2 moa or 1/4 moa?"

It really isn't calibrated so no one can honestly tell you. Depends on your methods, your rifle/chamber, your components, your shooting skill. And then there are the imponderables ....

Dave P
October 1, 2011, 04:02 PM
I contend that an SR shooter will not see a difference on target (maybe if you are a High Master). Although you may have more confidence in your loads.

murf
October 1, 2011, 10:56 PM
grab the current issue of handloader magazine and check out the article on page 60. mr. barsness answers your question quite nicely.

murf

Clark
October 2, 2011, 12:12 PM
The chamber throat will bend the neck straight by pushing on the bullet.

That happened to be 2.2 moa increase in group size calculated, and then verified in a SAMMI chambers 30-06 in the above work by the army, NRA and AA.

If you have some 6mmPPC chamber that is so tight that a spec of dust prevents easy chambering, then a crooked neck will have much less effect on accuracy. That non factory tight chamber will bend the cartridge almost perfectly straight.

Lovesbeer99
October 2, 2011, 01:07 PM
Clark - can you please elaborate? I'm not sure what you're saying.

T Bran
October 2, 2011, 02:13 PM
I beleive what Clark is saying is that if you have an overly tight chamber and cram a bullet that is not concentric into that chamber it will mechanically force the bulllet to streighten out. I agree also read Lees info on factory crimp dies they claim that the die will help to make the bullet more concentric when it is crimped. Wish I had a gague to test that claim but im not gonna buy one since my guns are more accurate than I am anyway. If any one has tested this I would be intrested in your results.
T

Walkalong
October 2, 2011, 02:31 PM
Even a tight chamber is not going to "straighten out" a crooked round. If you have a tight necked chamber and your round is so crooked that the chamber will "straighten" it out some, you need better reloading technique.

Benchrester shooters using tight necked chambers go to great lengths prepping brass, sizing it straight, and seating bullets straight. Yes, the loaded round does fit the chamber with very little "wiggle room", but the thought that you can just load em crooked and let the chamber straighten it out goes against everything a Benchrest shooter is striving for.

There are so many variables in shooting and reloading no one can tell you what amount of accuracy increase, if any, you will see by getting your loaded rounds more concentric.

Loaded straight ammo is a good thing, and that is a fact. :)

ranger335v
October 2, 2011, 03:22 PM
The Army's .30-06 runout tests simply showed the chamber itself must limit the amount of bullet runout it can accept. Obviously, once a certain degree of runout is reached, the throat WILL limit the runout to whatever the chamber 'slop' allows as the round is chambered and that's what the tests showed. (The degree of tilt-limiting depends on the individual throat, it's NOT held to what their single 1903 test rifle exhibited.) That self-limiting effect should not be considered a valid method of 'correcting runout' except at the extremes.

Walkalong
October 2, 2011, 06:52 PM
That self-limiting effect should not be considered a valid method of 'correcting runout' Exactly.

bigedp51
October 2, 2011, 09:23 PM
More reloaded ammunition is loaded inaccurately because the decapping rod/expander button is locked down off center than any other cause. A cheap lee Collet die will produce ammunition with less runout than any standard type die.

Bottom line, it doesn't do any good to turn your necks if the neck of the case has been pulled off center by the die.

Clark
October 3, 2011, 12:30 PM
In math terms, we have a linear relationship between accuracy and runout, until the runout is so large that it will not fit in the chamber that bent.

As the run out increases beyond what the chamber will straighten out, there is no change in accuracy, as at the time of firing all ammo has the same run out.... the most runout that fits the chamber.

The part of the chamber that affects this is the throat.

In a 30-06 the throat specification and ammo specification per the SAAMI drawings are so lose that the fit is unknown.

This is part of the reason that many rifles shoot better with the bullet jammed into the lands... at least one end of the bullet will be concentric.

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